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Earthquake

Earthquake

: An earthquake is a sudden and sometimes catastrophic movement of a part of the Earth's surface. Earthquakes result from the dynamic release of elastic strain energy that radiates seismic waves. Earthquakes typically result from the movement of faults, planar zones of deformation within the Earth's upper crust. The word earthquake is also widely used to indicate the source region itself. The Earth's lithosphere is a patch work of plates in slow but constant motion (see plate tectonics). Earthquakes occur where the stress resulting from the differential motion of these plates exceeds the strength of the crust. The highest stress (and possible weakest zones) are most often found at the boundaries of the tectonic plates and hence these locations are where the majority of earthquakes occur. Events located at plate boundaries are called interplate earthquakes; the less frequent events that occur in the interior of the lithospheric plates are called intraplate earthquakes (see New Madrid Seismic Zone). Earthquakes also occur in volcanic regions and as the result of a number of anthropogenic sources, such as reservoir induced seismicity, mining and the removal or injection of fluids into the crust. Seismic waves including some strong enough to be felt by humans can also be caused by explosions (chemical or nuclear), landslides, and collapse of old mine shafts, though these sources are not strictly earthquakes.

Characteristics

Large numbers of earthquakes occur on a daily basis on Earth, but the majority of them are detected only by seismometers and cause no damage ([http://neic.usgs.gov/neis/general/magnitude_intensity.html magnitude] 5). Most earthquakes occur in narrow regions around plate boundaries down to depths of a few tens of kilometres where the crust is rigid enough to support the elastic strain. Where the crust is thicker and colder they will occur at greater depths and the opposite in areas that are hot. At subduction zones where plates descend into the mantle earthquakes have been recorded to a depth of 600 km. Large earthquakes can cause serious destruction and massive loss of life through a variety of agents of damage, including fault rupture, vibratory ground motion (i.e., shaking), inundation (e.g., tsunami, seiche, dam failure), various kinds of permanent ground failure (e.g. liquefaction, landslide), and fire or a release of hazardous materials. In a particular earthquake, any of these agents of damage can dominate, and historically each has caused major damage and great loss of life, but for most of the earthquakes shaking is the dominant and most widespread cause of damage. There are four types of seismic waves that are all generated simultaneously and can be felt on the ground. S-waves (secondary or shear waves) and the two types of surfaces waves (Love waves and Rayleigh waves) are responsible for the shaking hazard. Rayleigh waves Rayleigh waves Most large earthquakes are accompanied by other, smaller ones, that can occur either before or after the principal quake — these are known as foreshocks or aftershocks, respectively. While almost all earthquakes have aftershocks, foreshocks are far less common occurring in only about 10% of events. The power of an earthquake is distributed over a significant area, but in the case of large earthquakes, it can spread over the entire planet. Ground motions caused by very distant earthquakes are called teleseisms. The Rayleigh waves from the Sumatra-Andaman Earthquake of 2004 caused ground motion of over 1 cm even at the seismometers that were located the greatest distance from it. Using such ground motion records from around the world it is possible to identify a point from which the earthquake's seismic waves appear to originate. That point is called its "focus" or "hypocenter" and usually proves to be the point at which the fault slip was initiated. The location on the surface directly above the hypocenter is known as the "epicenter". The total size of the fault that slips, the rupture zone, can be as large as 1000 km, for the biggest earthquakes. Just as a large loudspeaker can produce a greater volume of sound than a smaller one, large faults are capable of higher magnitude earthquakes than smaller faults are. Earthquakes, especially those that occur beneath oceans or seas (also called seaquake) and have large vertical displacements, can give rise to tsunamis, either as a direct result of the deformation of the sea bed due to the earthquake, or as a result of submarine landslips or "slides" indirectly triggered by it.

