1.EARTHQUAKE

An earthquake (also known as a quake, tremor or temblor) is the result of a sudden release of energy in the Earth's crust that creates seismic waves. The seismicity or seismic activity of an area refers to the frequency, type and size of earthquakes experienced over a period of time. Earthquakes are measured using observations from seismometers. The moment magnitude is the most common scale on which earthquakes larger than approximately 5 are reported for the entire globe. The more numerous earthquakes smaller than magnitude 5 reported by national seismological observatories are measured mostly on the local magnitude scale, also referred to as the Richter scale. These two scales are numerically similar over their range of validity. Magnitude 3 or lower earthquakes are mostly almost imperceptible and magnitude 7 and over potentially cause serious damage over large areas, depending on their depth. The largest earthquakes in historic times have been of magnitude slightly over 9, although there is no limit to the possible magnitude. The most recent large earthquake of magnitude 9.0 or larger was a 9.0 magnitude earthquake in Japan in 2011 (as of March 2011), and it was the largest Japanese earthquake since records began. Intensity of shaking is measured on the modified Mercalli scale. The shallower an earthquake, the more damage to structures it causes, all else being equal.^[1]

At the Earth's surface, earthquakes manifest themselves by shaking and sometimes displacement of the ground. When the epicenter of a large earthquake is located offshore, the seabed may be displaced sufficiently to cause a tsunami. Earthquakes can also trigger landslides, and occasionally volcanic activity.

In its most general sense, the word earthquake is used to describe any seismic event — whether natural or caused by humans — that generates seismic waves. Earthquakes are caused mostly by rupture of geological faults, but also by other events such as volcanic activity, landslides, mine blasts, and nuclear tests. An earthquake's point of initial rupture is called its focus or hypocenter. The epicenter is the point at ground level directly above the hypocenter.

Naturally occurring earthquakes

Fault types

Tectonic earthquakes occur anywhere in the earth where there is sufficient stored elastic strain energy to drive fracture propagation along a fault plane. The sides of a fault move past each other smoothly and aseismically only if there are no irregularities or asperities along the fault surface that increase the frictional resistance. Most fault surfaces do have such asperities and this leads to a form of stick-slip behaviour. Once the fault has locked, continued relative motion between the plates leads to increasing stress and therefore, stored strain energy in the volume around the fault surface. This continues until the stress has risen sufficiently to break through the asperity, suddenly allowing sliding over the locked portion of the fault, releasing the stored energy. This energy is released as a combination of radiated elastic strain seismic waves, frictional heating of the fault surface, and cracking of the rock, thus causing an earthquake. This process of gradual build-up of strain and stress punctuated by occasional sudden earthquake failure is referred to as the elastic-rebound theory. It is estimated that only 10 percent or less of an earthquake's total energy is radiated as seismic energy. Most of the earthquake's energy is used to power the earthquake fracture growth or is converted into heat generated by friction. Therefore, earthquakes lower the Earth's available elastic potential energy and raise its temperature, though these changes are negligible compared to the conductive and convective flow of heat out from the Earth's deep interior.^[2]

Earthquake fault types

Main article: Fault (geology)

There are three main types of fault that may cause an earthquake: normal, reverse (thrust) and strike-slip. Normal and reverse faulting are examples of dip-slip, where the displacement along the fault is in the direction of dip and movement on them involves a vertical component. Normal faults occur mainly in areas where the crust is being extended such as a divergent boundary. Reverse faults occur in areas where the crust is being shortened such as at a convergent boundary. Strike-slip faults are steep structures where the two sides of the fault slip horizontally past each other; transform boundaries are a particular type of strike-slip fault. Many earthquakes are caused by movement on faults that have components of both dip-slip and strike-slip; this is known as oblique slip.

Reverse faults, particularly those along convergent plate boundaries are associated with the most powerful earthquakes, including almost all of those of magnitude 8 or more. Strike-slip faults, particularly continental transforms can produce major earthquakes up to about magnitude 8. Earthquakes associated with normal faults are generally less than magnitude 7.

This is so because the energy released in an earthquake, and thus its magnitude, is proportional to the area of the fault that ruptures^[3] and the stress drop. Therefore, the longer the length and the wider the width of the faulted area, the larger the resulting magnitude. The topmost, brittle part of the Earth’s crust, and the cool slabs of the tectonic plates that are descending down into the hot mantle, are the only parts of our planet which can store elastic energy and release it in fault ruptures. Rocks hotter than about 300 degrees Celsius flow in response to stress, they do not rupture in earthquakes.^[4]^[5] The maximum observed lengths of ruptures and mapped faults, which may break in one go are approximately 1000 km. Examples are the earthquakes in Chile, 1960; Alaska, 1957; Sumatra, 2004, all in subduction zones. The longest earthquake ruptures on strike-slip faults, like the San Andreas Fault (1857, 1906), the North Anatolian Fault in Turkey (1939) and the Denali Fault in Alaska (2002), are about half to one third as long as the lengths along subducting plate margins, and those along normal faults are even shorter.

The most important parameter controlling the maximum earthquake magnitude on a fault is however not the maximum available length, but the available width because the latter varies by a factor of 20. Along converging plate margins, the dip angle of the rupture plane is very shallow, typically about 10 degrees.^[6] Thus the width of the plane within the top brittle crust of the Earth can become 50 to 100 km (Tohoku, 2011; Alaska, 1964), making the most powerful earthquakes possible.

Strike-slip faults tend to be oriented near vertically, resulting in an approximate width of 10 km within the brittle crust,^[7] thus earthquakes with magnitudes much larger than 8 are not possible. Maximum magnitudes along many normal faults are even more limited because many of them are located along spreading centers, as in Iceland, where the thickness of the brittle layer is only about 6 km.^[8]^[9]

In addition, there exists a hierarchy of stress level in the three fault types. Thrust faults are generated by the highest, strike slip by intermediate, and normal faults by the lowest stress levels.^[10] This can easily be understood by considering the direction of the greatest principal stress, the direction of the force that ‘pushes’ the rock mass during the faulting. In the case of normal faults, the rock mass is pushed down in a vertical direction, thus the pushing force (greatest principal stress) equals the weight of the rock mass itself. In the case of thrusting, the rock mass ‘escapes’ in the direction of the least principal stress, namely upward, lifting the rock mass up, thus the overburden equals the least principal stress. Strike-slip faulting is intermediate between the other two types described above. This difference in stress regime in the three faulting environments can contribute to differences in stress drop during faulting, which contributes to differences in the radiated energy, regardless of fault dimensions.

Earthquakes away from plate boundaries

Main article: Intraplate earthquake

Where plate boundaries occur within continental lithosphere, deformation is spread out over a much larger area than the plate boundary itself. In the case of the San Andreas fault continental transform, many earthquakes occur away from the plate boundary and are related to strains developed within the broader zone of deformation caused by major irregularities in the fault trace (e.g., the “Big bend” region). The Northridge earthquake was associated with movement on a blind thrust within such a zone. Another example is the strongly oblique convergent plate boundary between the Arabian and Eurasian plates where it runs through the northwestern part of the Zagros mountains. The deformation associated with this plate boundary is partitioned into nearly pure thrust sense movements perpendicular to the boundary over a wide zone to the southwest and nearly pure strike-slip motion along the Main Recent Fault close to the actual plate boundary itself. This is demonstrated by earthquake focal mechanisms.^[11]

All tectonic plates have internal stress fields caused by their interactions with neighbouring plates and sedimentary loading or unloading (e.g. deglaciation^[12]). These stresses may be sufficient to cause failure along existing fault planes, giving rise to intraplate earthquakes.^[13]

Shallow-focus and deep-focus earthquakes

Main article: Depth of focus (tectonics)

The majority of tectonic earthquakes originate at the ring of fire in depths not exceeding tens of kilometers. Earthquakes occurring at a depth of less than 70 km are classified as 'shallow-focus' earthquakes, while those with a focal-depth between 70 and 300 km are commonly termed 'mid-focus' or 'intermediate-depth' earthquakes. In subduction zones, where older and colder oceanic crust descends beneath another tectonic plate, deep-focus earthquakes may occur at much greater depths (ranging from 300 up to 700 kilometers).^[14] These seismically active areas of subduction are known as Wadati-Benioff zones. Deep-focus earthquakes occur at a depth where the subducted lithosphere should no longer be brittle, due to the high temperature and pressure. A possible mechanism for the generation of deep-focus earthquakes is faulting caused by olivine undergoing a phase transition into a spinel structure.^[15]

Earthquakes and volcanic activity

Earthquakes often occur in volcanic regions and are caused there, both by tectonic faults and the movement of magma in volcanoes. Such earthquakes can serve as an early warning of volcanic eruptions, as during the Mount St. Helens eruption of 1980.^[16] Earthquake swarms can serve as markers for the location of the flowing magma throughout the volcanoes. These swarms can be recorded by seismometers and tiltmeters (a device that measures ground slope) and used as sensors to predict imminent or upcoming eruptions.^[17]

Rupture dynamics

A tectonic earthquake begins by an initial rupture at a point on the fault surface, a process known as nucleation. The scale of the nucleation zone is uncertain, with some evidence, such as the rupture dimensions of the smallest earthquakes, suggesting that it is smaller than 100 m while other evidence, such as a slow component revealed by low-frequency spectra of some earthquakes, suggest that it is larger. The possibility that the nucleation involves some sort of preparation process is supported by the observation that about 40% of earthquakes are preceded by foreshocks. Once the rupture has initiated it begins to propagate along the fault surface. The mechanics of this process are poorly understood, partly because it is difficult to recreate the high sliding velocities in a laboratory. Also the effects of strong ground motion make it very difficult to record information close to a nucleation zone.^[18]

Rupture propagation is generally modelled using a fracture mechanics approach, likening the rupture to a propagating mixed mode shear crack. The rupture velocity is a function of the fracture energy in the volume around the crack tip, increasing with decreasing fracture energy. The velocity of rupture propagation is orders of magnitude faster than the displacement velocity across the fault. Earthquake ruptures typically propagate at velocities that are in the range 70–90 % of the S-wave velocity and this is independent of earthquake size. A small subset of earthquake ruptures appear to have propagated at speeds greater than the S-wave velocity. These supershear earthquakes have all been observed during large strike-slip events. The unusually wide zone of coseismic damage caused by the 2001 Kunlun earthquake has been attributed to the effects of the sonic boom developed in such earthquakes. Some earthquake ruptures travel at unusually low velocities and are referred to as slow earthquakes. A particularly dangerous form of slow earthquake is the tsunami earthquake, observed where the relatively low felt intensities, caused by the slow propagation speed of some great earthquakes, fail to alert the population of the neighbouring coast, as in the 1896 Meiji-Sanriku earthquake.^[18]

Tidal forces

Research work has shown a robust correlation between small tidally induced forces and non-volcanic tremor activity.^[19]^[20]^[21]^[22]

Earthquake clusters

Most earthquakes form part of a sequence, related to each other in terms of location and time.^[23] Most earthquake clusters consist of small tremors that cause little to no damage, but there is a theory that earthquakes can recur in a regular pattern.^[24]

Aftershocks

Main article: Aftershock

An aftershock is an earthquake that occurs after a previous earthquake, the mainshock. An aftershock is in the same region of the main shock but always of a smaller magnitude. If an aftershock is larger than the main shock, the aftershock is redesignated as the main shock and the original main shock is redesignated as a foreshock. Aftershocks are formed as the crust around the displaced fault plane adjusts to the effects of the main shock.^[23]

Earthquake swarms

Main article: Earthquake swarm

Earthquake swarms are sequences of earthquakes striking in a specific area within a short period of time. They are different from earthquakes followed by a series of aftershocks by the fact that no single earthquake in the sequence is obviously the main shock, therefore none have notable higher magnitudes than the other. An example of an earthquake swarm is the 2004 activity at Yellowstone National Park.^[25]

Earthquake storms

Main article: Earthquake storm

Sometimes a series of earthquakes occur in a sort of earthquake storm, where the earthquakes strike a fault in clusters, each triggered by the shaking or stress redistribution of the previous earthquakes. Similar to aftershocks but on adjacent segments of fault, these storms occur over the course of years, and with some of the later earthquakes as damaging as the early ones. Such a pattern was observed in the sequence of about a dozen earthquakes that struck the North Anatolian Fault in Turkey in the 20th century and has been inferred for older anomalous clusters of large earthquakes in the Middle East.^[26]^[27]

Size and frequency of occurrence

It is estimated that around 500,000 earthquakes occur each year, detectable with current instrumentation. About 100,000 of these can be felt.^[28]^[29] Minor earthquakes occur nearly constantly around the world in places like California and Alaska in the U.S., as well as in Guatemala, Chile, Peru, Indonesia, Iran, Pakistan, the Azores in Portugal, Turkey, New Zealand, Greece, Italy, and Japan, but earthquakes can occur almost anywhere, including New York City, London, and Australia.^[30] Larger earthquakes occur less frequently, the relationship being exponential; for example, roughly ten times as many earthquakes larger than magnitude 4 occur in a particular time period than earthquakes larger than magnitude 5. In the (low seismicity) United Kingdom, for example, it has been calculated that the average recurrences are: an earthquake of 3.7–4.6 every year, an earthquake of 4.7–5.5 every 10 years, and an earthquake of 5.6 or larger every 100 years.^[31] This is an example of the Gutenberg-Richter law.

The Messina earthquake and tsunami took as many as 200,000 lives on December 28, 1908 in Sicily and Calabria.^[32]

The number of seismic stations has increased from about 350 in 1931 to many thousands today. As a result, many more earthquakes are reported than in the past, but this is because of the vast improvement in instrumentation, rather than an increase in the number of earthquakes. The United States Geological Survey estimates that, since 1900, there have been an average of 18 major earthquakes (magnitude 7.0–7.9) and one great earthquake (magnitude 8.0 or greater) per year, and that this average has been relatively stable.^[33] In recent years, the number of major earthquakes per year has decreased, though this is probably a statistical fluctuation rather than a systematic trend. More detailed statistics on the size and frequency of earthquakes is available from the United States Geological Survey (USGS).^[34] Alternatively, some scientists suggest that the recent increase in major earthquakes could be explained by a cyclical pattern of periods of intense tectonic activity, interspersed with longer periods of low-intensity. However, accurate recordings of earthquakes only began in the early 1900s, so it is too early to categorically state that this is the case.^[35]

Most of the world's earthquakes (90%, and 81% of the largest) take place in the 40,000 km long, horseshoe-shaped zone called the circum-Pacific seismic belt, known as the Pacific Ring of Fire, which for the most part bounds the Pacific Plate.^[36]^[37] Massive earthquakes tend to occur along other plate boundaries, too, such as along the Himalayan Mountains.^[38]

With the rapid growth of mega-cities such as Mexico City, Tokyo and Tehran, in areas of high seismic risk, some seismologists are warning that a single quake may claim the lives of up to 3 million people.^[39]

Induced seismicity

Main article: Induced seismicity

While most earthquakes are caused by movement of the Earth's tectonic plates, human activity can also produce earthquakes. Four main activities contribute to this phenomenon: storing large amounts of water behind a dam (and possibly building an extremely heavy building), drilling and injecting liquid into wells, and by coal mining and oil drilling.^[40] Perhaps the best known example is the 2008 Sichuan earthquake in China's Sichuan Province in May; this tremor resulted in 69,227 fatalities and is the 19th deadliest earthquake of all time. The Zipingpu Dam is believed to have fluctuated the pressure of the fault 1,650 feet (503 m) away; this pressure probably increased the power of the earthquake and accelerated the rate of movement for the fault.^[41] The greatest earthquake in Australia's history is also claimed to be induced by humanity, through coal mining. The city of Newcastle was built over a large sector of coal mining areas. The earthquake has been reported to be spawned from a fault that reactivated due to the millions of tonnes of rock removed in the mining process.^[42]

Measuring and locating earthquakes

Main article: Seismology

Earthquakes can be recorded by seismometers up to great distances, because seismic waves travel through the whole Earth's interior. The absolute magnitude of a quake is conventionally reported by numbers on the Moment magnitude scale (formerly Richter scale, magnitude 7 causing serious damage over large areas), whereas the felt magnitude is reported using the modified Mercalli intensity scale (intensity II–XII).

Every tremor produces different types of seismic waves, which travel through rock with different velocities:

Longitudinal P-waves (shock- or pressure waves)
Transverse S-waves (both body waves)
Surface waves — (Rayleigh and Love waves)

Propagation velocity of the seismic waves ranges from approx. 3 km/s up to 13 km/s, depending on the density and elasticity of the medium. In the Earth's interior the shock- or P waves travel much faster than the S waves (approx. relation 1.7 : 1). The differences in travel time from the epicentre to the observatory are a measure of the distance and can be used to image both sources of quakes and structures within the Earth. Also the depth of the hypocenter can be computed roughly.

In solid rock P-waves travel at about 6 to 7 km per second; the velocity increases within the deep mantle to ~13 km/s. The velocity of S-waves ranges from 2–3 km/s in light sediments and 4–5 km/s in the Earth's crust up to 7 km/s in the deep mantle. As a consequence, the first waves of a distant earth quake arrive at an observatory via the Earth's mantle.

Rule of thumb: On the average, the kilometer distance to the earthquake is the number of seconds between the P and S wave times 8.^[43] Slight deviations are caused by inhomogeneities of subsurface structure. By such analyses of seismograms the Earth's core was located in 1913 by Beno Gutenberg.

Earthquakes are not only categorized by their magnitude but also by the place where they occur. The world is divided into 754 Flinn-Engdahl regions (F-E regions), which are based on political and geographical boundaries as well as seismic activity. More active zones are divided into smaller F-E regions whereas less active zones belong to larger F-E regions.

Effects of earthquakes

1755 copper engraving depicting Lisbon in ruins and in flames after the 1755 Lisbon earthquake, which killed an estimated 60,000 people. A tsunami overwhelms the ships in the harbor.

The effects of earthquakes include, but are not limited to, the following:

Shaking and ground rupture

Shaking and ground rupture are the main effects created by earthquakes, principally resulting in more or less severe damage to buildings and other rigid structures. The severity of the local effects depends on the complex combination of the earthquake magnitude, the distance from the epicenter, and the local geological and geomorphological conditions, which may amplify or reduce wave propagation.^[44] The ground-shaking is measured by ground acceleration.

Specific local geological, geomorphological, and geostructural features can induce high levels of shaking on the ground surface even from low-intensity earthquakes. This effect is called site or local amplification. It is principally due to the transfer of the seismic motion from hard deep soils to soft superficial soils and to effects of seismic energy focalization owing to typical geometrical setting of the deposits.

Ground rupture is a visible breaking and displacement of the Earth's surface along the trace of the fault, which may be of the order of several metres in the case of major earthquakes. Ground rupture is a major risk for large engineering structures such as dams, bridges and nuclear power stations and requires careful mapping of existing faults to identify any likely to break the ground surface within the life of the structure.^[45]

Landslides and avalanches

Main article: Landslide

Earthquakes, along with severe storms, volcanic activity, coastal wave attack, and wildfires, can produce slope instability leading to landslides, a major geological hazard. Landslide danger may persist while emergency personnel are attempting rescue.^[46]

Fires

Fires of the 1906 San Francisco earthquake

Earthquakes can cause fires by damaging electrical power or gas lines. In the event of water mains rupturing and a loss of pressure, it may also become difficult to stop the spread of a fire once it has started. For example, more deaths in the 1906 San Francisco earthquake were caused by fire than by the earthquake itself.^[47]

Soil liquefaction

Main article: Soil liquefaction

Soil liquefaction occurs when, because of the shaking, water-saturated granular material (such as sand) temporarily loses its strength and transforms from a solid to a liquid. Soil liquefaction may cause rigid structures, like buildings and bridges, to tilt or sink into the liquefied deposits. This can be a devastating effect of earthquakes. For example, in the 1964 Alaska earthquake, soil liquefaction caused many buildings to sink into the ground, eventually collapsing upon themselves.^[48]

Tsunami

Main article: Tsunami

The tsunami of the 2004 Indian Ocean earthquake

Tsunamis are long-wavelength, long-period sea waves produced by the sudden or abrupt movement of large volumes of water. In the open ocean the distance between wave crests can surpass 100 kilometers (62 mi), and the wave periods can vary from five minutes to one hour. Such tsunamis travel 600-800 kilometers per hour (373–497 miles per hour), depending on water depth. Large waves produced by an earthquake or a submarine landslide can overrun nearby coastal areas in a matter of minutes. Tsunamis can also travel thousands of kilometers across open ocean and wreak destruction on far shores hours after the earthquake that generated them.^[49]

Ordinarily, subduction earthquakes under magnitude 7.5 on the Richter scale do not cause tsunamis, although some instances of this have been recorded. Most destructive tsunamis are caused by earthquakes of magnitude 7.5 or more.^[49]

Floods

Main article: Flood

A flood is an overflow of any amount of water that reaches land.^[50] Floods occur usually when the volume of water within a body of water, such as a river or lake, exceeds the total capacity of the formation, and as a result some of the water flows or sits outside of the normal perimeter of the body. However, floods may be secondary effects of earthquakes, if dams are damaged. Earthquakes may cause landslips to dam rivers, which collapse and cause floods.^[51]

The terrain below the Sarez Lake in Tajikistan is in danger of catastrophic flood if the landslide dam formed by the earthquake, known as the Usoi Dam, were to fail during a future earthquake. Impact projections suggest the flood could affect roughly 5 million people.^[52]

Human impacts

Damaged infrastructure, one week after the 2007 Peru earthquake

An earthquake may cause injury and loss of life, road and bridge damage, general property damage (which may or may not be covered by earthquake insurance), and collapse or destabilization (potentially leading to future collapse) of buildings. The aftermath may bring disease, lack of basic necessities, and higher insurance premiums.

Major earthquakes

Main article: List of earthquakes

One of the most devastating earthquakes in recorded history occurred on 23 January 1556 in the Shaanxi province, China, killing more than 830,000 people (see 1556 Shaanxi earthquake).^[53] Most of the population in the area at the time lived in yaodongs, artificial caves in loess cliffs, many of which collapsed during the catastrophe with great loss of life. The 1976 Tangshan earthquake, with death toll estimated to be between 240,000 to 655,000, is believed to be the largest earthquake of the 20th century by death toll.^[54]

The largest earthquake that has been measured on a seismograph reached 9.5 magnitude, occurring on 22 May 1960.^[28]^[29] Its epicenter was near Cañete, Chile. The energy released was approximately twice that of the next most powerful earthquake, the Good Friday Earthquake, which was centered in Prince William Sound, Alaska.^[55]^[56] The ten largest recorded earthquakes have all been megathrust earthquakes; however, of these ten, only the 2004 Indian Ocean earthquake is simultaneously one of the deadliest earthquakes in history.

Earthquakes that caused the greatest loss of life, while powerful, were deadly because of their proximity to either heavily populated areas or the ocean, where earthquakes often create tsunamis that can devastate communities thousands of kilometers away. Regions most at risk for great loss of life include those where earthquakes are relatively rare but powerful, and poor regions with lax, unenforced, or nonexistent seismic building codes.

Prediction

Main article: Earthquake prediction

Many different methods have been developed for predicting the time and place in which earthquakes will occur. Despite considerable research efforts by seismologists, scientifically reproducible predictions cannot yet be made to a specific day or month.^[57] However, for well-understood faults the probability that a segment may rupture during the next few decades can be estimated.^[58]

Earthquake warning systems have been developed that can provide regional notification of an earthquake in progress, but before the ground surface has begun to move, potentially allowing people within the system's range to seek shelter before the earthquake's impact is felt.

Preparedness

The objective of earthquake engineering is to foresee the impact of earthquakes on buildings and other structures and to design such structures to minimize the risk of damage. Existing structures can be modified by seismic retrofitting to improve their resistance to earthquakes. Earthquake insurance can provide building owners with financial protection against losses resulting from earthquakes.

Emergency management strategies can be employed by a government or organization to mitigate risks and prepare for consequences.

Historical views

An image from a 1557 book

From the lifetime of the Greek philosopher Anaxagoras in the 5th century BCE to the 14th century CE, earthquakes were usually attributed to "air (vapors) in the cavities of the Earth."^[59] Thales of Miletus, who lived from 625–547 (BCE) was the only documented person who believed that earthquakes were caused by tension between the earth and water.^[59] Other theories existed, including the Greek philosopher Anaxamines' (585–526 BCE) beliefs that short incline episodes of dryness and wetness caused seismic activity. The Greek philosopher Democritus (460–371 BCE) blamed water in general for earthquakes.^[59] Pliny the Elder called earthquakes "underground thunderstorms."^[59]

Earthquakes in culture

Mythology and religion

In Norse mythology, earthquakes were explained as the violent struggling of the god Loki. When Loki, god of mischief and strife, murdered Baldr, god of beauty and light, he was punished by being bound in a cave with a poisonous serpent placed above his head dripping venom. Loki's wife Sigyn stood by him with a bowl to catch the poison, but whenever she had to empty the bowl the poison dripped on Loki's face, forcing him to jerk his head away and thrash against his bonds, which caused the earth to tremble.^[60]

In Greek mythology, Poseidon was the cause and god of earthquakes. When he was in a bad mood, he struck the ground with a trident, causing earthquakes and other calamities. He also used earthquakes to punish and inflict fear upon people as revenge.^[61]

In Japanese mythology, Namazu (鯰) is a giant catfish who causes earthquakes. Namazu lives in the mud beneath the earth, and is guarded by the god Kashima who restrains the fish with a stone. When Kashima lets his guard fall, Namazu thrashes about, causing violent earthquakes.

Popular culture

In modern popular culture, the portrayal of earthquakes is shaped by the memory of great cities laid waste, such as Kobe in 1995 or San Francisco in 1906.^[62] Fictional earthquakes tend to strike suddenly and without warning.^[62] For this reason, stories about earthquakes generally begin with the disaster and focus on its immediate aftermath, as in Short Walk to Daylight (1972), The Ragged Edge (1968) or Aftershock: Earthquake in New York (1998).^[62] A notable example is Heinrich von Kleist's classic novella, The Earthquake in Chile, which describes the destruction of Santiago in 1647. Haruki Murakami's short fiction collection after the quake depicts the consequences of the Kobe earthquake of 1995.

The most popular single earthquake in fiction is the hypothetical "Big One" expected of California's San Andreas Fault someday, as depicted in the novels Richter 10 (1996) and Goodbye California (1977) among other works.^[62] Jacob M. Appel's widely anthologized short story, A Comparative Seismology, features a con artist who convinces an elderly woman that an apocalyptic earthquake is imminent.^[63] In Pleasure Boating in Lituya Bay, one of the stories in Jim Shepard's Like You'd Understand, Anyway, the "Big One" leads to an even more devastating tsunami.

In the film 2012 (2009), solar flares (geologically implausibly) affecting the Earth's core caused massive destabilization of the Earth's crust layers. This created destruction planet-wide with earthquakes and tsunamis, foreseen by the Mayan culture and myth surrounding the last year noted in the Mesoamerican calendar — 2012.

Contemporary depictions of earthquakes in film are variable in the manner in which they reflect human psychological reactions to the actual trauma that can be caused to directly afflicted families and their loved ones.^[64] Disaster mental health response research emphasizes the need to be aware of the different roles of loss of family and key community members, loss of home and familiar surroundings, loss of essential supplies and services to maintain survival.^[65]^[66] Particularly for children, the clear availability of caregiving adults who are able to protect, nourish, and clothe them in the aftermath of the earthquake, and to help them make sense of what has befallen them has been shown even more important to their emotional and physical health than the simple giving of provisions.^[67] As was observed after other disasters involving destruction and loss of life and their media depictions, such as those of the 2001 World Trade Center Attacks or Hurricane Katrina—and has been recently observed in the 2010 Haiti earthquake, it is also important not to pathologize the reactions to loss and displacement or disruption of governmental administration and services, but rather to validate these reactions, to support constructive problem-solving and reflection as to how one might improve the conditions of those affected.^[68]

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^ ^a ^b National Research Council (U.S.). Committee on the Science of Earthquakes (2003). "5. Earthquake Physics and Fault-System Science". Living on an Active Earth: Perspectives on Earthquake Science. Washington D.C.: National Academies Press. p. 418. ISBN 9780309065627. Retrieved 8 July 2010.
^ Thomas, Amanda M.; Nadeau, Robert M.; Bürgmann, Roland (December 24, 2009). "Tremor-tide correlations and near-lithostatic pore pressure on the deep San Andreas fault". Nature 462 (7276): 1048–51. Bibcode 2009Natur.462.1048T. doi:10.1038/nature08654. PMID 20033046.
^ "Gezeitenkräfte: Sonne und Mond lassen Kalifornien erzittern" SPIEGEL online, 29.12.2009
^ Tamrazyan, Gurgen P. (1967). "Tide-forming forces and earthquakes". Icarus 7 (1–3): 59–65. Bibcode 1967Icar....7...59T. doi:10.1016/0019-1035(67)90047-4.
^ Tamrazyan, Gurgen P. (1968). "Principal regularities in the distribution of major earthquakes relative to solar and lunar tides and other cosmic forces". Icarus 9 (1–3): 574–92. Bibcode 1968Icar....9..574T. doi:10.1016/0019-1035(68)90050-X.
^ ^a ^b "What are Aftershocks, Foreshocks, and Earthquake Clusters?".
^ "Repeating Earthquakes". United States Geological Survey. January 29, 2009. Retrieved May 11, 2009.
^ "Earthquake Swarms at Yellowstone". United States Geological Survey. Retrieved 2008-09-15.
^ Amos Nur (2000). "Poseidon's Horses: Plate Tectonics and Earthquake Storms in the Late Bronze Age Aegean and Eastern Mediterranean". Journal of Archaeological Science 27 (1): 43–63. doi:10.1006/jasc.1999.0431. ISSN 0305-4403.
^ "Earthquake Storms". Horizon. 1 April 2003. Retrieved 2007-05-02.
^ ^a ^b "Earthquake Facts". United States Geological Survey. Retrieved 2010-04-25.
^ ^a ^b Pressler, Margaret Webb (14 April 2010). "More earthquakes than usual? Not really.". KidsPost (Washington Post: Washington Post): pp. C10.
^ "Earthquake Hazards Program". United States Geological Survey. Retrieved 2006-08-14.
^ "Seismicity and earthquake hazard in the UK". Quakes.bgs.ac.uk. Retrieved 2010-08-23.
^ "Italy's earthquake history." BBC News. October 31, 2002.
^ "Common Myths about Earthquakes". United States Geological Survey. Retrieved 2006-08-14.
^ "Earthquake Facts and Statistics: Are earthquakes increasing?". United States Geological Survey. Retrieved 2006-08-14.
^ The 10 biggest earthquakes in history, Australian Geographic, March 14, 2011.
^ "Historic Earthquakes and Earthquake Statistics: Where do earthquakes occur?". United States Geological Survey. Retrieved 2006-08-14.
^ "Visual Glossary — Ring of Fire". United States Geological Survey. Retrieved 2006-08-14.
^ Jackson, James, "Fatal attraction: living with earthquakes, the growth of villages into megacities, and earthquake vulnerability in the modern world," Philosophical Transactions of the Royal Society, doi: 10.1098/rsta.2006.1805 Phil. Trans. R. Soc. A 15 August 2006 vol. 364 no. 1845 1911–1925.
^ "Global urban seismic risk." Cooperative Institute for Research in Environmental Science.
^ Madrigal, Alexis (4 June 2008). "Top 5 Ways to Cause a Man-Made Earthquake". Wired News (CondéNet). Retrieved 2008-06-05.
^ "How Humans Can Trigger Earthquakes". National Geographic. February 10, 2009. Retrieved April 24, 2009.
^ Brendan Trembath (January 9, 2007). "Researcher claims mining triggered 1989 Newcastle earthquake". Australian Broadcasting Corporation. Retrieved April 24, 2009.
^ "Speed of Sound through the Earth". Hypertextbook.com. Retrieved 2010-08-23.
^ "On Shaky Ground, Association of Bay Area Governments, San Francisco, reports 1995,1998 (updated 2003)". Abag.ca.gov. Retrieved 2010-08-23.
^ "Guidelines for evaluating the hazard of surface fault rupture, California Geological Survey". California Department of Conservation. 2002.
^ "Natural Hazards — Landslides". United States Geological Survey. Retrieved 2008-09-15.
^ "The Great 1906 San Francisco earthquake of 1906". United States Geological Survey. Retrieved 2008-09-15.
^ "Historic Earthquakes — 1946 Anchorage Earthquake". United States Geological Survey. Retrieved 2008-09-15.
^ ^a ^b Noson, Qamar, and Thorsen (1988). Washington Division of Geology and Earth Resources Information Circular 85. Washington State Earthquake Hazards.
^ MSN Encarta Dictionary. Flood. Retrieved on 2006-12-28. Archived 2009-10-31.
^ "Notes on Historical Earthquakes". British Geological Survey. Retrieved 2008-09-15.
^ "Fresh alert over Tajik flood threat". BBC News. 2003-08-03. Retrieved 2008-09-15.
^ "Earthquakes with 50,000 or More Deaths". U.S. Geological Survey
^ Spignesi, Stephen J. [2005] (2005). Catastrophe!: The 100 Greatest Disasters of All Time. ISBN 0806525584
^ Kanamori Hiroo. "The Energy Release in Great Earthquakes". Journal of Geophysical Research. Retrieved 2010-10-10.
^ USGS. "How Much Bigger?". United States Geological Survey. Retrieved 2010-10-10.
^ Earthquake Prediction. Ruth Ludwin, U.S. Geological Survey.
^ Working Group on California Earthquake Probabilities in the San Francisco Bay Region, 2003 to 2032, 2003, http://earthquake.usgs.gov/regional/nca/wg02/index.php.
^ ^a ^b ^c ^d "Earthquakes". Encyclopedia of World Environmental History. 1. Encyclopedia of World Environmental History. 2003. pp. 358–364.
^ Sturluson, Snorri (1220). Prose Edda. ISBN 1156786215.
^ Sellers, Paige (1997-03-03). "Poseidon". Encyclopedia Mythica. Retrieved 2008-09-02.
^ ^a ^b ^c ^d Van Riper, A. Bowdoin (2002). Science in popular culture: a reference guide. Westport: Greenwood Press. p. 60. ISBN 0313318220.
^ JM Appel. A Comparative Seismology. Weber Studies (first publication), Volume 18, Number 2.
^ Goenjian, Najarian; Pynoos, Steinberg; Manoukian, Tavosian; Fairbanks, AM; Manoukian, G; Tavosian, A; Fairbanks, LA (1994). "Posttraumatic stress disorder in elderly and younger adults after the 1988 earthquake in Armenia". Am J Psychiatry 151 (6): 895–901. PMID 8185000.
^ Wang, Gao; Shinfuku, Zhang; Zhao, Shen; Zhang, H; Zhao, C; Shen, Y (2000). "Longitudinal Study of Earthquake-Related PTSD in a Randomly Selected Community Sample in North China". Am J Psychiatry 157 (8): 1260–1266. doi:10.1176/appi.ajp.157.8.1260. PMID 10910788.
^ Goenjian, Steinberg; Najarian, Fairbanks; Tashjian, Pynoos (2000). "Prospective Study of Posttraumatic Stress, Anxiety, and Depressive Reactions After Earthquake and Political Violence". Am J Psychiatry 157 (6): 911–895. doi:10.1176/appi.ajp.157.6.911.
^ Coates SW, Schechter D (2004). Preschoolers’ traumatic stress post-9/11: relational and developmental perspectives. Disaster Psychiatry Issue. Psychiatric Clinics of North America, 27(3), 473–489.
^ Schechter, DS; Coates, SW; First, E (2002). "Observations of acute reactions of young children and their families to the World Trade Center attacks". Journal of ZERO-TO-THREE: National Center

2. VOLCANO

A volcano is an opening, or rupture, in a planet's surface or crust, which allows hot magma, volcanic ash and gases to escape from below the surface.