Earthquake Size

The first method of quantifying earthquakes was intensity scales. In the United States the Mercalli (or Modified Mercalli, MM) scale, is commonly used while Japan (shindo) and the EU (European Macroseismic Scale) each have their own scales. These assign a numeric value (different for each scale) to a location based on the size of the shaking experienced there. The values 6 (normally denoted ‘’VI’’) in the MM scale for example is: Everyone feels movement. People have trouble walking. Objects fall from shelves. Pictures fall off walls. Furniture moves. Plaster in walls might crack. Trees and bushes shake. Damage is slight in poorly built buildings. No structural damage. The problem with these scales is the measurement is subjective, often based on the worst damage in an area and influenced by local effects like site conditions that make it a poor measure for the relative size of different events in different places. For some tasks related to engineering and local planning it is still useful for the very same reasons and thus still collected. If you feel an earthquake in the US you can report the effects to the USGS here: [http://pasadena.wr.usgs.gov/shake/ Did you feel it?] The first attempt to qualitatively define one value to describe the size of earthquakes was the magnitude scale (the name being taking from similar formed scales used on the brightness of stars). In the 1930s, a California seismologist named Charles F. Richter devised a simple numerical scale (which he called the magnitude) to describe the relative sizes of earthquakes in Southern California. This is known as the “Richter scale”, “Richter Magnitude” or “Local Magnitude” (M_L). It is obtained by measuring the maximum amplitude of a recording on a Wood-Anderson torsion seismometer (or one calibrated to it) at a distance of 600km from the earthquake. Other more recent Magnitude measurements include: body wave magnitude (m_b), surface wave magnitude (M_s) and duration magnitude (M_D). Each of these is scaled to gives values similar to the values given by the Richter scale. However as each is also based on the measurement of one part of the seismogram they do not measure the overall power of the source and can suffer from saturation at higher magnitude values (larger events fail to produce higher magnitude values).These scales are also empirical and as such there is no physical meaning to the values. They are still useful however as they can be rapidly calculated, there are catalogues of them dating back many years and are they are familiar to the public. Seismologists now favor a measure called the seismic moment, related to the concept of moment in physics, to measure the size of a seismic source. The seismic moment is calculated from seismograms but can also by obtained from geologic estimates of the size of the fault rupture and the displacement. The values of moments for different earthquakes ranges over several order of magnitude. As a result the moment magnitude (M_W) scale was introduced by Hiroo Kanamori, which is comparable to the other magnitude scales but will not saturate at higher values. seismogram on February 28 2001.]] 2001 of the shaking of the Nisqually earthquake on February 28 2001.]]

Causes

Most earthquakes are powered by the release of the elastic strain that accumulate over time, typically, at the boundaries of the plates that make up the Earth's lithosphere via a process called Elastic-rebound theory. The Earth is made up of tectonic plates driven by the heat in the Earth's core. these plates collide against each other all the time but sometimes the gaps between them are stressed. Eventually, the plates make way and all that energy is sent out in the form of seismic waves. Deep focus earthquakes, at depths of 100's km, are possibly generated as subducted lithospheric material catastrophically undergoes a phase transition since at the pressures and temperatures present at such depth elastic strain cannot be supported. Some earthquakes are also caused by the movement of magma in volcanoes, and such quakes can be an early warning of volcanic eruptions. A rare few earthquakes have been associated with the build-up of large masses of water behind dams, such as the Kariba Dam in Zambia, Africa, and with the injection or extraction of fluids into the Earth's crust (e.g. at certain geothermal power plants and at the Rocky Mountain Arsenal). Such earthquakes occur because the strength of the Earth's crust can be modified by fluid pressure. Earthquakes have also been known to be caused by the removal of natural gas from subsurface deposits, for instance in the northern Netherlands. Finally, ground shaking can also result from the detonation of explosives. Thus scientists have been able to monitor, using the tools of seismology, nuclear weapons tests performed by governments that were not disclosing information about these tests along normal channels. Earthquakes such as these, that are caused by human activity, are referred to by the term induced seismicity. Another type of movement of the Earth is observed by terrestrial spectroscopy. These oscillations of the earth are either due to the deformation of the Earth by tide caused by the Moon or the Sun, or other phenomena.