Volcanoes are generally found where tectonic plates are diverging or converging. A mid-oceanic ridge, for example the Mid-Atlantic Ridge, has examples of volcanoes caused by divergent tectonic plates pulling apart; the Pacific Ring of Fire has examples of volcanoes caused by convergent tectonic plates coming together. By contrast, volcanoes are usually not created where two tectonic plates slide past one another. Volcanoes can also form where there is stretching and thinning of the Earth's crust in the interiors of plates, e.g., in the East African Rift, the Wells Gray-Clearwater volcanic field and the Rio Grande Rift in North America. This type of volcanism falls under the umbrella of "Plate hypothesis" volcanism.^[1]

Intraplate volcanism has also been postulated to be caused by mantle plumes. These so-called "hotspots", for example Hawaii, are postulated to arise from upwelling diapirs from the core-mantle boundary, 3,000 km deep in the Earth.

Etymology

The word volcano is derived from the name of Vulcano, a volcanic island in the Aeolian Islands of Italy whose name in turn originates from Vulcan, the name of a god of fire in Roman mythology.^[2] The study of volcanoes is called volcanology, sometimes spelled vulcanology.

Plate tectonics

Map showing the divergent plate boundaries (OSR – Oceanic Spreading Ridges) and recent sub aerial volcanoes.

Divergent plate boundaries

Main article: Divergent boundary

At the mid-oceanic ridges, two tectonic plates diverge from one another. New oceanic crust is being formed by hot molten rock slowly cooling and solidifying. The crust is very thin at mid-oceanic ridges due to the pull of the tectonic plates. The release of pressure due to the thinning of the crust leads to adiabatic expansion, and the partial melting of the mantle causing volcanism and creating new oceanic crust. Most divergent plate boundaries are at the bottom of the oceans, therefore most volcanic activity is submarine, forming new seafloor. Black smokers or deep sea vents are an example of this kind of volcanic activity. Where the mid-oceanic ridge is above sea-level, volcanic islands are formed, for example, Iceland.

Mount Rinjani eruption in 1994, in Lombok, Indonesia

Lava enters the Pacific at the Big Island of Hawaii

Convergent plate boundaries

Main article: Convergent boundary

Subduction zones are places where two plates, usually an oceanic plate and a continental plate, collide. In this case, the oceanic plate subducts, or submerges under the continental plate forming a deep ocean trench just offshore. Water released from the subducting plate lowers the melting temperature of the overlying mantle wedge, creating magma. This magma tends to be very viscous due to its high silica content, so often does not reach the surface and cools at depth. When it does reach the surface, a volcano is formed. Typical examples for this kind of volcano are Mount Etna and the volcanoes in the Pacific Ring of Fire.

"Hotspots"

Main article: Hotspot (geology)

"Hotspots" is the name given to volcanic provinces postulated to be formed by mantle plumes. These are postulated to comprise columns of hot material that rise from the core-mantle boundary. They are suggested to be hot, causing large-volume melting, and to be fixed in space. Because the tectonic plates move across them, each volcano becomes dormant after a while and a new volcano is then formed as the plate shifts over the postulated plume. The Hawaiian Islands have been suggested to have been formed in such a manner, as well as the Snake River Plain, with the Yellowstone Caldera being the part of the North American plate currently above the hot spot. This theory is currently under criticism, however.^[1]

Volcanic features

Conical Mount Fuji in Japan, at sunrise from Lake Kawaguchi (2005)

The most common perception of a volcano is of a conical mountain, spewing lava and poisonous gases from a crater at its summit. This describes just one of many types of volcano, and the features of volcanoes are much more complicated. The structure and behavior of volcanoes depends on a number of factors. Some volcanoes have rugged peaks formed by lava domes rather than a summit crater, whereas others present landscape features such as massive plateaus. Vents that issue volcanic material (lava, which is what magma is called once it has escaped to the surface, and ash) and gases (mainly steam and magmatic gases) can be located anywhere on the landform. Many of these vents give rise to smaller cones such as Puʻu ʻŌʻō on a flank of Hawaii's Kīlauea.

Lakagigar fissure vent in Iceland, source of the major world climate alteration of 1783–84.

Skjaldbreiður, a shield volcano whose name means "broad shield"

January 2009 image of the rhyolitic lava dome of Chaitén Volcano, southern Chile during its 2008–2009 eruption

Mayon, near-perfect stratovolcano in the Philippines

Mud volcano on Taman Peninsula, Russia

Other types of volcano include cryovolcanoes (or ice volcanoes), particularly on some moons of Jupiter, Saturn and Neptune; and mud volcanoes, which are formations often not associated with known magmatic activity. Active mud volcanoes tend to involve temperatures much lower than those of igneous volcanoes, except when a mud volcano is actually a vent of an igneous volcano.

Fissure vents

Main article: Fissure vent

Volcanic fissure vents are flat, linear cracks through which lava emerges.

Shield volcanoes

Main article: Shield volcano

Shield volcanoes, so named for their broad, shield-like profiles, are formed by the eruption of low-viscosity lava that can flow a great distance from a vent, but not generally explode catastrophically. Since low-viscosity magma is typically low in silica, shield volcanoes are more common in oceanic than continental settings. The Hawaiian volcanic chain is a series of shield cones, and they are common in Iceland, as well.

Lava domes

Main article: Lava dome

Lava domes are built by slow eruptions of highly viscous lavas. They are sometimes formed within the crater of a previous volcanic eruption (as in Mount Saint Helens), but can also form independently, as in the case of Lassen Peak. Like stratovolcanoes, they can produce violent, explosive eruptions, but their lavas generally do not flow far from the originating vent.

Cryptodomes

Cryptodomes are formed when viscous lava forces its way up and causes a bulge. The 1980 eruption of Mount St. Helens was an example. Lava was under great pressure and forced a bulge in the mountain, which was unstable and slid down the north side.

Volcanic cones (cinder cones)

Main articles: volcanic cone and Cinder cone

Holocene cinder cone volcano on State Highway 18 near Veyo, Utah

Volcanic cones or cinder cones are the result from eruptions that erupt mostly small pieces of scoria and pyroclastics (both resemble cinders, hence the name of this volcano type) that build up around the vent. These can be relatively short-lived eruptions that produce a cone-shaped hill perhaps 30 to 400 meters high. Most cinder cones erupt only once. Cinder cones may form as flank vents on larger volcanoes, or occur on their own. Parícutin in Mexico and Sunset Crater in Arizona are examples of cinder cones. In New Mexico, Caja del Rio is a volcanic field of over 60 cinder cones.

Stratovolcanoes (composite volcanoes)

Main article: Stratovolcano

Stratovolcanoes or composite volcanoes are tall conical mountains composed of lava flows and other ejecta in alternate layers, the strata that give rise to the name. Stratovolcanoes are also known as composite volcanoes, created from several structures during different kinds of eruptions. Strato/composite volcanoes are made of cinders, ash and lava. Cinders and ash pile on top of each other, lava flows on top of the ash, where it cools and hardens, and then the process begins again. Classic examples include Mt. Fuji in Japan, Mayon Volcano in the Philippines, and Mount Vesuvius and Stromboli in Italy.

In recorded history, explosive eruptions by stratovolcanoes have posed the greatest hazard to civilizations, as ash is produced by an explosive eruption. No supervolcano erupted in recorded history. Shield volcanoes have not an enormous pressure build up from the lava flow. Fissure vents and monogenetic volcanic fields (volcanic cones) have not powerful explosive eruptions, as they are many times under extension. Stratovolcanoes (30–35°) are steeper than shield volcanoes (generally 5–10°), their loose tephra are material for dangerous lahars.^[3]

The Lake Toba volcano created a caldera 100 km long

Supervolcanoes

Main article: Supervolcano

A supervolcano is a large volcano that usually has a large caldera and can potentially produce devastation on an enormous, sometimes continental, scale. Such eruptions would be able to cause severe cooling of global temperatures for many years afterwards because of the huge volumes of sulfur and ash erupted. They are the most dangerous type of volcano. Examples include Yellowstone Caldera in Yellowstone National Park and Valles Caldera in New Mexico (both western United States), Lake Taupo in New Zealand, Lake Toba in Sumatra, Indonesia and Ngorogoro Crater in Tanzania, Krakatoa near Java and Sumatra, Indonesia. Supervolcanoes are hard to identify centuries later, given the enormous areas they cover. Large igneous provinces are also considered supervolcanoes because of the vast amount of basalt lava erupted, but are non-explosive.

Submarine volcanoes

Main article: Submarine volcano

Submarine volcanoes are common features on the ocean floor. Some are active and, in shallow water, disclose their presence by blasting steam and rocky debris high above the surface of the sea. Many others lie at such great depths that the tremendous weight of the water above them prevents the explosive release of steam and gases, although they can be detected by hydrophones and discoloration of water because of volcanic gases. Pumice rafts may also appear. Even large submarine eruptions may not disturb the ocean surface. Because of the rapid cooling effect of water as compared to air, and increased buoyancy, submarine volcanoes often form rather steep pillars over their volcanic vents as compared to above-surface volcanoes. They may become so large that they break the ocean surface as new islands. Pillow lava is a common eruptive product of submarine volcanoes. Hydrothermal vents are common near these volcanoes, and some support peculiar ecosystems based on dissolved minerals.

Herðubreið, one of the tuyas in Iceland

Subglacial volcanoes

Main article: Subglacial volcano

Subglacial volcanoes develop underneath icecaps. They are made up of flat lava which flows at the top of extensive pillow lavas and palagonite. When the icecap melts, the lavas on the top collapse, leaving a flat-topped mountain. These volcanoes are also called table mountains, tuyas or (uncommonly) mobergs. Very good examples of this type of volcano can be seen in Iceland, however, there are also tuyas in British Columbia. The origin of the term comes from Tuya Butte, which is one of the several tuyas in the area of the Tuya River and Tuya Range in northern British Columbia. Tuya Butte was the first such landform analyzed and so its name has entered the geological literature for this kind of volcanic formation. The Tuya Mountains Provincial Park was recently established to protect this unusual landscape, which lies north of Tuya Lake and south of the Jennings River near the boundary with the Yukon Territory.

Mud volcanoes

Main article: Mud volcano

Mud volcanoes or mud domes are formations created by geo-excreted liquids and gases, although there are several processes which may cause such activity. The largest structures are 10 kilometers in diameter and reach 700 meters high.

Erupted material

Pāhoehoe lava from Kīlauea, Hawaii

Pāhoehoe Lava flow on Hawaii. The picture shows overflows of a main lava channel.

The Stromboli volcano off the coast of Sicily has erupted continuously for thousands of years, giving rise to the term strombolian eruption.

Mafic basalt lava flows created the Deccan Traps near Matheran, east of Mumbai, one of the largest volcanic features on Earth.

Lava composition

Another way of classifying volcanoes is by the composition of material erupted (lava), since this affects the shape of the volcano. Lava can be broadly classified into 4 different compositions (Cas & Wright, 1987):

If the erupted magma contains a high percentage (>63%) of silica, the lava is called felsic.
- Felsic lavas (dacites or rhyolites) tend to be highly viscous (not very fluid) and are erupted as domes or short, stubby flows. Viscous lavas tend to form stratovolcanoes or lava domes. Lassen Peak in California is an example of a volcano formed from felsic lava and is actually a large lava dome.
- Because siliceous magmas are so viscous, they tend to trap volatiles (gases) that are present, which cause the magma to erupt catastrophically, eventually forming stratovolcanoes. Pyroclastic flows (ignimbrites) are highly hazardous products of such volcanoes, since they are composed of molten volcanic ash too heavy to go up into the atmosphere, so they hug the volcano's slopes and travel far from their vents during large eruptions. Temperatures as high as 1,200 °C are known to occur in pyroclastic flows, which will incinerate everything flammable in their path and thick layers of hot pyroclastic flow deposits can be laid down, often up to many meters thick. Alaska's Valley of Ten Thousand Smokes, formed by the eruption of Novarupta near Katmai in 1912, is an example of a thick pyroclastic flow or ignimbrite deposit. Volcanic ash that is light enough to be erupted high into the Earth's atmosphere may travel many kilometres before it falls back to ground as a tuff.
If the erupted magma contains 52–63% silica, the lava is of intermediate composition.
- These "andesitic" volcanoes generally only occur above subduction zones (e.g. Mount Merapi in Indonesia).
- Andesitic lava is typically formed at convergent boundary margins of tectonic plates, by several processes:
  - Hydration melting of peridotite and fractional crystallization
  - Melting of subducted slab containing sediments^{[citation needed]}
  - Magma mixing between felsic rhyolitic and mafic basaltic magmas in an intermediate reservoir prior to emplacement or lava flow.
If the erupted magma contains <52% and >45% silica, the lava is called mafic (because it contains higher percentages of magnesium (Mg) and iron (Fe)) or basaltic. These lavas are usually much less viscous than rhyolitic lavas, depending on their eruption temperature; they also tend to be hotter than felsic lavas. Mafic lavas occur in a wide range of settings:
- At mid-ocean ridges, where two oceanic plates are pulling apart, basaltic lava erupts as pillows to fill the gap;
- Shield volcanoes (e.g. the Hawaiian Islands, including Mauna Loa and Kilauea), on both oceanic and continental crust;
- As continental flood basalts.
Some erupted magmas contain <=45% silica and produce ultramafic lava. Ultramafic flows, also known as komatiites, are very rare; indeed, very few have been erupted at the Earth's surface since the Proterozoic, when the planet's heat flow was higher. They are (or were) the hottest lavas, and probably more fluid than common mafic lavas.

Lava texture

Two types of lava are named according to the surface texture: ʻAʻa (pronounced [ˈʔaʔa]) and pāhoehoe ([paːˈho.eˈho.e]), both Hawaiian words. ʻAʻa is characterized by a rough, clinkery surface and is the typical texture of viscous lava flows. However, even basaltic or mafic flows can be erupted as ʻaʻa flows, particularly if the eruption rate is high and the slope is steep.

Pāhoehoe is characterized by its smooth and often ropey or wrinkly surface and is generally formed from more fluid lava flows. Usually, only mafic flows will erupt as pāhoehoe, since they often erupt at higher temperatures or have the proper chemical make-up to allow them to flow with greater fluidity.

Volcanic activity

Fresco of Bacchus and Agathodaemon with Mount Vesuvius, as seen in Pompeii's House of the Centenary.

Active volcano Mount St. Helens shortly after the eruption of 18 May 1980

Damavand, the highest volcano in Asia, is a potentially active volcano with fumaroles and solfatara near its summit.

Fourpeaked volcano, Alaska, in September 2007, after being thought extinct for over 10,000 years.

Popular classification of volcanoes

Active

A popular way of classifying magmatic volcanoes is by their frequency of eruption, with those that erupt regularly called active, those that have erupted in historical times but are now quiet called dormant, and those that have not erupted in historical times called extinct. However, these popular classifications—extinct in particular—are practically meaningless to scientists. They use classifications which refer to a particular volcano's formative and eruptive processes and resulting shapes, which was explained above.

There is no real consensus among volcanologists on how to define an "active" volcano. The lifespan of a volcano can vary from months to several million years, making such a distinction sometimes meaningless when compared to the lifespans of humans or even civilizations. For example, many of Earth's volcanoes have erupted dozens of times in the past few thousand years but are not currently showing signs of eruption. Given the long lifespan of such volcanoes, they are very active. By human lifespans, however, they are not.

Scientists usually consider a volcano to be erupting or likely to erupt if it is currently erupting, or showing signs of unrest such as unusual earthquake activity or significant new gas emissions. Most scientists consider a volcano active if it has erupted in holocene times. Historic times is another timeframe for active.^[4] But it is important to note that the span of recorded history differs from region to region. In China and the Mediterranean, recorded history reaches back more than 3,000 years but in the Pacific Northwest of the United States and Canada, it reaches back less than 300 years, and in Hawaii and New Zealand, only around 200 years.^[5] The Smithsonian Global Volcanism Program's definition of active is having erupted within the last 10,000 years (the 'holocene' period).

Presently there are about 500 active volcanoes in the world – the majority following along the Pacific 'Ring of Fire' – and around 50 of these erupt each year.^[6] The United States is home to 50 active volcanoes.^[7] There are more than 1,500 potentially active volcanoes.^[8] An estimated 500 million people live near active volcanoes.^[9]

Extinct

Extinct volcanoes are those that scientists consider unlikely to erupt again, because the volcano no longer has a lava supply. Examples of extinct volcanoes are many volcanoes on the Hawaiian – Emperor seamount chain in the Pacific Ocean, Hohentwiel, Shiprock and the Zuidwal volcano in the Netherlands. Edinburgh Castle in Scotland is famously located atop an extinct volcano. Otherwise, whether a volcano is truly extinct is often difficult to determine. Since "supervolcano" calderas can have eruptive lifespans sometimes measured in millions of years, a caldera that has not produced an eruption in tens of thousands of years is likely to be considered dormant instead of extinct.

Dormant

It is difficult to distinguish an extinct volcano from a dormant one. Volcanoes are often considered to be extinct if there are no written records of its activity. Nevertheless, volcanoes may remain dormant for a long period of time. For example, Yellowstone has a repose/recharge period of around 700 ka, and Toba of around 380 ka.^[10] Vesuvius was described by Roman writers as having been covered with gardens and vineyards before its famous eruption of AD 79, which destroyed the towns of Herculaneum and Pompeii. Before its catastrophic eruption of 1991, Pinatubo was an inconspicuous volcano, unknown to most people in the surrounding areas. Two other examples are the long-dormant Soufrière Hills volcano on the island of Montserrat, thought to be extinct before activity resumed in 1995 and Fourpeaked Mountain in Alaska, which, before its September 2006 eruption, had not erupted since before 8000 BC and had long been thought to be extinct.

Technical classification of volcanoes

Volcanic-alert level

The three common popular classifications of volcanoes can be subjective and some volcanoes thought to have been extinct have announced to the world they were just pretending.^[11] To help prevent citizens from falsely believing they are not at risk when living on or near a volcano, countries have adopted new classifications to describe the various levels and stages of volcanic activity.^[12] Some alert systems use different numbers or colors to designate the different stages. Other systems use colors and words. Some systems use a combination of both.

Volcano warning schemes of the United States

The United States Geological Survey (USGS) has adopted a common system nationwide for characterizing the level of unrest and eruptive activity at volcanoes. The new volcano alert-level system classifies volcanoes now as being in a normal, advisory, watch or warning stage. Additionally, colors are used to denote the amount of ash produced. Details of the US system can be found at Volcano warning schemes of the United States.

Notable volcanoes

Koryaksky volcano towering over Petropavlovsk-Kamchatsky on Kamchatka Peninsula, Far Eastern Russia.

Mount Teide on the island of Tenerife (Spain).

Main articles: Lists of volcanoes and Decade Volcanoes

The 16 current Decade Volcanoes are:

Avachinsky-Koryaksky, Kamchatka, Russia
Nevado de Colima, Jalisco and Colima, Mexico
Mount Etna, Sicily, Italy
Galeras, Nariño, Colombia
Mauna Loa, Hawaii, USA
Mount Merapi, Central Java, Indonesia
Mount Nyiragongo, Democratic Republic of the Congo
Mount Rainier, Washington, USA

Effects of volcanoes

Volcanic "injection"

Solar radiation reduction from volcanic eruptions

Sulfur dioxide emissions by volcanoes.

Average concentration of sulfur dioxide over the Sierra Negra Volcano (Galapagos Islands) from October 23 – November 1, 2005

There are many different types of volcanic eruptions and associated activity: phreatic eruptions (steam-generated eruptions), explosive eruption of high-silica lava (e.g., rhyolite), effusive eruption of low-silica lava (e.g., basalt), pyroclastic flows, lahars (debris flow) and carbon dioxide emission. All of these activities can pose a hazard to humans. Earthquakes, hot springs, fumaroles, mud pots and geysers often accompany volcanic activity.

The concentrations of different volcanic gases can vary considerably from one volcano to the next. Water vapor is typically the most abundant volcanic gas, followed by carbon dioxide and sulfur dioxide. Other principal volcanic gases include hydrogen sulfide, hydrogen chloride, and hydrogen fluoride. A large number of minor and trace gases are also found in volcanic emissions, for example hydrogen, carbon monoxide, halocarbons, organic compounds, and volatile metal chlorides.

Large, explosive volcanic eruptions inject water vapor (H₂O), carbon dioxide (CO₂), sulfur dioxide (SO₂), hydrogen chloride (HCl), hydrogen fluoride (HF) and ash (pulverized rock and pumice) into the stratosphere to heights of 16–32 kilometres (10–20 mi) above the Earth's surface. The most significant impacts from these injections come from the conversion of sulfur dioxide to sulfuric acid (H₂SO₄), which condenses rapidly in the stratosphere to form fine sulfate aerosols. The aerosols increase the Earth's albedo—its reflection of radiation from the Sun back into space – and thus cool the Earth's lower atmosphere or troposphere; however, they also absorb heat radiated up from the Earth, thereby warming the stratosphere. Several eruptions during the past century have caused a decline in the average temperature at the Earth's surface of up to half a degree (Fahrenheit scale) for periods of one to three years — sulfur dioxide from the eruption of Huaynaputina probably caused the Russian famine of 1601–1603.^[13]

One proposed volcanic winter happened c. 70,000 years ago following the supereruption of Lake Toba on Sumatra island in Indonesia.^[14] According to the Toba catastrophe theory to which some anthropologists and archeologists subscribe, it had global consequences,^[15] killing most humans then alive and creating a population bottleneck that affected the genetic inheritance of all humans today.^[16] The 1815 eruption of Mount Tambora created global climate anomalies that became known as the "Year Without a Summer" because of the effect on North American and European weather.^[17] Agricultural crops failed and livestock died in much of the Northern Hemisphere, resulting in one of the worst famines of the 19th century.^[18] The freezing winter of 1740–41, which led to widespread famine in northern Europe, may also owe its origins to a volcanic eruption.^[19]

It has been suggested that volcanic activity caused or contributed to the End-Ordovician, Permian-Triassic, Late Devonian mass extinctions, and possibly others. The massive eruptive event which formed the Siberian Traps, one of the largest known volcanic events of the last 500 million years of Earth's geological history, continued for a million years and is considered to be the likely cause of the "Great Dying" about 250 million years ago,^[20] which is estimated to have killed 90% of species existing at the time.^[21]

The sulfate aerosols also promote complex chemical reactions on their surfaces that alter chlorine and nitrogen chemical species in the stratosphere. This effect, together with increased stratospheric chlorine levels from chlorofluorocarbon pollution, generates chlorine monoxide (ClO), which destroys ozone (O₃). As the aerosols grow and coagulate, they settle down into the upper troposphere where they serve as nuclei for cirrus clouds and further modify the Earth's radiation balance. Most of the hydrogen chloride (HCl) and hydrogen fluoride (HF) are dissolved in water droplets in the eruption cloud and quickly fall to the ground as acid rain. The injected ash also falls rapidly from the stratosphere; most of it is removed within several days to a few weeks. Finally, explosive volcanic eruptions release the greenhouse gas carbon dioxide and thus provide a deep source of carbon for biogeochemical cycles.

Rainbow and volcanic ash with sulfur dioxide emissions from Halema`uma`u vent.

Gas emissions from volcanoes are a natural contributor to acid rain. Volcanic activity releases about 130 to 230 teragrams (145 million to 255 million short tons) of carbon dioxide each year.^[22] Volcanic eruptions may inject aerosols into the Earth's atmosphere. Large injections may cause visual effects such as unusually colorful sunsets and affect global climate mainly by cooling it. Volcanic eruptions also provide the benefit of adding nutrients to soil through the weathering process of volcanic rocks. These fertile soils assist the growth of plants and various crops. Volcanic eruptions can also create new islands, as the magma cools and solidifies upon contact with the water.

Ash thrown into the air by eruptions can present a hazard to aircraft, especially jet aircraft where the particles can be melted by the high operating temperature. Dangerous encounters in 1982 after the eruption of Galunggung in Indonesia, and 1989 after the eruption of Mount Redoubt in Alaska raised awareness of this phenomenon. Nine Volcanic Ash Advisory Centers were established by the International Civil Aviation Organization to monitor ash clouds and advise pilots accordingly. The 2010 eruptions of Eyjafjallajökull caused major disruptions to air travel in Europe.

Volcanoes on other planetary bodies

The Tvashtar volcano erupts a plume 330 km (205 mi) above the surface of Jupiter's moon Io.

Olympus Mons (Latin, "Mount Olympus") is the tallest known mountain in our solar system, located on the planet Mars.

Main articles: Geology of the Moon, Geology of Mars, Volcanism on Io, and Volcanism on Venus

The Earth's Moon has no large volcanoes and no current volcanic activity, although recent evidence suggests it may still possess a partially molten core.^[23] However, the Moon does have many volcanic features such as maria (the darker patches seen on the moon), rilles and domes.

The planet Venus has a surface that is 90% basalt, indicating that volcanism played a major role in shaping its surface. The planet may have had a major global resurfacing event about 500 million years ago,^[24] from what scientists can tell from the density of impact craters on the surface. Lava flows are widespread and forms of volcanism not present on Earth occur as well. Changes in the planet's atmosphere and observations of lightning have been attributed to ongoing volcanic eruptions, although there is no confirmation of whether or not Venus is still volcanically active. However, radar sounding by the Magellan probe revealed evidence for comparatively recent volcanic activity at Venus's highest volcano Maat Mons, in the form of ash flows near the summit and on the northern flank.

There are several extinct volcanoes on Mars, four of which are vast shield volcanoes far bigger than any on Earth. They include Arsia Mons, Ascraeus Mons, Hecates Tholus, Olympus Mons, and Pavonis Mons. These volcanoes have been extinct for many millions of years,^[25] but the European Mars Express spacecraft has found evidence that volcanic activity may have occurred on Mars in the recent past as well.^[25]

Jupiter's moon Io is the most volcanically active object in the solar system because of tidal interaction with Jupiter. It is covered with volcanoes that erupt sulfur, sulfur dioxide and silicate rock, and as a result, Io is constantly being resurfaced. Its lavas are the hottest known anywhere in the solar system, with temperatures exceeding 1,800 K (1,500 °C). In February 2001, the largest recorded volcanic eruptions in the solar system occurred on Io.^[26] Europa, the smallest of Jupiter's Galilean moons, also appears to have an active volcanic system, except that its volcanic activity is entirely in the form of water, which freezes into ice on the frigid surface. This process is known as cryovolcanism, and is apparently most common on the moons of the outer planets of the solar system.

In 1989 the Voyager 2 spacecraft observed cryovolcanoes (ice volcanoes) on Triton, a moon of Neptune, and in 2005 the Cassini–Huygens probe photographed fountains of frozen particles erupting from Enceladus, a moon of Saturn.^[27] The ejecta may be composed of water, liquid nitrogen, dust, or methane compounds. Cassini–Huygens also found evidence of a methane-spewing cryovolcano on the Saturnian moon Titan, which is believed to be a significant source of the methane found in its atmosphere.^[28] It is theorized that cryovolcanism may also be present on the Kuiper Belt Object Quaoar.

A 2010 study of the exoplanet COROT-7b, which was detected by transit in 2009, studied that tidal heating from the host star very close to the planet and neighboring planets could generate intense volcanic activity similar to Io.^[29]

Traditional beliefs about volcanoes

Many ancient accounts ascribe volcanic eruptions to supernatural causes, such as the actions of gods or demigods. To the ancient Greeks, volcanoes' capricious power could only be explained as acts of the gods, while 16th/17th-century German astronomer Johannes Kepler believed they were ducts for the Earth's tears.^[30] One early idea counter to this was proposed by Jesuit Athanasius Kircher (1602–1680), who witnessed eruptions of Mount Etna and Stromboli, then visited the crater of Vesuvius and published his view of an Earth with a central fire connected to numerous others caused by the burning of sulfur, bitumen and coal.

Various explanations were proposed for volcano behavior before the modern understanding of the Earth's mantle structure as a semisolid material was developed. For decades after awareness that compression and radioactive materials may be heat sources, their contributions were specifically discounted. Volcanic action was often attributed to chemical reactions and a thin layer of molten rock near the surface.

Panoramas

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Crater of Mount Tangkuban Perahu, West Java, Indonesia.

Irazú Volcano, Costa Rica.

Black Rock Volcano an extinct cinder cone near Fillmore, Utah.

Vulcano island with the north coast of Sicily in the background.

References

^ ^a ^b Foulger, G.R. (2010). Plates vs. Plumes: A Geological Controversy. Wiley-Blackwell. ISBN 978-1-4051-6148-0.
^ Douglas Harper (November 2001). "Volcano". Online Etymology Dictionary. Retrieved 2009-06-11.
^ Lockwood, John P.; Hazlett, Richard W. (2010). Volcanoes: Global Perspectives. p. 552. ISBN 978-1-4051-6250-0.
^ "Volcanoes". U.S. Department of the Interior, U.S. Geological Survey.
^ "Mountains of fire: the nature of volcanoes". Robert Wayne Decker, Barbara Decker (1991). p.7. ISBN 0521312906
^ "Volcanoes". European Space Agency.
^ "Volcano Environments". U.S. Geological Survey.
^ "Sensing Remote Volcanoes". NASA Earth Observatory.
^ "Volcanoes". Reuters. December 12, 2009.
^ Chesner, C.A.; Westgate, J.A.; Rose, W.I.; Drake, R.; Deino, A. (March 1991). "Eruptive History of Earth's Largest Quaternary caldera (Toba, Indonesia) Clarified". Geology 19 (3): 200–203. doi:10.1130/0091-7613(1991)019<0200:EHOESL>2.3.CO;2. Retrieved 2010-01-20.
^ "Formerly "Extinct" El Chichon Volcano In Mexico Erupts". Vulkaner.no. Retrieved 2011-08-22.
^ "Volcanic Alert Levels of Various Countries". Volcanolive.com. Retrieved 2011-08-22.
^ University of California – Davis (2008, April 25). "Volcanic Eruption Of 1600 Caused Global Disruption". ScienceDaily.
^ "Supervolcano Eruption – In Sumatra – Deforested India 73,000 Years Ago". ScienceDaily. November 24, 2009.
^ "The new batch – 150,000 years ago". BBC – Science & Nature – The evolution of man.
^ "When humans faced extinction". BBC. 2003-06-09. Retrieved 2007-01-05.
^ "Volcanoes in human history: the far-reaching effects of major eruptions". Jelle Zeilinga de Boer, Donald Theodore Sanders (2002). Princeton University Press. p.155. ISBN 0691050813
^ Oppenheimer, Clive (2003). "Climatic, environmental and human consequences of the largest known historic eruption: Tambora volcano (Indonesia) 1815". Progress in Physical Geography 27 (2): 230–259. doi:10.1191/0309133303pp379ra.
^ "Ó Gráda, C.: Famine: A Short History". Princeton University Press.
^ "Yellowstone's Super Sister". Discovery Channel.
^ Benton M J (2005). When Life Nearly Died: The Greatest Mass Extinction of All Time. Thames & Hudson. ISBN 978-0500285732.
^ "Volcanic Gases and Their Effects". U.S. Geological Survey. Retrieved 2007-06-16.
^ M. A. Wieczorek, B. L. Jolliff, A. Khan, M. E. Pritchard, B. P. Weiss, J. G. Williams, L. L. Hood, K. Righter, C. R. Neal, C. K. Shearer, I. S. McCallum, S. Tompkins, B. R. Hawke, C. Peterson, J, J. Gillis, B. Bussey (2006). "The Constitution and Structure of the Lunar Interior". Reviews in Mineralogy and Geochemistry 60 (1): 221–364. doi:10.2138/rmg.2006.60.3.
^ D.L. Bindschadler (1995). "Magellan: A new view of Venus' geology and geophysics". American Geophysical Union. Retrieved 2006-09-04.
^ ^a ^b "Glacial, volcanic and fluvial activity on Mars: latest images". European Space Agency. 2005-02-25. Retrieved 2006-08-17.
^ Exceptionally Bright Eruption on lo Rivals Largest in Solar System, Nov. 13, 2002^{[dead link]}
^ "Cassini Finds an Atmosphere on Saturn's Moon Enceladus'". Pparc.ac.uk. Retrieved 2010-10-24.
^ "Hydrocarbon volcano discovered on Titan". Newscientist.com. June 8, 2005. Retrieved 2010-10-24.
^ Jaggard, Victoria (2010-02-05). ""Super Earth" May Really Be New Planet Type: Super-Io". National Geographic web site daily news. National Geographic Society. Retrieved 2010-03-11.
^ Micheal Williams (11-2007). "Hearts of fire". Morning Calm (Korean Air Lines Co., Ltd.) (11–2007): 6.

/ virus pictures / influenza pictures (influenza A): Birds & Bird Flu graphics showing avian influenza (H5N1) mixing with other 'flu strains; image of bird flu viruses and human flu viruses entering the same cell and pandemic influenza emerging, diagram of replication of viruses showing reassortment of viral RNA genome segments (genetic mixing or recombination) creating a new viral strain (reassortant) with the potential to create a catastrophic new flu pandemic against which no flu vaccine will protect.

BIRD FLU INFECTING A PERSON: graphic artwork (above) showing avian influenza viruses (H5N1 shown in green) infecting a person (with lungs and other organs visible). H5N1 seems to provoke an extreme immune response (a cytokine storm) which might account for the very high death rate from this strain. Above image measures 500 pixels across but the original image is 4000 x 5657 pixels. See all RKM flu images

bird flu H5N1 (avain influenza) emerging from birds and human influenza viruse entering a cell leading to a reassortment of genetic material with the creation of a new virulent pandemic influenza (graphic)

BIRD FLU & HUMAN FLU REPLICATION IN A SINGLE CELL: graphic artwork (above) showing avian influenza viruses (H5N1 shown in green) emerging from birds and infecting a cell. A human strain (shown in blue) infects the same cell. The genome segments (green and blue dashes) enter the nucleus (purple curved surface) and are copied. The new copies exit the nucleus but get jumbled together and form the genome of a new viral strain (red-yellow virions) that might be as lethal as the bird flu and as easily spread from person to person as the human flu. This seems to be the recipe for the next influenza pandemic. Above image measures 500 pixels across but the original image is 4000 x 5657 pixels.

bird flu H5N1 and pandemic flu: avain influenza emerging from birds and mixing with human influenza viruses (graphic)

BIRDS & BIRD FLU GRAPHIC #1: artwork (above) showing avian influenza viruses (H5N1) emerging from birds and mixing with other strains of influenza virus. Above image measures 500 pixels across but the original image is 4000 x 5657 pixels.

bird flu H5N and pandemic flu: avain influenza emerging from birds (swans) and mixing with human influenza viruses to create a pandemic strain (graphic)

BIRDS & BIRD FLU GRAPHIC #2: artwork (above) showing avian influenza viruses (H5N1 - shown in green) emerging from birds (swans) and mixing with other strains (blue) of influenza virus, creating the potential for a new strain (red & yellow) which can pass easily from person to person, thereby creating the potential for a pandemic. Above image measures 500 pixels across but the original image is 4000 x 5657 pixels.

BIRDS & BIRD FLU GRAPHIC #3: artwork (above) showing avian influenza viruses (H5N1 - shown in green) emerging from birds and mixing with other strains (blue) of influenza virus, creating the potential for a new strain (red & yellow) which can pass easily from person to person, thereby creating the potential for a pandemic. Above image measures 500 pixels across but the original image is 5657 x 4000 pixels.

bird flu H5N1 and pandemic flu: infection of a cell with bird flu and human flu leading to the creation of a new pandemic strain (graphic)

PANDEMIC FLU GRAPHIC: illustration above shows a cell being infected by bird flu (H5N1) and a human flu virus at the same time. Inside the cell, a reassortment of flu RNA might occur leading to the creation of a new pandemic flu type that can spread rapidly from person to person. Above image measures 500 pixels across but the original image is 4000 x 5657 pixels and contains no text. The design allows text and labels to be easily inserted. The image is also available on a white background:

PANDEMIC FLU GRAPHIC ON WHITE: Above image measures 500 pixels across but the original image is 4000 x 5657 pixels and contains no text. The design allows text and labels to be easily inserted so that you can customise the graphic for your needs.