Preparation for earthquakes

- Emergency preparedness
- Household seismic safety
- Seismic retrofit
- Earthquake prediction

Specific fault articles

- Alpine Fault
- Calaveras Fault
- Hayward Fault Zone
- North Anatolian Fault Zone
- New Madrid Fault Zone
- San Andreas Fault

Specific earthquake articles

- Shaanxi Earthquake (1556). Deadliest known earthquake in history, estimated to have killed 830,000 in China.
- Cascadia Earthquake (1700).
- Kamchatka earthquakes (1737 and 1952).
- Lisbon earthquake (1755).
- New Madrid Earthquake (1811).
- Fort Tejon Earthquake (1857).
- Charleston earthquake (1886). Largest earthquake in the Southeast and killed 100.
- San Francisco Earthquake (1906).
- Great Kantō earthquake (1923). On the Japanese island of Honshu, killing over 140,000 in Tokyo and environs.
- Kamchatka earthquakes (1952 and 1737).
- Great Chilean Earthquake (1960). Biggest earthquake ever recorded, 9.5 on Moment magnitude scale.
- Good Friday Earthquake (1964) Alaskan earthquake.
- Ancash earthquake (1970). Caused a landslide that buried the town of Yungay, Peru; killed over 40,000 people.
- Sylmar earthquake (1971). Caused great and unexpected destruction of freeway bridges and flyways in the San Fernando Valley, leading to the first major seismic retrofits of these types of structures, but not at a sufficient pace to avoid the next California freeway collapse in 1989.
- Tangshan earthquake (1976). The most destructive earthquake of modern times. The official death toll was 255,000, but many experts believe that two or three times that number died.
- Great Mexican Earthquake (1985). 8.1 on the Ritcher Scale, killed over 6,500 people (though it is believed as many as 30,000 may have died, due to missing people never reappearing.)
- Whittier Narrows earthquake (1987).
- Armenian earthquake (1988). Killed over 25,000.
- Loma Prieta earthquake (1989). Severely affecting Santa Cruz, San Francisco and Oakland in California. Revealed necessity of accelerated seismic retrofit of road and bridge structures.
- Northridge, California earthquake (1994). Damage showed seismic resistance deficiencies in modern low-rise apartment construction.
- Great Hanshin earthquake (1995). Killed over 6,400 people in and around Kobe, Japan.
- İzmit earthquake (1999) Killed over 17,000 in northwestern Turkey.
- Düzce earthquake (1999)
- Chi-Chi earthquake (1999).
- Nisqually Earthquake (2001).
- Gujarat Earthquake (2001).
- Dudley Earthquake (2002).
- Bam Earthquake (2003).
- Parkfield, California earthquake (2004). Not large (6.0), but the most anticipated and intensely instrumented earthquake ever recorded and likely to offer insights into predicting future earthquakes elsewhere on similar slip-strike fault structures.
- Chuetsu Earthquake (2004).
- Indian Ocean Earthquake (2004). One of the largest earthquakes ever recorded at 9.0. Epicenter off the coast of the Indonesian island Sumatra. Triggered a tsunami which caused nearly 300,000 deaths spanning several countries.
- Sumatran Earthquake (2005).
- Fukuoka earthquake (2005).
- Kashmir earthquake (2005). Killed over 79,000 people. Many more at risk from the Kashmiri winter.
- Lake Tanganyika earthquake (2005). See also List of earthquakes