BIRD FLU VACCINE: considerable effort is being devoted to create large quantities of influenza vaccine to fight a potential bird flu pandemic. Vaccines rely on the immune system to fight the disease. A vaccine primes the immune system by exposing the body to antigens present on the infecting agent. Influenza vaccines have to be regularly updated because the influenza virus constantly changes its surface antigens. The flu virus is a moving target, antigenically speaking, and so vaccine manufacture lags viral innovation. The problem with vaccinating against a flu pandemic is that we do not know what the new pandemic influenza strain will be like. Avian influenza vaccines might not work against a novel pandemic viral strain. A vaccination program will require a huge investment of resources. There is a race on to develop vaccines very fast and one option is DNA vaccines. DNA vaccination works by introducing DNA that codes for the antigen. The host cells create copies of the antigen which provoke an immune response thereby priming the person's immune system against a future attack by the real pathogen.

bird flu: reassortment of viral RNA segments creating new influenza virus strain (graphic)

FLU FLU REASSORTMENT GRAPHIC: illustration above shows reassortment of viral RNA segments in a cell infected by two strains of influenza virus (e.g. human and bird flu) leading to a new and potentially dangerous strain that could spread easily from human to human and so trigger a deadly worldwide epidemic. Such genetic mixing might occur in pigs, since a pig might be infected by both strains and then pass the new virus on to humans. Above image measures 500 pixels across, original image is 4744 pixels across.

SHORT EXPLANATION OF BOTTOM PICTURE

Influenza A virus has its RNA genome (genetic material) split into 8 segments. If two different viral types infect the same cell, then segments from both types can get jumbled together (they reassort) as the new virus particles are assembled. Consequently, new viral strains can emerge that contain a mixture of the parental genes. Image shows two different viral strains (BLUE genome at upper right and ORANGE genome at upper middle) infecting the same cell (at lower right). During replication, new viral particles may emerge that contain segments sourced form both the BLUE and the ORANGE strains. The new strain (BLUE & ORANGE STRIPED genome, shown at left) has the potential to spread rapidly.

LONG EXPLANATION OF BOTTOM PICTURE

CELL ENTRY: At upper right, a BLUE influenza virus particle (representing an avian flu virus) is shown landing on the cell surface. The virus docks with cell membrane when the red spikes (haemagglutinin, shown in red) link to molecules on the cell surface. The cell surface folds inwards causing the virus particle to sink into the cell. The virus sinks deeper into the cell until it is completely wrapped up in cell membrane. The resulting membranous "bubble" (or vesicle) breaks free from the surface of the cell and transports its contained virus into the cell. The netlike structure beneath the docking virus and the cage-like structure around the resultant vesicle represent clathrin, a protein that forms an external scaffold that causes the cell membrane to invaginate and finally form the vesicle (this entry mechanism is called receptor mediated endocytosis please see our VIRAL ENTRY graphic). Further to the left an ORANGE influenza virus particle (representing a human flu virus) is shown landing on the cell surface.

UNCOATING OF VIRUS AND RELEASE OF GENOME INTO CELL: The clathrin coat is then lost and the virus in its naked vesicle can be seen half out of frame at the right of the image. The engulfed virus then appears in an endosome (the large irregular yellow vesicle). It is more acidic in the endosome and this modifies the haemagglutinin spikes. The altered haemagglutinin draws the membranes of the virus and endosome together and they merge, creating a hole through which the viral contents are poured into the cytoplasm. These contents include the viral matrix protein (purple) and the nucleocapsid (BLUE segments). Some matrix protein is shown travelling to the nucleus. The nucleocapsid segments, which contain the viral genetic information, migrate to the nucleus. They move into the nucleus via nuclear pores (the flower like structures on the curved surface of the nucleus) and so deliver the viral genome to the nucleus (which contains the cell's own genetic material).

INSIDE THE NUCLEUS: In the nucleus, the viral genetic material (-ve sense RNA) produces viral messenger RNAs of various kinds (vmRNA) which travel out through the nuclear pores. (Messenger RNA, or mRNA, carries the genetic information that is used to direct protein manufacture.) Some vmRNA directs the synthesis of nucleoprotein (green dots) that travel back into the nucleus. Other vmRNA directs the production of matrix protein (purple dots) shown emerging from a viral polyribosome (several ribosomes strung together along a length of viral mRNA) in the middle of the picture. Some matrix protein travels to the nucleus and some collects beneath the cell membrane. Other vmRNAs direct the production of external (transmembrane) viral proteins. The manufacture of such "external" proteins follows a different route. Production starts in the rough endoplasmic reticulum and progresses through the Golgi apparatus. The haemagglutinin (red) is shown progressing through the Golgi at lower left, finally being discharged onto the cell surface from a vesicle (the sphere containing red dots that is delivering its contents onto the cell surface through a hole). The neuraminidase (yellow) is shown (for clarity) going through the Golgi in parallel but above the haemagglutinin.

NEW VIRAL RNA SEGMENTS: In the nucleus, the viral -ve sense genome also produces +ve sense copies of itself. These are then used to create further copies of the viral genome. These new -ve sense viral genomic RNAs become associated with nucleoproteins and some matrix proteins that have migrated into the nucleus. Such newly formed nucleocapsids and their associated M proteins exit the nucleus via nuclear pores (BLUE and ORANGE segments can be seen streaming across the cell).

VIRAL ASSEMBLY AT CELL SURFACE: Just beneath the cell surface, these individual BLUE and ORANGE ribonucleoprotein segments are shown associating together to form the helical nucleocapsid (the BLUE and ORANGE barrel-like structure). Around the new nucleocapsid, the matrix proteins are shown collected beneath the cell membrane (the haze of purple particles marked), while above the cell membrane, haemagglutinin and neuraminidase have coated the surface. With all these viral elements now in place, the newly forming virus particle (which contains segments derived from both the BLUE and ORANGE strains) can begin to take shape and to bud from the cell surface. The cell membrane that envelopes the emerging nucleocapsid and matrix protein becomes the viral envelope (complete with projecting spikes) and the virus particle is released. The new virus particle is now ready to infect another cell. Because it contains a new mix of genes, this reassortant can pose serious dangers. This dramatic change in the genotype is called antigenic shift to distinguish it from the more minor changes that occur due to mutation or poor fidelity RNA copying, which are called antigenic drift.

FOR INFORMATION ON VIRUSES ENTERING CELLS please see virus entry into animal cells from Ed Rybicki.

REPRODUCTION
FEMALE REPRODUCTION.

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The Mammary Glands

The mammary glands are accessory organs of the female reproductive system that are specialized to secrete milk following pregnancy. They are located in the subcutaneous tissue of the front thorax within the elevations which are called breasts. A "nipple" is located near the tip of each breast, and it is surrounded by a circular area of pigmented skin called the "areola." A mammary gland is composed of fifteen to twenty irregularly shaped lobes, each of which includes alveolar glands, and a duct (lactiferous duct) that leads to the nipple and opens to the outside. The lobes are separated by dense connective tissues that support the glands and attach them to the tissues on the underlying pectoral muscles. Other connective tissue, which forms dense strands called "suspensory ligaments," extends inward from the skin of the breast to the pectoral tissue to support the weight of the breast. The breasts are really modified sweat glands, which are made up of fibrous tissues and fat that provide support and contain nerves, blood vessels and lymphatic vessels.

Cervix

The lower one-third of the uterus is the tubular "cervix," which extends downward into the upper portion of the vagina. The cervix surrounds the opening called the "cervical orifice," through which the uterus communicates with the vagina.

Fallopian Tubes

The fallopian tube extends from the uterus to the ovary. This tube carries eggs and sperm and is where fertilization of the egg, or "ovum" takes place. The fallopian tubes lie in the pelvic portion of the abdominal cavity and each tube reaches from an ovary to become the upper part of the uterus. This funnel-shaped tube is about three inches in length. The larger end of the funnel is divided into feathery, finger-like projections which lie close to the ovary. These beating projections, along with muscle contractions, force the ovum down the funnel's small end, which opens into the uterus. After sexual intercourse, sperm swim up this funnel from the uterus. The lining of the tube and its secretions sustain both the egg and the sperm, encouraging fertilization and nourishing the egg until it reaches the uterus. If an egg splits in two after fertilization, identical or "maternal" twins are produced. If separate eggs are fertilized by different sperm, the mother gives birth to un-identical or "fraternal" twins.

Labia Minor

The labia (singular, labium) minor are flattened lengthwise into folds located with the cleft between the labia major. These folds extend along either side of the vestibule. They are composed of connective tissue that is richly supplied with blood vessels, causing a pinkish appearance. In the back, near the anus, the labia minor merge with the labia major, while in the front they converge to form a hood-like covering around the clitoris.

Ovary Ligaments

Each ovary is attached to several ligaments that help to hold it in position. The largest of these, formed by a fold of peritoneum, is called the "broad ligament." It is also attached to the uterine tubes and to the uterus. At its upper end, the ovary is held by a small fold of peritoneum, called the "suspensory ligament," which contains the ovarian blood vessels and nerves. At its lower end, it is attached to the uterus by a rounded, cord-like thickening of the broad ligament, called the "ovarian ligament." The "peritoneum" is a two-layered membrane that supports the abdominal organs, produces lubricating fluid that allows the organs to flow smoothly over each other, and protects against infection.

Ovaries

The ovaries are a pair of oval or almond-shaped glands which lie on either side of the uterus and just below the opening to the fallopian tubes. In addition to producing eggs or "ova," the ovaries produce female sex hormones called estrogen and progesterone. The ovaries produce a female hormone, called estrogen, and store female sex cells or "ova." The female, unlike the male, does not manufacture the sex cells. A girl baby is born with about 60,000 of these cells, which are contained in sac-like depressions in the ovaries. Each of these cells may have the potential to mature for fertilization, but in actuality, only about 400 ripen during the woman's lifetime. Pregnant and prenatal both come from the same Latin roots. "Prae" means "before" and "nascor" means "to be born". Nascor is also the derivative of nature, innate and native. Only a few years ago, the word, "pregnant" was seldom used in mixed company. Polite society referred to a pregnant woman as "expecting" or "being in the family way."

Uterus

The uterus or "womb" is a hollow, muscular organ in which a fertilized egg, called the "zygote," becomes embedded and in which the egg is nourished and allowed to develop until birth. It lies in the pelvic cavity behind the bladder and in front of the bowel. The uterus usually tilts forward at a ninety degree angle to the vagina, although in about 20%% of women, it tilts backwards. The uterus is lined with tissues which change during the menstrual cycle. These tissues build under the influence of hormones from the ovary. When the hormones withdraw after the menstrual cycle, the blood supply is cut off and the tissues and unfertilized egg are shed as waste. During pregnancy, the uterus stretches from three to four inches in length to a size which will accommodate a growing baby. During this time, muscular walls increase from two to three ounces to about two pounds and these powerful muscles release the baby through the birth canal with great force. The womb shrinks back to half its pregnant weight before a baby is a week old. By the time the baby is a month old, the uterus may be as small as when the egg first entered. Superstition, myth or ignorance have surrounded the menstrual period since the beginning of time. This is largely due to a primitive fear of blood. The word, "taboo," may stem from the Polynesian word for menstruation, but not all legends are negative; a girl's first menses is celebrated in some societies, because it is a sign that she can bear children.

Vagina

The vagina is a muscular passage which forms a part of the female sex organs and which connects the neck of the uterus (called the "cervix") with the external genitals. The vagina, which is approximately two and one-half to four inches long, has muscular walls which are supplied with numerous blood vessels. These walls become erect when a woman is aroused as extra blood is pumped into these vessels. The vagina has three functions: as a receptacle for the penis during love-making; as a outlet for blood during menstruation; and as a passageway for the baby to pass through at birth. According to The Guiness Book of World Records, a Russian peasant woman who lived in the 18th Century holds the record for the most children born to one mother. She had sixty-nine children within forty years. She produced sixteen pairs of twins, seven sets of triplets, and four sets of quadruplets!

Vulva

The vulva is made up of several female organs which are external. These include a small, rounded pad of fat which protects the pubic bone. Reaching down almost to the anus are two folds of fatty tissue, called the "larger lips," to protect the inner genitals. Just inside are two "smaller lips," which enclose the opening of the urethra (which comes down from the bladder) and the vagina. At the upper end, are small projections, called the "prepuce," that protect the clitoris. The clitoris is a very small, sensitive organ with numerous nerve endings that, like the penis, contain tissues which fill with blood when sexually aroused.

Human Physiology/The female reproductive system

< Human Physiology

← The male reproductive system — Human Physiology — Pregnancy and birth →

Homeostasis — Cells — Integumentary — Nervous — Senses — Muscular — Blood — Cardiovascular — Immune — Urinary — Respiratory — Gastrointestinal — Nutrition — Endocrine — Reproduction (male) — Reproduction (female) — Pregnancy — Genetics — Development — Answers

Introduction

All living things reproduce. This is something that sets the living apart from non-living. Even though the reproductive system is essential to keeping a species alive, it is not essential to keeping an individual alive. This chapter describes the different parts of the female reproductive system: the organs involved in the process of reproduction, hormones that regulate a woman's body, the menstrual cycle, ovulation and pregnancy, the female's role in genetic division, birth control, sexually transmitted diseases and other diseases and disorders.

Reproduction

Reproduction can be defined as the process by which an organism continues its species. In the human reproductive process, two kinds of sex cells ( gametes), are involved: the male gamete (sperm), and the female gamete (egg or ovum). These two gametes meet within the female's uterine tubes located one on each side of the upper pelvic cavity, and begin to create a new individual. The female needs a male to fertilize her egg; she then carries offspring through pregnancy and childbirth.

Similarities between male and female reproductive systems

The reproductive systems of the male and female have some basic similarities and some specialized differences. They are the same in that most of the reproductive organs of both sexes develop from similar embryonic tissue, meaning they are homologous. Both systems have gonads that produce (sperm and egg or ovum) and sex organs. And both systems experience maturation of their reproductive organs, which become functional during puberty as a result of the gonads secreting sex hormones.

The human male reproductive system

Cross-sectional diagram of the female reproductive organs.

In short, this is a known list of sex organs that evolve from the same tissues in a human life.

Undifferentiated	Male	Female
Gonad	Testis	Ovary
Mullerian duct	Appendix testis	Fallopian tubes
Mullerian duct	Prostatic utricle	Uterus, proximal
Wolffian duct	Rete testis	Rete ovarii
Mesonephric tubules	Efferent ducts	Epoophoron
Wolffian duct	Epididymis	Gartner's duct
Wolffian duct	Vas deferens
Wolffian duct	Seminal vesicle
Wolffian duct	Prostate	Skene's glands
Urogenital sinus	Bladder, urethra	Bladder, urethra, distal
Urogenital sinus	Bulbourethral gland	Bartholin's gland
Genital swelling	Scrotum	Labia majora
Urogenital folds	Distal urethra	Labia minora
Genital tubercle	Penis	Clitoris
Prepuce		Clitoral hood
	Bulb of penis	Vestibular bulbs
	Glans penis	Clitoral glans
	Crus of penis	Clitoral crura

Differences between male and female reproductive systems

The differences between the female and male reproductive systems are based on the functions of each individual's role in the reproduction cycle. A male who is healthy, and sexually mature, continuously produces sperm. The development of women's "eggs" are arrested during fetal development. This means she is born with a predetermined number of oocytes and cannot produce new ones.

At about 5 months gestation, the ovaries contain approximately six to seven million oogonia, which initiate meiosis. The oogonia produce primary oocytes that are arrested in prophase I of meiosis from the time of birth until puberty. After puberty, during each menstrual cycle, one or several oocytes resume meiosis and undergo their first meiotic division during ovulation. This results in the production of a secondary oocyte and one polar body. The meiotic division is arrested in metaphase II. Fertilization triggers completion of the second meiotic division and the result is one ovum and an additional polar body.

The ovaries of a newborn baby girl contain about one million oocytes. This number declines to 400,000 to 500,000 by the time puberty is reached. On average, 500-1000 oocytes are ovulated during a woman's reproductive lifetime.

When a young woman reaches puberty around age 10 to 13, a promary oocyte is discharged from one of the ovaries every 28 days. This continues until the woman reaches menopause, usually around the age of 50 years. Occytes are present at birth, and age as a woman ages.

Female Reproductive System

Produces eggs (ova)
Secretes sex hormones
Receives the male spermatazoa during
Protects and nourishes the fertilized egg until it is fully developed
Delivers fetus through birth canal
Provides nourishment to the baby through milk secreted by mammary glands in the breast

External Genitals

Vulva

The external female genitalia is referred to as vulva. It consists of the labia majora and labia minora (while these names translate as "large" and "small" lips, often the "minora" can protrude outside the "majora"), mons pubis, clitoris, opening of the urethra (meatus), vaginal vestibule, vestibular bulbs, vestibular glands.

The term "vagina" is often improperly used as a generic term to refer to the vulva or female genitals, even though - strictly speaking - the vagina is a specific internal structure and the vulva is the exterior genitalia only. Calling the vulva the vagina is akin to calling the mouth the throat.

Mons Veneris

The mons veneris, Latin for "mound of Venus" (Roman Goddess of love) is the soft mound at the front of the vulva (fatty tissue covering the pubic bone). It is also referred to as the mons pubis. The mons veneris protects the pubic bone and vulva from the impact of sexual intercourse. After puberty, it is covered with pubic hair, usually in a triangular shape. Heredity can play a role in the amount of pubic hair an individual grows.

Labia Majora

The labia majora are the outer "lips" of the vulva. They are pads of loose connective and adipose tissue, as well as some smooth muscle. The labia majora wrap around the vulva from the mons pubis to the perineum. The labia majora generally hides, partially or entirely, the other parts of the vulva. There is also a longitudinal separation called the pudendal cleft. These labia are usually covered with pubic hair. The color of the outside skin of the labia majora is usually close to the overall color of the individual, although there may be some variation. The inside skin is usually pink to light brown. They contain numerous sweat and oil glands. It has been suggested that the scent from these oils are sexually arousing.

Labia Minora

Medial to the labia majora are the labia minora. The labia minora are the inner lips of the vulva. They are thin stretches of tissue within the labia majora that fold and protect the vagina, urethra, and clitoris. The appearance of labia minora can vary widely, from tiny lips that hide between the labia majora to large lips that protrude. There is no pubic hair on the labia minora, but there are sebaceous glands. The two smaller lips of the labia minora come together longitudinally to form the prepuce, a fold that covers part of the clitoris. The labia minora protect the vaginal and urethral openings. Both the inner and outer labia are quite sensitive to touch and pressure.

Clitoris

The clitoris, visible as the small white oval between the top of the labia minora and the clitoral hood, is a small body of spongy tissue that functions solely for sexual pleasure. Only the tip or glans of the clitoris shows externally, but the organ itself is elongated and branched into two forks, the crura, which extend downward along the rim of the vaginal opening toward the perineum. Thus the clitoris is much larger than most people think it is, about 4" long on average.

The clitoral glans or external tip of the clitoris is protected by the prepuce, or clitoral hood, a covering of tissue similar to the foreskin of the male penis. However, unlike the penis, the clitoris does not contain any part of the urethra.

During sexual excitement, the clitoris erects and extends, the hood retracts, making the clitoral glans more accessible. The size of the clitoris is variable between women. On some, the clitoral glans is very small; on others, it is large and the hood does not completely cover it.

Urethra

The opening to the urethra is just below the clitoris. Although it is not related to sex or reproduction, it is included in the vulva. The urethra is actually used for the passage of urine. The urethra is connected to the bladder. In females the urethra is 1.5 inches long, compared to males whose urethra is 8 inches long. Because the urethra is so close to the anus, women should always wipe themselves from front to back to avoid infecting the vagina and urethra with bacteria. This location issue is the reason for bladder infections being more common among females.

Hymen

The hymen is a thin fold of mucous membrane that separates the lumen of the vagina from the urethral sinus. Sometimes it may partially cover the vaginal orifice. The hymen is usually perforated during later fetal development.

Because of the belief that first vaginal penetration would usually tear this membrane and cause bleeding, its "intactness" has been considered a guarantor of virginity. However, the hymen is a poor indicator of whether a woman has actually engaged in sexual intercourse because a normal hymen does not completely block the vaginal opening. The normal hymen is never actually "intact" since there is always an opening in it. Furthermore, there is not always bleeding at first vaginal penetration. The blood that is sometimes, but not always, observed after first penetration can be due to tearing of the hymen, but it can also be from injury to nearby tissues.

A tear to the hymen, medically referred to as a "transection," can be seen in a small percentage of women or girls after first penetration. A transection is caused by penetrating trauma. Masturbation and tampon insertion can, but generally are not forceful enough to cause penetrating trauma to the hymen. Therefore, the appearance of the hymen is not a reliable indicator of virginity or chastity.

Perineum

The perineum is the short stretch of skin starting at the bottom of the vulva and extending to the anus. It is a diamond shaped area between the symphysis pubis and the coccyx. This area forms the floor of the pelvis and contains the external sex organs and the anal opening. It can be further divided into the urogenital triangle in front and the anal triangle in back.

The perineum in some women may tear during the birth of an infant and this is apparently natural. Some physicians however, may cut the perineum preemptively on the grounds that the "tearing" may be more harmful than a precise cut by a scalpel. If a physician decides the cut is necessary, they will perform it. The cut is called an episiotomy.

Internal Genitals

Vagina

The vagina is a muscular, hollow tube that extends from the vaginal opening to the cervix of the uterus. It is situated between the urinary bladder and the rectum. It is about three to five inches long in a grown woman. The muscular wall allows the vagina to expand and contract. The muscular walls are lined with mucous membranes, which keep it protected and moist. A thin sheet of tissue with one or more holes in it, called the hymen, partially covers the opening of the vagina. The vagina receives sperm during sexual intercourse from the penis. The sperm that survive the acidic condition of the vagina continue on through to the fallopian tubes where fertilization may occur.

The vagina is made up of three layers, an inner mucosal layer, a middle muscularis layer, and an outer fibrous layer. The inner layer is made of vaginal rugae that stretch and allow penetration to occur. These also help with stimulation of the penis. microscopically the vaginal rugae has glands that secrete an acidic mucus (pH of around 4.0.) that keeps bacterial growth down. The outer muscular layer is especially important with delivery of a fetus and placenta.

Purposes of the Vagina

Receives a male's erect penis and semen during sexual intercourse.
Pathway through a woman's body for the baby to take during childbirth.
Provides the route for the menstrual blood (menses) from the uterus, to leave the body.
May hold forms of birth control, such as a diaphragm, FemCap, Nuva Ring, or female condom.

Clinical Application:
Pelvic inflammatory disease (PID) is a widespread infection that originates in the vagina and uterus and spreads to the uterine tubes, ovaries, and ultimately the pelvic peritoneum. This condition, which occurs in about 10% of women is usually caused by chlamydial or gonorrheal infection, other bacteria infecting the vagina may be involved as well. Signs and symptoms include tenderness of the lower abdomen, fever, and a vaginal discharge. Even a single episode of PID can cause infertility, due to scarring that blocks the uterine tubes. Therefore, patients are immediately given broad-spectrum antibiotics whenever PID is suspected.

Cervix

The cervix (from Latin "neck") is the lower, narrow portion of the uterus where it joins with the top end of the vagina. Where they join together forms an almost 90 degree curve. It is cylindrical or conical in shape and protrudes through the upper anterior vaginal wall. Approximately half its length is visible with appropriate medical equipment; the remainder lies above the vagina beyond view. It is occasionally called "cervix uteri", or "neck of the uterus".

During menstruation, the cervix stretches open slightly to allow the endometrium to be shed. This stretching is believed to be part of the cramping pain that many women experience. Evidence for this is given by the fact that some women's cramps subside or disappear after their first vaginal birth because the cervical opening has widened.

The portion projecting into the vagina is referred to as the portio vaginalis or ectocervix. On average, the ectocervix is three cm long and two and a half cm wide. It has a convex, elliptical surface and is divided into anterior and posterior lips. The ectocervix's opening is called the external os. The size and shape of the external os and the ectocervix varies widely with age, hormonal state, and whether the woman has had a vaginal birth. In women who have not had a vaginal birth the external os appears as a small, circular opening. In women who have had a vaginal birth, the ectocervix appears bulkier and the external os appears wider, more slit-like and gaping.

The passageway between the external os and the uterine cavity is referred to as the endocervical canal. It varies widely in length and width, along with the cervix overall. Flattened anterior to posterior, the endocervical canal measures seven to eight mm at its widest in reproductive-aged women. The endocervical canal terminates at the internal os which is the opening of the cervix inside the uterine cavity.

During childbirth, contractions of the uterus will dilate the cervix up to 10 cm in diameter to allow the child to pass through. During orgasm, the cervix convulses and the external os dilates.

Uterus

The uterus is shaped like an upside-down pear, with a thick lining and muscular walls. Located near the floor of the pelvic cavity, it is hollow to allow a blastocyte, or fertilized egg, to implant and grow. It also allows for the inner lining of the uterus to build up until a fertilized egg is implanted, or it is sloughed off during menses.

The uterus contains some of the strongest muscles in the female body. These muscles are able to expand and contract to accommodate a growing fetus and then help push the baby out during labor. These muscles also contract rhythmically during an orgasm in a wave like action. It is thought that this is to help push or guide the sperm up the uterus to the fallopian tubes where fertilization may be possible.

The uterus is only about three inches long and two inches wide, but during pregnancy it changes rapidly and dramatically. The top rim of the uterus is called the fundus and is a landmark for many doctors to track the progress of a pregnancy. The uterine cavity refers to the fundus of the uterus and the body of the uterus.

Helping support the uterus are ligaments that attach from the body of the uterus to the pelvic wall and abdominal wall. During pregnancy the ligaments prolapse due to the growing uterus, but retract after childbirth. In some cases after menopause, they may lose elasticity and uterine prolapse may occur. This can be fixed with surgery.

Some problems of the uterus include uterine fibroids, pelvic pain (including endometriosis, adenomyosis), pelvic relaxation (or prolapse), heavy or abnormal menstrual bleeding, and cancer. It is only after all alternative options have been considered that surgery is recommended in these cases. This surgery is called hysterectomy. Hysterectomy is the removal of the uterus, and may include the removal of one or both of the ovaries. Once performed it is irreversible. After a hysterectomy, many women begin a form of alternate hormone therapy due to the lack of ovaries and hormone production.

Fallopian Tubes

At the upper corners of the uterus are the fallopian tubes. There are two fallopian tubes, also called the uterine tubes or the oviducts. Each fallopian tube attaches to a side of the uterus and connects to an ovary. They are positioned between the ligaments that support the uterus. The fallopian tubes are about four inches long and about as wide as a piece of spaghetti. Within each tube is a tiny passageway no wider than a sewing needle. At the other end of each fallopian tube is a fringed area that looks like a funnel. This fringed area, called the infundibulum, lies close to the ovary, but is not attached. The ovaries alternately release an egg. When an ovary does ovulate, or release an egg, it is swept into the lumen of the fallopian tube by the fimbriae.

Once the egg is in the fallopian tube, tiny hairs in the tube's lining help push it down the narrow passageway toward the uterus. The oocyte, or developing egg cell, takes four to five days to travel down the length of the fallopian tube. If enough sperm are ejaculated during sexual intercourse and there is an oocyte in the fallopian tube, fertilization will occur. After fertilization occurs, the zygote, or fertilized egg, will continue down to the uterus and implant itself in the uterine wall where it will grow and develop.

If a zygote doesn't move down to the uterus and implants itself in the fallopian tube, it is called a ectopic or tubal pregnancy. If this occurs, the pregnancy will need to be terminated to prevent permanent damage to the fallopian tube, possible hemorrhage and possible death of the mother.

Mammary glands

Cross section of the breast of a human female.

Mammary glands are the organs that produce milk for the sustenance of a baby. These exocrine glands are enlarged and modified sweat glands.

Structure

The basic components of the mammary gland are the alveoli (hollow cavities, a few millimetres large) lined with milk-secreting epithelial cells and surrounded by myoepithelial cells. These alveoli join up to form groups known as lobules, and each lobule has a lactiferous duct that drains into openings in the nipple. The myoepithelial cells can contract, similar to muscle cells, and thereby push the milk from the alveoli through the lactiferous ducts towards the nipple, where it collects in widenings (sinuses) of the ducts. A suckling baby essentially squeezes the milk out of these sinuses.

The development of mammary glands is controlled by hormones. The mammary glands exist in both sexes, but they are rudimentary until puberty when - in response to ovarian hormones - they begin to develop in the female. Estrogen promotes formation, while testosterone inhibits it.

At the time of birth, the baby has lactiferous ducts but no alveoli. Little branching occurs before puberty when ovarian estrogens stimulate branching differentiation of the ducts into spherical masses of cells that will become alveoli. True secretory alveoli only develop in pregnancy, where rising levels of estrogen and progesterone cause further branching and differentiation of the duct cells, together with an increase in adipose tissue and a richer blood flow.

Colostrum is secreted in late pregnancy and for the first few days after giving birth. True milk secretion (lactation) begins a few days later due to a reduction in circulating progesterone and the presence of the hormone prolactin. The suckling of the baby causes the release of the hormone oxytocin which stimulates contraction of the myoepithelial cells.

The cells of mammary glands can easily be induced to grow and multiply by hormones. If this growth runs out of control, cancer results. Almost all instances of breast cancer originate in the lobules or ducts of the mammary glands.

STRUCTURE	LOCATION & DESCRIPTION	FUNCTION
Breasts	Upper chest one on each side containing alveolar cells (milk production), myoepithelial cells (contract to expel milk), and duct walls (help with extraction of milk).	Lactation milk/nutrition for newborn.
Cervix	The lower narrower portion of the uterus.	During childbirth, contractions of the uterus will dilate the cervix up to 10 cm in diameter to allow the child to pass through. During orgasm, the cervix convulses and the external os dilates
Clitoris	Small erectile organ directly in front of the vestibule.	Sexual excitation, engorged with blood.
Fallopian tubes	Extending upper part of the uterus on either side.	Egg transportation from ovary to uterus (fertilization usually takes place here).
Hymen	Thin membrane that partially covers the vagina in young females.
Labia majora	Outer skin folds that surround the entrance to the vagina.	Lubrication during mating.
Labia minora	Inner skin folds that surround the entrance to the vagina.	Lubrication during mating.
Mons	Mound of skin and underlying fatty tissue, central in lower pelvic region
Ovaries (female gonads)	Pelvic region on either side of the uterus.	Provides an environment for maturation of oocyte. Synthesizes and secretes sex hormones (estrogen and progesterone).
Perineum	Short stretch of skin starting at the bottom of the vulva and extending to the anus.
Urethra	Pelvic cavity above bladder, tilted.	Passage of urine.
Uterus	Center of pelvic cavity.	To house and nourish developing human.
Vagina	Canal about 10-8 cm long going from the cervix to the outside of the body.	Receives penis during mating. Pathway through a womans body for the baby to take during childbirth. Provides the route for the menstrual blood (menses) from the uterus, to leave the body. May hold forms of birth control, such as an IUD, diaphragm, neva ring, or female condom
Vulva	Surround entrance to the reproductive tract.(encompasses all external genitalia)
Endometrium	The innermost layer of uterine wall.	Contains glands that secrete fluids that bathe the utrine lining.
Myometrium	Smooth muscle in uterine wall.	Contracts to help expel the baby.

The Female Reproductive Cycle

Towards the end of puberty, girls begin to release eggs as part of a monthly period called the female reproductive cycle, or menstrual cycle (menstrual referring to "monthly"). Approximately every 28 days, during ovulation, an ovary sends a tiny egg into one of the fallopian tubes. Unless the egg is fertilized by a sperm while in the fallopian in the two to three days following ovulation, the egg dries up and leaves the body about two weeks later through the vagina. This process is called menstruation. Blood and tissues from the inner lining of the uterus (the endometrium) combine to form the menstrual flow, which generally lasts from four to seven days. The first period is called menarche. During menstruation arteries that supply the lining of the uterus constrict and capillaries weaken. Blood spilling from the damaged vessels detaches layers of the lining, not all at once but in random patches. Endometrium mucus and blood descending from the uterus, through the liquid creates the menstruation flow.

Menstrual cycle

The reproductive cycle can be divided into an ovarian cycle and a uterine cycle (compare ovarian histology and uterine histology in the diagram on the right). During the uterine cycle, the endometrial lining of the uterus builds up under the influence of increasing levels of estrogen (labeled as estradiol in the image). Follicles develop, and within a few days one matures into an ovum, or egg. The ovary then releases this egg, at the time of ovulation. After ovulation the uterine lining enters a secretory phase, or the ovarian cycle, in preparation for implantation, under the influence of progesterone. Progesterone is produced by the corpus luteum (the follicle after ovulation) and enriches the uterus with a thick lining of blood vessels and capillaries so that it can sustain the growing fetus. If fertilization and implantation occur, the embryo produces Human Chorionic Gonadotropin (HCG), which maintains the corpus luteum and causes it to continue producing progesterone until the placenta can take over production of progesterone. Hence, progesterone is "pro gestational" and maintains the uterine lining during all of pregnancy. If fertilization and implantation do not occur the corpus luteum degenerates into a corpus albicans, and progesterone levels fall. This fall in progesterone levels cause the endometrium lining to break down and sluff off through the vagina. This is called menstruation, which marks the low point for estrogen activity and is the starting point of a new cycle.

Common usage refers to menstruation and menses as a period. This bleeding serves as a sign that a woman has not become pregnant. However, this cannot be taken as certainty, as sometimes there is some bleeding in early pregnancy. During the reproductive years, failure to menstruate may provide the first indication to a woman that she may have become pregnant.

Menstruation forms a normal part of a natural cyclic process occurring in healthy women between puberty and the end of the reproductive years. The onset of menstruation, known as menarche, occurs at an average age of 12, but is normal anywhere between 8 and 16. Factors such as heredity, diet, and overall health can accelerate or delay the onset of menarche.

Signs of ovulation

The female body produces outward signs that can be easily recognized at the time of ovulation. The two main signs are thinning of the cervical mucus and a slight change in body temperature.

Thinning of the Cervical Mucus

After menstruation and right before ovulation, a woman will experience an increase of cervical mucus. At first, it will be thick and yellowish in color and will not be very plentiful. Leading up to ovulation, it will become thinner and clearer. On or around the day of ovulation, the cervical mucus will be very thin, clear and stretchy. It can be compared to the consistency of egg whites. This appearance is known as 'spinnbarkeit'.

Temperature Change

A woman can also tell the time of ovulation by taking her basal body temperature daily. This is a temperature taken with a very sensitive thermometer first thing in the morning before the woman gets out of bed. The temperature is then tracked to show changes. In the uterine cycle, a normal temperature will be around 97.0 – 98.0. The day of ovulation the temperature spikes down, usually into the 96.0 – 97.0 range and then the next morning it will spike up to normal of around 98.6 and stay in that range until menstruation begins.

Both of these methods are used for conception and contraception. They are more efficient in conception due to the fact that sperm can live for two to three days inside of the fallopian tubes. A woman could be off by a couple of days in her calculations and still become pregnant.

Menopause is the physiological cessation of menstrual cycles associated with advancing age. Menopause is sometimes referred to as "the change of life" or climacteric. Menopause occurs as the ovaries stop producing estrogen, causing the reproductive system to gradually shut down. As the body adapts to the changing levels of natural hormones, vasomotor symptoms such as hot flashes and palpitations, psychological symptoms such as increased depression, anxiety, irritability, mood swings and lack of concentration, and atrophic symptoms such as vaginal dryness and urgency of urination appear. Together with these symptoms, the woman may also have increasingly scanty and erratic menstrual periods.

Technically, menopause refers to the cessation of menses; the gradual process through which this occurs, which typically takes a year but may last as little as six months or more than five years, is known as climacteric. A natural or physiological menopause is that which occurs as a part of a woman's normal aging process. However, menopause can be surgically induced by such procedures as hysterectomy.

The average onset of menopause is 50.5 years, but some women enter menopause at a younger age, especially if they have suffered from cancer or another serious illness and undergone chemotherapy. Premature menopause is defined as menopause occurring before the age of 40, and occurs in 1% of women. Other causes of premature menopause include autoimmune disorders, thyroid disease, and diabetes mellitus.