External links

- [http://www.eqnet.org/ EQNET: Earthquake Information Network]
- [http://neic.usgs.gov/ The U.S. National Earthquake Information Center]
- [http://earthquake.usgs.gov/faq/ USGS Earthquake FAQs]
- [http://www.ssn.unam.mx/ Mexican Sismological Service] Reports earthquakes in Mexico. Updated regularly.
- [http://wapi.isu.edu/envgeo/EG5_earthqks/eg_mod5.htm Environmental Geology - GEOL 406/506 (Earthquakes)]
- [http://www.quakes.bgs.ac.uk/hazard/ems1.htm The European Macroseismic Scale]
- [http://simscience.org/crackling/Advanced/Earthquakes/GutenbergRichter.html Gutenberg-Richter] power law of earthquake frequency against magnitude
- [http://www.guardian.co.uk/flash/0,5860,1121610,00.html Interactive guide: Earthquakes] an educational presentation on why earthquakes happen by Guardian Unlimited
- [http://www.geowall.org Geowall]- an educational 3d presentation system for looking at and understanding earthquake data
- [http://www.sciencecourseware.com/VirtualEarthquake/ Virtual Earthquake] educational site explaining how epicenters are located and magnitude is determined
- [http://www.pbs.org/newshour/science/earthquake/ PBS NewsHour - Predicting Earthquakes]
- [http://www.lamit.ro/earthquake-early-warning-system.htm Earthquake Warning System] Personal Earthquake warning system. Highly advanced detector, featuring sos signals and carrying strip.
- [http://www.data.scec.org/ Southern California Earthquake Data Center]
- [http://www.emsc-csem.org/ European-Mediterranean Seismological Centre (EMSC)]
- [http://www.gfz-potsdam.de/geofon/seismon/globmon.html Global Seismic Monitor at GFZ Potsdam]
- [http://earthquake.usgs.gov/bytopic/eqmonitoring/history/part09.php USGS Earthquake Monitoring History]
- [http://tsunami.geo.ed.ac.uk/local-bin/quakes/mapscript/demo_run.pl Global Earthquake Report – chart updated with each new earthquake or aftershock]
- [http://hraun.vedur.is/ja/englishweb/index.html Earthquakes in Iceland during the last 48 hours], updated automatically once every 2 minutes.
- [http://www.data.scec.org/recenteqs/Quakes/quakes0.html Recent earthquakes in California and Nevada ]
- [http://neic.usgs.gov/neis/eqlists/10maps_world.html USGS – Largest earthquakes in the world since 1900]
- [http://www.armageddononline.org/earthquake.php The Destruction of Earthquakes - and a List of the Worst ever recorded]
- [http://www.losangelesearthquakes.com Los Angeles Earthquakes plotted on a Google map]
- [http://rev.seis.sc.edu Seismograms for recent earthquakes via REV, the Rapid Earthquake Viewer]
- [http://www.iris.edu Incorporated Research Institutions for Seismology (IRIS)], earthquake database and software
- [http://www.iris.edu/seismon/ IRIS Seismic Monitor], world map of recent earthquakes
- [http://www.iris.edu/seismo/ SeismoArchives], Seismogram Archives of Significant Earthquakes of the World Category:Seismology Category:Geological hazards ko:지진 ms:Gempa bumi ja:地震 simple:Earthquake th:แผ่นดินไหว

Seismic wave

right right right A seismic wave is a wave that travels through the Earth, often as the result of an earthquake or explosion. Seismic waves are studied by seismologists, and measured by a seismograph or seismometer.
- Body waves travel through the interior of the Earth. They follow curved paths because of the varying density and composition of the Earth's interior. This effect is similar to the refraction of light waves. Body waves transmit the preliminary tremors of an earthquake but have little destructive effect. Body waves are divided into two types: primary (P-waves) and secondary (S-waves

Crust (geology)

to exosphere. Partially to scale.]] In geology, a crust is the outer layer of a planet, part of its lithosphere. Planetary crusts are generally composed of a less dense material than that of its deeper layers. The crust of the Earth is composed mainly of basalt and granite. It is cooler and more rigid than the deeper layers of the mantle and core. On partially-molten planets, such as Earth, the lithosphere is floating on fluid interior layers. Because of the partially-fluid upper mantle, or asthenosphere, underneath, floating lithospheres can be broken into tectonic plates that move. Sea floor crust is different from that of the continents. The oceanic crust (sima) is 5 to 10 km thick and is composed primarily of a dark, dense rock called basalt. The continental crust (sial) is 20-70 km deep and is composed of a variety of less dense rocks.

External links

- [http://quake.wr.usgs.gov/research/structure/CrustalStructure/ USGS Crust Thickness Map] Category:Geology Category:Plate tectonics ms:Kerak bumi ja:地殻 simple:Crust th:เปลือกโลก