Premature menopause is diagnosed by measuring the levels of follicle stimulating hormone (FSH) and luteinizing hormone (LH). The levels of these hormones will be higher if menopause has occurred. Rates of premature menopause have been found to be significantly higher in both fraternal and identical twins; approximately 5% of twins reach menopause before the age of 40. The reasons for this are not completely understood. Post-menopausal women are at increased risk of osteoporosis.

Perimenopause refers to the time preceding menopause, during which the production of hormones such as estrogen and progesterone diminish and become more irregular. During this period fertility diminishes. Menopause is arbitrarily defined as a minimum of twelve months without menstruation. Perimenopause can begin as early as age 35, although it usually begins much later. It can last for a few months or for several years. The duration of perimenopause cannot be predicted in advance.

Premenstrual Syndrome (PMS) It is common for women to experience some discomfort in the days leading up to their periods. PMS usually is at its worst the seven days before a period starts and can continue through the end of the period. PMS includes both physical and emotional symptoms: acne, bloating, fatigue, backaches, sore breasts, headaches, constipation, diarrhea, food cravings, depression, irritability, difficulty concentrating or handling stress.

Ovarian and Uterine Cycles in the Nonpregnant Woman

An ovary about to release an egg.

Ovarian Cycle	Events	Uterine Cycle	Events
Follicular phase - Days 1-13	FSH secretion begins.	Menstruation - Days 2-5	Endometrium breaks down.
	Follicle maturation occurs.	Proliferative phase - Days 6-13	Endometrium rebuilds.
	Estrogen secretion is prominent.
Ovulation - Day 14*	LH spike occurs.
Luteal phase - Days 15-28	LH secretion continues.	Secretory phase - Days 15-28	Endometrial thickens, and glands are secretory.
	Corpus luteum forms.
	Progesterone secretion is prominent.

(*)Assuming a 28 day cycle.

There are two phases of the ovarian cycle the follicular phase and the luteal phase. In the follicular phase about 10-25 follicles are taken from preantral or early antrial follicles to develop further. Seven days later the dominant follicle is selected to develop to full maturity. This is the pre-cursor for ovulation. Follicles themselves secrete FSH and estrogen, and these two hormones stimulate follicular growth and development. Ovulation marks the beginning of the luteal phase. This is started by the wall of the Graffian follicle to rupture and cause a flow of antral fluid that will carry the oocyte to the ovary's surface. The ruptured follicle is then turned into a gland (corpus luteum). Which secretes estrogens and progesterone. This is all triggered by and abrupt change in plasma LH levels. After ovulation the released oocyte enters the uterine tube, where it will be either fertilized or discarded.

The uterine cycle operates in sync with the ovarian cycle and is divided into three phases. The first phase in the menstrual phase. It is named the menstrual phase because in corresponds with the shedding the the uterine lining or more commonly called menstruation. The corpus luteum degenerates causing plasma estrogen and progesterone levels to decrease and in turn causes menstruation. Blood vessels in the outer most layer of the endometrium constrict and decrease blood flow to the tissues killing these tissues. After the tissues die they start to separate from the underlying endometrail tissues. Eventually the dead tissue is shed. This shedding of the tissues ruptures blood vessels and causes bleeding. Now we have the proliferative phase. During this phase the uterus renews itself and prepares for pregnancy. The endomitrial tissue that is left after menstruation begins to grow. The endometrial glands grow and enlarge causing more blood vessels. The cervical canal has glands that secrete a thin mucous that helps deposited sperm. Estrogen promotes uterine changes in this phase. The last phase is the secretory phase. This is where the endometrium is transformed to make it the best environment for implantation and subsequent housing and nourishment of the developing embryo. By doing this the endometrium will do things like have an enriched blood supply, begin to secrete fluids rich in glycogen, and even form a plug at the end of the cervical canal so that microorganisms can not enter. These changes in the uterus are caused by progesterone, due to the corpus luteum. At the end of the secretory phase the corpus luteum degenerates, and progesterone levels fall. This will trigger menstruation.

Sexual Reproduction

Sexual reproduction is a type of reproduction that results in increasing genetic diversity of the offspring. In sexual reproduction, genes from two individuals are combined in random ways with each new generation. Sex hormones released into the body by the endocrine system signal the body when it is time to start puberty. The female and male reproductive systems are the only systems so vastly different that each sex has their own different organs. All other systems have "unisex" organs.

Reproduction is characterized by two processes. The first, meiosis, involves the halving of the 46 of chromosomes. The second process, fertilization, leads the fusion of two gametes and the restoration of the original number of chromosomes: 23 chromosomes from the paternal side and 23 from the maternal side. During meiosis, the chromosomes of each pair usually cross over to achieve genetic recombination.

Sexual reproduction cannot happen without the sexual organs called gonads. Both sexes have gonads: in females, the gonads are the ovaries. The female gonads produce female gametes (eggs); the male gonads produce male gametes (sperm). After an egg is fertilized by the sperm, the fertilized egg is called the zygote.

The fertilization usually occurs in the oviducts, but can happen in the uterus itself. The zygote then implants itself in the wall of the uterus, where it begins the processes of embryogenesis and morphogenesis. The womens body carries out this process of reproduction for 40 weeks, until delivery of the fetus from the uterus through the vagina (birth canal). Even after birth, the female continues with the reproduction process by supplying the milk to nourish the infant.

Infertility

Infertility is the inability to naturally conceive a child or the inability to carry a pregnancy to term. There are many reasons why a couple may not be able to conceive without medical assistance. Infertility affects approximately 15% of couples. Roughly 40% of cases involve a male contribution or factor, 40% involve a female factor, and the remaining 20% involve both sexes. Healthy couples in their mid-20s having regular sex have a one-in-four chance of getting pregnant in any given month. This is called "Fecundity".

Primary vs. secondary

According to the American Society for Reproductive Medicine, infertility affects about 6.1 million people in the United States, equivalent to 10% of the reproductive age population. Female infertility accounts for one third of infertility cases, male infertility for another third, combined male and female infertility for another 15%, and the remainder of cases are "unexplained.

"Secondary infertility" is difficulty conceiving after already having conceived and carried a normal pregnancy. Apart from various medical conditions (e.g. hormonal), this may come as a result of age and stress felt to provide a sibling for their first child. Technically, secondary infertility is not present if there has been a change of partners.

Factors of Infertility

Factors relating to female infertility are:

General factors
- Diabetes mellitus,thyroid disorders,adrenal disease
- Significant liver,kidney disease
- Psychological factors
Hypothalamic-pituitary factors:
- Kallmann syndrome
- Hypothalamic dysfunction
- Hyperprolactinemia
- Hypopituitarism
Ovarian factors
- Polycystic ovary syndrome
- Anovulation
- Diminished ovarian reserve
- Luteal dysfunction
- Premature menopause
- Gonadal dysgenesis (Turner syndrome)
- Ovarian neoplasm
Tubal/peritoneal factors
- Endometriosis
- Pelvic adhesions
- Pelvic inflammatory disease(PID, usually due to chlamydia)
- Tubal occlusion
Uterine factors
- Uterine malformations
- Uterine fibroids (leiomyoma)
- Asherman's Syndrome
Cervical factors
- Cervical stenosis
- Antisperm antibodies
- Insufficient cervical mucus (for the travel and survival of sperm)
Vaginal factors
- Vaginismus
- Vaginal obstruction
Genetic factors
- Various intersexuality|intersexed conditions, such as androgen insensitivity syndrome

Combined Infertility

In some cases, both the man and woman may be infertile or sub-fertile, and the couple's infertility arises from the combination of these factors. In other cases, the cause is suspected to be immunological or genetic; it may be that each partner is independently fertile but the couple cannot conceive together without assistance.

Unexplained Infertility

In about 15% of cases of infertility, investigation will show no abnormalities. In these cases abnormalities are likely to be present but not detected by current methods. Possible problems could be that the egg is not released at the optimum time for fertilization, that it may not enter the fallopian tube, sperm may not be able to reach the egg, fertilization may fail to occur, transport of the zygote may be disturbed, or implantation fails. It is increasingly recognized that egg quality is of critical importance.

Diagnosis of Infertility

Diagnosis of infertility begins with a medical history and physical exam. The healthcare provider may order tests, including the following:

an endometrial biopsy, which tests the lining of the uterus
hormone testing, to measure levels of female hormones
laparoscopy, which allows the provider to see the pelvic organs
ovulation testing, which detects the release of an egg from the ovary
Pap smear, to check for signs of infection
pelvic exam, to look for abnormalities or infection
a postcoital test, which is done after sex to check for problems with secretions
special X-ray tests

Treatment

Fertility medication which stimulates the ovaries to "ripen" and release eggs (e.g. Clomifene|clomifene citrate, which stimulates ovulation)
Surgery to restore potency of obstructed fallopian tubes (tuboplasty)
Donor insemination which involves the woman being artificially inseminated or artificially inseminated with donor sperm.
In vitro fertilization (IVF) in which eggs are removed from the woman, fertilized and then placed in the woman's uterus, bypassing the fallopian tubes. Variations on IVF include:
- Use of donor eggs and/or sperm in IVF. This happens when a couple's eggs and/or sperm are unusable, or to avoid passing on a genetic disease.
- Intracytoplasmic sperm injection (ICSI) in which a single sperm is injected directly into an egg; the fertilized egg is then placed in the woman's uterus as in IVF.
- Zygote intrafallopian transfer(ZIFT) in which eggs are removed from the woman, fertilized and then placed in the woman's fallopian tubes rather than the uterus.
- Gamete intrafallopian transfer(GIFT) in which eggs are removed from the woman, and placed in one of the fallopian tubes, along with the man's sperm. This allows fertilization to take place inside the woman's body.
Other assisted reproductive technology (ART):
- Assisted hatching
- Fertility preservation
- Freezing (cryopreservation) of sperm, eggs, & reproductive tissue
- Frozen embryo transfer (FET)
Alternative and complimentary treatments
- Acupuncture Recent controlled trials published in Fertility and Sterility have shown acupuncture to increase the success rate of IVF by as much as 60%. Acupuncture was also reported to be effective in the treatment of female anovular infertility, World Health Organization, Acupuncture: Review and Analysis of Reports on Controlled Trials (2002).
- Diet and supplements
- Healthy lifestyle

Types of Birth Control

Birth control is a regimen of one or more actions, devices, or medications followed in order to deliberately prevent or reduce the likelihood of a woman becoming pregnant. Methods and intentions typically termed birth control may be considered a pivotal ingredient to family planning. Mechanisms which are intended to reduce the likelihood of the fertilization of an ovum by a sperm may more specifically be referred to as contraception. Contraception differs from abortion in that the former prevents fertilization, while the latter terminates an already established pregnancy. Methods of birth control which may prevent the implantation of an embryo if fertilization occurs are medically considered to be contraception. It is advised to talk with a doctor before choosing a contraceptive. If you have genetics problems or blood conditions, such as factor V leiden, certain contraceptives can be deadly.

Type	Procedure	Method	Effectiveness	Risks
Abstinence	Refrain from sexual intercourse	No sperm in vagina	100%	None
Rhythm Method	Intercourse is avoided for about an 8-day span every month in middle of her cycle, from about five days before ovulation to three days after ovulation.	fertilization is only possible during 8-day span in middle of menstrual cycle	70-80%	None
Withdrawal	The man withdraws his penis from the vagina at just the right moment before ejaculation.	sperm are unable to enter vagina if male penis is removed at the right time	70-80%	None
Tubal Ligation (Vasectomy)	Oviducts are cut and tied	No eggs in oviduct	Almost 99%	About 75% Irreversible
Hormonal IUD (intrauterine device)	Flexible, plastic coil inserted by physician	Releases small amounts of estrogen. In most cases, stops egg from developing and being released	About 99%	May cause infections, uterine perforation
Oral Contraceptive	Hormone medication taken daily	Stops release of FSH and LH	More than 90%	Blood clots, especially in smokers
Contraceptive Implants	Tubes of progesterone implanted under the skin	Stops release of FSH and LH	More than 90%	None known
Contraceptive Injections	Injections of hormones	Stops release of FSH and LH	About 99%	Possible osteoporosis
Diaphragm	Latex cup inserted into vagina to cover cervix before intercourse	Blocks entrance of sperm into uterus	With spermicide, about 90%	Latex or spermicide allergy
Cervical Cap	Latex cup held by suction over cervix	Delivers spermicide near cervix	Almost 85%	UTI, latex or spermicide allergy
Female Condom	Polyurethane liner fitted inside vagina	Blocks entrance of sperm into uterus and prevents STD’s	Almost 85%	None
Male Condom	soft sheath, made of latex or animal membrane, encloses penis, trapping ejaculated sperm	Blocks entrance of sperm into vagina and prevents STD's	90%	None
Jellies, Cream, Foams	Spermicidal products inserted before intercourse	Kills large number of sperm	About 75%	UTI, allergy to spermicides
Natural Family Planning	Keep record of ovulation using various methods	Avoid sexual intercourse near ovulation	About 70%	None known
Douche	Vagina cleansed after intercourse	Washes out sperm	Less than 70%	None known
Plan B Pill	Pill taken after intercourse	Prevents release of egg, fertilization of egg, or egg from attaching to uterus	About 89%	Same as oral contraceptive

Sexually Transmitted Diseases

Sexually transmitted diseases (STDs) are diseases or infections that have a significant probability of transmission between humans by means of sexual contact: vaginal intercourse, oral sex, and/or anal sex. Many STDs are (more easily) transmitted through the mucous membranes of the penis, vulva, and (less often) the mouth. The visible membrane covering the head of the penis is a mucous membrane, though, for those who are circumcised it is usually dry and produces no mucus (similar to the lips of the mouth). Mucous membranes differ from skin in that they allow certain pathogens (viruses or bacteria) into the body (more easily).

The probability of transmitting infections through sex is far greater than by more casual means of transmission, such as non-sexual contact—touching, sharing cutlery, and shaking hands. Although mucous membranes exist in the mouth as well as in the genitals, many STDs are more likely to be transmitted through oral sex than through deep kissing. Many infections that are easily transmitted from the mouth to the genitals or from the genitals to the mouth, are much harder to transmit from one mouth to another. With HIV, genital fluids happen to contain a great deal more of the pathogen than saliva. Some infections labeled as STDs can be transmitted by direct skin contact. Herpes simplex and HPV are both examples. Depending on the STD, a person who has has the disease but has no symptoms may or may not be able to spread the infection. For example, a person is much more likely to spread herpes infection when blisters are present than when they are absent. However, a person can spread HIV infection at any time, even if he/she has not developed symptoms of AIDS.

All sexual behaviors that involve contact with the bodily fluids of another person should be considered to hold some risk of transmission of sexually transmitted diseases. Most attention has focused on controlling HIV, which causes AIDS, but each STD presents a different situation.

As may be noted from the name, sexually transmitted diseases are transmitted from one person to another by certain sexual activities rather than being actually caused by those sexual activities. Bacteria, fungi, protozoa or viruses are still the causative agents. It is not possible to catch any sexually transmitted disease from a sexual activity with a person who is not carrying a disease; conversely, a person who has an STD received it from contact (sexual or otherwise) with someone who is infected.

Although the likelihood of transmitting diseases by sexual activities varies a great deal, in general, all sexual activities between two (or more) people should be considered as being a two-way route for the transmission of STDs (i.e. "giving" or "receiving" are both risky).

Prevention of Sexually Transmitted Diseases

Although healthcare professionals suggest that safer sex, such as the use of condoms, as the most reliable way of decreasing the risk of contracting sexually transmitted diseases during sexual activity, safer sex should by no means be considered an absolute safeguard. The transfer of and exposure to bodily fluids, such as blood transfusions and other blood products, sharing injection needles, needle-stick injuries (when medical staff are inadvertently jabbed or pricked with needles during medical procedures), sharing tattoo needles, and childbirth are all avenues of transmission. These means put certain groups, such as doctors, haemophiliacs and drug users, particularly at risk.

Human Papillomavirus (HPV)

There are over 100 types of this virus which is often asymptomatic. Nearly 3 out of 4 Americans between ages 15 and 49 have been infected. It can be contracted through one partner and remain dormant allowing it to be transmitted to another. Some types can cause cervical cancer.

Genital HPV infection is a sexually transmitted disease that is caused by human papillomavirus. Human papillomavirus is the name of a group of viruses that includes more than 100 different strains. More than 30 of these are sexually transmitted and they can infect the genital area of men and women. Approximately 20 million people are currently infected with HPV and at least 50% of sexually active men and women will acquire HPV at some point in their lives. By age 50 at least 80% of women will have acquired HPV and about 6.2 million Americans get a new HPV infection each year. Most people who have HPV don't know that they are infected. The virus lives in the skin or mucous membranes and usually causes no symptoms. Commonly some people get visible genital warts or have pre-cancerous changes in the cervix, vulva, anus, or penis. Very rarely, HPV results in anal or genital cancers. Genital warts usually appear soft, moist, pink, or flesh colored swellings. They can be raised, flat, single, or multiple, small or large and sometimes cauliflower shaped. Warts may not appear for weeks or months or not at all and the only way to diagnose them is by visible inspection. Most women are diagnosed with HPV on the basis of abnormal pap tests and there are no tests available for men. There is no cure for HPV. The surest way to eliminate risk for HPV is to refrain from any genital contact with another individual. For those who choose to be sexually active, a long term monogamous relationship with an uninfected partner is the strategy most likely to prevent future HPV infections.The next best way to help reduce risk is using a condom but the effectiveness is unknown.

What is the connection between HPV and cervical cancer? All types of HPV cause mild pap test abnormalities which do not have serious consequences. Approximately 10 of the 30 identified HPV types can lead to development of cervical cancer. Research as shown that for most women, 90% cervical HPV infection becomes undetectable within two years. Although only a small proportion of women have persistent infection, persistent infection with the high risk types of HPV is the main risk factor for cervical cancer.

A pap test can detect pre-cancerous and cancerous cells on the cervix. Regular pap testing and careful medical follow up, with treatment if necessary, can help ensure that pre-cancerous changes in the cervix caused by HPV infection do not develop into life threatening cervical cancer. The pap test used in the U.S. cervical cancer screening programs is responsible for greatly reducing deaths from cervical cancer.

Diseases and Disorders of the Female Reproductive System

Women are commonly dealing with many different diseases and disorders that pertain to the reproductive system. Here are some of the most common:

Vulvovaginitis (pronounced:vul-vo-vah-juh-ni-tus) is an inflammation of the vulva and vagina. It may be caused by irritating substances such as laundry soap, bubble baths or poor hygiene such as wiping from back to front. Symptoms include redness and itching in these areas and sometimes vaginal discharge. It can also be caused by an overgrowth of candida, a fungus normally present in the vagina.
Nonmenstrual vaginal bleeding is most commonly due to the presence of a foreign body in the vagina. It may also be due to urethral prolapse, a condition in which the mucous membranes of the urethra protrude into the vagina and forms a tiny, donut shaped mass of tissue that bleeds easily. It can also be due to a straddle injury or vaginal trauma from sexual abuse.
Ectopic Pregnancy occurs when a fertilized egg or zygote doesn't travel into the uterus, but instead grows rapidly in the fallopian tube. Women with this condition can develop severe abdominal pain and should see a doctor because surgery may be necessary.
Ovarian tumors,although rare, can occur. Women with ovarian tumors may have abdominal pain and masses that can be felt in the abdomen. Surgery may be needed to remove the tumor.
Ovarian cysts are noncancerous sacs filled with fluid or semi-solid material. Although they are common and generally harmless, they can become a problem if they grow very large. Large cysts may push on surrounding organs, causing abdominal pain. In most cases, cysts will pass or disappear on their own and treatment is not necessary. If the cysts are painful and occur frequently, a doctor may prescribe birth control pills to alter their growth and occurrences. Surgery is also an option if they need to be removed.
Polycystic ovary syndrome is a hormone disorder in which too many hormones are produced by the ovaries. This condition causes the ovaries to become enlarged and develop many fluid filled sacs or cysts. It often first appears during the teen years. Depending on the type and the severity of the condition, it may be treated with drugs to regulate hormone balance and menstruation.
Trichomonas vaginalis inflammatory condition of the vagina usually a bacterial infection also called vaginosis.
Dysmenorrhea is painful periods.
Menorrhagia is when a woman has very heavy periods with excess bleeding.
Oligomenorrhea is when a woman misses or has infrequent periods, even though she has been menstruating for a while and is not pregnant.
Amenorrhea is when a girl has not started her period by the time she is 16 years old or 3 years after puberty has started, has not developed signs of puberty by 14, or has had normal periods but has stopped menstruating for some reasons other than pregnancy.
Toxic shock syndrome is caused by toxins released into the body during a type of bacterial infection that is more likely to develop if a tampon is left in too long. It can produce high fever, diarrhea, vomiting, and shock.
Candidasis symptoms of yeast infections include itching, burning and discharge. Yeast organisms are always present in all people, but are usually prevented from "overgrowth" (uncontrolled multiplication resulting in symptoms) by naturally occurring microorganisms.

At least three quarters of all women will experience candidiasis at some point in their lives. The Candida albicans organism is found in the vaginas of almost all women and normally causes no problems. However, when it gets out of balance with the other "normal flora," such as lactobacilli (which can also be harmed by using douches), an overgrowth of yeast can result in noticeable symptoms. Pregnancy, the use of oral contraceptives, engaging in vaginal sex after anal sex in an unhygienic manner, and using lubricants containing glycerin have been found to be causally related to yeast infections. Diabetes mellitus and the use of antibiotics are also linked to an increased incidence of yeast infections. Candidiasis can be sexually transmitted between partners. Diet has been found to be the cause in some animals. Hormone Replacement Therapy and Infertility Treatment may be factors.

There are also cancer's of the female reproductive system, such as:

Cervical cancer
Ovarian cancer
Uterine cancer
Breast cancer

Endometriosis

Endometriosis is the most common gynecological diseases, affecting more than 5.5 million women in North America alone! The two most common symptoms are pain and infertility. In this disease a specialized type of tissue that normally lines the inside of the uterus,(the endometrium) becomes implanted outside the uterus, most commonly on the fallopian tubes, ovaries, or the tissue lining the pelvis. During the menstrual cycle, hormones signal the lining of the uterus to thicken to prepare for possible pregnancy. If a pregnancy doesn't occur, the hormone levels decrease, causing the thickened lining to shed.

When endometrial tissue is located in other parts it continues to act in it's normal way: It thickens, breaks down and bleeds each month as the hormone levels rise and fall. However, because there's nowhere for the blood from this mislocated tissue to exit the body, it becomes trapped and surrounding tissue becomes irritated. Trapped blood may lead to growth of cysts. Cysts in turn may form scar tissue and adhesions. This causes pain in the area of the misplaced tissue, usually the pelvis. Endometriosis can cause fertility problems. In fact, scars and adhesions on the ovaries or fallopian tubes can prevent pregnancy. Endometriosis can be mild, moderate or severe and tends to get worse over time without treatment. The most common symptoms are:

Painful periods Pelvic pain and severe cramping, intense back pain and abdominal pain.
Pain at other times Women may experience pelvic pain during ovulation, sharp deep pain in pelvis during intercourse, or pain during bowel movements or urination.
Excessive bleeding Heavy periods or bleeding between periods.
Infertility Approximately 30-40% of women

The cause of endometriosis remains mysterious. Scientists are studying the roles that hormones and the immune system play in this condition. One theory holds that menstrual blood containing endometrial cells flows back through the fallopian tubes, takes root and grows. Another hypothesis proposes that the bloodstream carries endometrial cells to other sites in the body. Still another theory speculates that a predisposition toward endometriosis may be carried in the genes of certain families.

Other researchers believe that certain cells present within the abdomen in some women retain their ability to specialize into endometrial cells. These same cells were responsible for the growth of the woman's reproductive organs when she was an embryo. It is believed that genetic or environmental influences in later life allow these cells to give rise to endometrial tissue outside the uterus.

Experts estimate that up to one in ten American women of childbearing age have endometriosis. There is some thinking that previous damage to cells that line the pelvis can lead to endometriosis. There are several ways to diagnose endometriosis:

Pelvic exam
Ultrasound
Laparoscopy Usually used, most correct diagnosis
Blood test

Endometriosis can be treated with:

Pain medication
Hormone therapy
1. Oral contraceptives
2. Gonadotropin-releasing hormone(Gn-Rh)agonists and antagonists
3. Danazol(Danocrine)
4. Medroxyprogesterone(Depo-Provera)
Conservative surgery which removes endometrial growths.
Hysterectomy

Check Your Understanding

Answers for these questions can be found here

1. In homology, the ___________ in the female is equal to the penis in the male

A) labia majora

B) clitoral hood

C) clitoris

D) labia minora

E) none of the above

2. This contains some of the strongest muscles in the human body

A) uterus

B) clitoris

C) cervix

D) labia majora

3. This protects the vaginal and urethral openings

A) labia majora

B) labia minora

C) clitoris

D) urethra

4. Sally has noticed that her cervical mucus has changed and now resembles egg whites- from this Sally could assume

A) her period will begin soon

B) nothing, this is a normal occurrence

C) she has a yeast infection

D) she is ovulating

5. Debbie recently went to the OBGYN and was diagnosed with PCOD (polycystic ovary syndrome) because of this she has

A) nothing, its normal in women

B) antisperm antibodies

C) an overproduction of LH

D) leaking of milk from her mammary glands

E) problems becoming pregnant

6. Angie went to the doctor because she has had pain in her leg recently- this could be caused by

A) ovulation pain

B) her period that will be starting tomorrow

C) premenstrual syndrome

D) a blood clot resulting from her birth control pill

7. Sue recently started her period and has noticed that they are very heavy and painful, and that they are inconsistent in their timing. One explanation could be

A) endometriosis

B) ovarian cancer

C) candidasis

D) toxic shock syndrome

E) amenorrhea

8. Mary is getting married and is not ready to become a mother- she chooses this birth control because of its high effectiveness

A) natural family planning

B) a diaphragm

C) contraceptive injections

D) a spermicide foam

9. The release of LH in woman causes

A) menstration

B) ovulation

C) increase of endometrial lining

D) decrease of endometrial lining

E) nothing LH only does something in the male reproductive system

10. When the ovaries stop producing estrogen, this occurs

A) ovulation

B) implantation

C) premenstrual syndrome

D) menopause

11. Infertility affects what percentage of couples?

A) 5%

B) 10%

C) 15%

D) 20%

12. What is the only 100% effective form of birth control?

A) Tubal ligation

B) IUD

C) Natural family planning

D) Abstinence

Glossary

Adhesions: Abnormal tissue that binds organs together

Alveoli: Basic components of the mammary glands; lined with milk-secreting epithelial cells

Birth Control: regimen of one or more actions, devices, or medications followed in order to deliberately prevent or reduce the likelihood of a woman becoming pregnant

Cervical Mucus: Mucus secreted by the cervix, near ovulation it helps to lower the acidity of the vagina

Cervix: Lower, narrow portion of the uterus where it joins with the top of the vagina

Clitoris: Small body of spongy tissue that functions solely for sexual pleasure

Chromosomes: Structures in the nucleus that contain the genes for genetic expression

Ectocervix: Portion of the cervix projecting into vagina

Endocervical Canal: Passageway between the external os and the uterine cavity

Endometrium: The inner lining of the uterus

Fallopian Tubes: Located at the upper end of the vagina, passage way for the egg from the ovary

Factor V Leiden: This is the name given to a variant of human factor V that causes a hypercoagulability disorder. In this disorder the Leiden variant of factor V, cannot be inactivated by activated protein C. Factor V Leiden is the most common hereditary hypercoagulability disorder amongst Eurasians. It is named after the city Leiden (The Netherlands), where it was first identified in 1994 by Prof R. Bertina et al.

Gamete: A haploid sex cell; either an egg cell or a sperm cell

Gene: That portion of the DNA of a chromosome containing the information needed to synthesize a particular protein molecule

Gonad: A reproductive organ, testis or ovary that produces gametes and sex hormones

Hormone: A chemical substance produced in an endocrine gland and secreted into the bloodstream that acts on target cells to produce a specific effect

Hymen: Thin fold of mucous membrane that separates the lumen of the vagina from the urethral sinus

Infertility: Inability to naturally conceive a child or the inability to carry a pregnancy to term

Labia Majora: Outer "lips" of the vulva, made of loose connective tissue and adipose tissue with some smooth muscle

Labia Minora: Inner lips of the vulva, folds and protects the vagina, urethra and clitoris

Mammary Glands: Organs that produce milk for the sustenance of a baby

Meiosis: A specialized type of cell division by which gametes, or haploid sex cells, are formed

Menarche: The first menstrual discharge; occurs normally between the ages of 9 and 17

Menopause: The period marked by the cessation of menstrual periods in the human female

Menstrual Cycle: The rhythmic female reproductive cycle characterized by physical changes in the uterine lining

Menstruation: The discharge of blood and tissue from the uterus at the end of menstrual cycle

Mittelschmerz: Pain near the lower abdomen site at the time of ovulation; German for ovulation pain

Mons Veneris: soft mound at the front of the vulva (fatty tissue covering the pubic bone)

Ovarian Cycle: Last phase of the reproductive cycle; if no implantation occurs, causes the breakdown of the endometrial lining and causes menstruation

Ovulation: The rupture of an ovarian follicle with the release of an ovum

Perineum: External region between the scrotum and the anus in a male or between the vulva and anus in a female

Premenstrual Syndrome (PMS): Time leading up to menstruation; includes both physical and emotional symptoms: acne, bloating, fatigue, backaches, sore breasts, headaches, constipation, diarrhea, food cravings, depression, irritability, difficulty concentrating or handling stress

Puberty: The period of development in which the reproductive organs become functional and the secondary sex characteristics are expressed

Reproduction: Process by which an organism continues its species

Sexually transmitted diseases (STDs): diseases or infections that have a significant probability of transmission between humans by means of sexual contact

Urethra: Located below the clitoris, used for the passage of urine

Uterine Cycle: First part of the reproductive cycle; the time when the endrometrial lining builds up and follicles develop

Uterus: Major reproductive organ, receives fertilized eggs which become implanted in the lining, the lining (endometrium) provides nourishment to developing fetus; contains some of the strongest muscles in the female body and is able to stretch during fetus development

Vagina: Muscular, hollow tube that extends from the vaginal opening to the cervix

Vulva: External female genitals, includes labia majora, labia minora, mons pubis, clitoris, meatus, vaginal vestibule, vestibule bulbs and vestibular glands

References

Essentials of Anatomy and Physiology. Fourth Edition. Valerie C. Scanlon and Tina Sanders.
Human Anatomy. Sixth Edition. Van De Graaff.
Wikibook: Sexual Health
http://www.fda.gov/cder/drug/infopage/planBQandAhtm
http://www.goplanb.com/Forconsumers
American Social Health Association;ashastd.org
http://www.cdc.gov
http://www.mayoclinic.com

MALE REPRODUCTION

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Cowper's (Bulbourethral) Glands

The bulbourethral, or Cowper's, glands, are two small structures about the size of peas, which are located below the prostate gland. They are composed of many tubes whose linings secrete a fluid that is released in response to sexual stimulation to provide some lubrication to the end of the penis in preparation for sexual intercourse. Most of the lubricating, however, is provided by the female reproductive organs.

Ductus Deferens

The "ductus deferens" (also called the "vas deferens") is a muscular tube that begins at the lower end of the epididymis and passes upward along the side of the testis to become part of the spermatic cord. It passes through the inguinal canal, enters the abdominal cavity, and courses over the pelvic brim. From there, it extends back into the pelvic cavity, where it ends behind the urinary bladder. Near its termination, it becomes dilated into a portion called the "ampulla." Just outside the prostate gland, the tube becomes slender again and unites with the duct of a seminal vesicle. The fusion of these two ducts forms an ejaculatory duct, which passes through the substance of the prostate gland and empties into the urethra through a slit-like opening.

Penis

The penis is the external sex organ of the male through which both urine and semen pass. It consists of three cylinder-shaped bodies of spongy tissue filled with tiny blood vessels, which run the length of the organ. Two of these bodies lie side by side in the upper portion of the penis. The third is a tube which lies centrally beneath the others and expands at the end to form the tip of the penis, which is called the "glans." The penis transfers sperm to the woman's body during sexual intercourse and is a duct for the disposal of fluid waste. The penis becomes erect during sexual excitement, because extra blood is pumped into spongy tissues, resulting in enlargement and hardening which allows penetration into the female organ. The "head" of the penis is called the "glans" and is normally covered with a protective, retractable skin or "hood." This skin is often cut off at birth, or "circumcised," and it is believed that its removal lessens risk of cancer and bacterial infections. At the center of the penis is a tube which carries urine from the bladder and semen from the prostate gland, called the "urethra." During sexual intercourse, reflexes prevent urine from entering this duct and alkaline solutions are produced and secreted to flush out any traces of urine from the urethra before semen is secreted.

Prostate Gland

The prostate gland is a solid, chestnut-shaped organ surrounding the first part of the urethra (tube which carries the urine and semen) in the male. It produces secretions which form a part of the semen. The prostate gland lies just under the bladder and in front of the rectum. It consists of two main zones: the inner zone, which produces secretions to keep the lining of the male urethra moist, and the outer zone, which produces seminal fluids to facilitate the passage of semen into the female. The "urethra" is a two-stemmed duct leading from the bladder and from the prostate gland into the penis. The word, "aphrodisiac," is derived from Aphrodite, the Greek goddess of love and sexual pleasure. The ancient Greeks thought honey would produce an increase in sexual powers and they believed the same of the hair from a wolf's tail and ground snake bones. The French used an "aphrodisia" which they called the "love apple" - actually a tomato. Today, the hope lies in oysters. Sadly, the fallacy that a ground rhinoceros horn will help in this plight has put this animal on the endangered species list. Actually, aphrodisiacs are in the mind and operate only by the power of suggestion - if at all.

Testicles

The scrotum is a sac that hangs under the penis and holds the testes. It is divided internally into two halves by a membrane; each half containing a testis. It has an outer layer of thin, wrinkled skin over a layer of tissue which contains muscle. The testicle lies inside the scrotum and produces as many as 12 trillion sperm in a male's lifetime, about 400 million of which are ejaculated in one average intercourse. Each sperm takes about seventy-two days to mature and its maturity is overseen by a complex interaction of hormones. The scrotum has a built-in thermostat, which keeps the sperm at the correct temperature. It may be surprising that the testicles should lie in such a vulnerable place, outside the body, but it is too hot inside. The sperm production needs a temperature which is three to five degrees below body temperature. If it becomes too cool on the outside, the scrotum will contract to bring the testes closer the body for warmth.

WELCOME TO THE SITE OF ALBERTO

Contents

Naturally occurring earthquakes

Earthquake fault types

Earthquakes away from plate boundaries

Shallow-focus and deep-focus earthquakes

Earthquakes and volcanic activity

Rupture dynamics

Tidal forces

Earthquake clusters

Aftershocks

Earthquake swarms

Earthquake storms

Size and frequency of occurrence

Induced seismicity

Measuring and locating earthquakes

Effects of earthquakes

Shaking and ground rupture

Landslides and avalanches

Fires

Soil liquefaction

Tsunami

Floods

Human impacts

Major earthquakes

Prediction

Preparedness

Historical views

Earthquakes in culture

Mythology and religion

Popular culture

See also

References

Contents

Etymology

Plate tectonics

Divergent plate boundaries

Convergent plate boundaries

"Hotspots"

Volcanic features

Fissure vents

Shield volcanoes

Lava domes

Cryptodomes

Volcanic cones (cinder cones)

Stratovolcanoes (composite volcanoes)

Supervolcanoes

Submarine volcanoes

Subglacial volcanoes

Mud volcanoes

Erupted material

Lava composition

Lava texture

Volcanic activity

Popular classification of volcanoes

Active

Extinct

Dormant

Technical classification of volcanoes

Volcanic-alert level

Volcano warning schemes of the United States

Notable volcanoes

Effects of volcanoes

Volcanoes on other planetary bodies

Traditional beliefs about volcanoes

Panoramas

See also

References

Further reading

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The Mammary Glands

Cervix