Plate tectonics

Plate tectonics (from the Greek word for "one who constructs", τεκτων, tekton) is a theory of geology developed to explain the phenomenon of continental drift, and is currently the theory accepted by the vast majority of scientists working in this area. In the theory of plate tectonics the outermost part of the Earth's interior is made up of two layers, the outer lithosphere and the inner asthenosphere. The lithosphere essentially "floats" on the asthenosphere and is broken-up into ten major plates: African, Antarctic, Australian, Eurasian, North American, South American, Pacific, Cocos, Nazca, and the Indian plates. These plates (and the more numerous minor plates) move in relation to one another at one of three types of plate boundaries: convergent (two plates push against one another), divergent (two plates move away from each other), and transform (two plates slide past one another). Earthquakes, volcanic activity, mountain-building, and oceanic trench formation occur along plate boundaries (most notably around the so-called "Pacific Ring of Fire"). Plate tectonic theory arose out of two separate geological observations: continental drift, noticed in the early 20th century, and seafloor spreading, noticed in the 1960s. The theory itself was developed during the late 1960s and has since almost universally been accepted by scientists and has revolutionized the Earth sciences (akin to the development of the periodic table for chemistry, the discovery of the genetic code for genetics, or evolution in biology). biology

Key principles

The division of the Earth's interior into lithospheric and asthenospheric components is based on their mechanical differences. The lithosphere is cooler and more rigid, whilst the asthenosphere is hotter and mechanically weaker. This division should not be confused with the chemical subdivision of the Earth into (from innermost to outermost) core, mantle, and crust. The key principle of plate tectonics is that the lithosphere exists as separate and distinct tectonic plates, which "float" on the fluid-like asthenosphere. The relative fluidity of the asthenosphere allows the tectonic plates to undergo motion in different directions. One plate meets another along a plate boundary, and plate boundaries are commonly associated with geological events such as earthquakes and the creation of topographic features like mountains, volcanoes and oceanic trenches. The majority of the world's active volcanoes occur along plate boundaries, with the Pacific Plate's Ring of Fire being most active and famous. These boundaries are discussed in further detail below. Tectonic plates are comprised of two types of lithosphere: continental and oceanic lithospheres; for example, the African Plate includes the continent and parts of the floor of the Atlantic and Indian Oceans. The distinction is based on the density of constituent materials; oceanic lithospheres are denser than continental ones due to their greater mafic mineral content. As a result, the oceanic lithospheres generally lie below sea level (for example the entire Pacific Plate, which carries no continent), while the continental ones project above sea level (see isostasy for explanation of this principle, which is essentially a largescale version of Archimedes' Bath).

Types of plate boundaries

Archimedes There are three types of plate boundaries, characterised by the way the plates move relative to each other. They are associated with different types of surface phenomena. The different types of plate boundaries are: # Transform boundaries occur where plates slide, or perhaps more accurately grind, past each other along transform faults. The relative motion of the two plates is either sinistral or dextral. # Divergent boundaries occur where two plates slide apart from each other. # Convergent boundaries (or active margins) occur where two plates slide towards each other commonly forming either a subduction zone (if one plate moves underneath the other) or an orogenic belt (if the two simply collide and compress). Plate boundary zones occur in more complex situations where three or more plates meet and exhibit a mixture of the above three boundary types.

Transform (conservative) boundaries

The left- or right-lateral motion of one plate against another along transformstrike slip faults can cause highly visible surface effects. Because of friction, the plates cannot simply glide past each other. Rather, stress builds up in both plates and when it reaches a level that exceeds the slipping-point of rocks on either side of the transform-faults the accumulated potential energy is released as strain, or motion along the fault. The massive amounts of energy that are released are the cause of earthquakes, a common phenomenon along transform boundaries. A good example of this type of plate boundary is the San Andreas Fault complex, which is found in the western coast of North America and is one part of a highly complex system of faults in this area. At this location, the Pacific and North American plates move relative to each other such that the Pacific plate is moving north with respect to North America.

Divergent (constructive) boundaries

At divergent boundaries, two plates move apart from each other and the space that this creates is filled with new crustal material sourced from molten magma that forms below. The origin of new divergent boundaries at triple junctions is sometimes thought to be associated with the phenomenon known as hotspots. Here, exceedingly large convective cells bring very large quantities of hot asthenospheric material near the surface and the kinetic energy is thought to be sufficient to break apart the lithosphere. The hot spot which may have initiated the Mid-Atlantic Ridge system currently underlies Iceland which is widening at a rate of a few centimetres per century. Divergent boundaries are typified in the oceanic lithosphere by the rifts of the oceanic ridge system, including the Mid-Atlantic Ridge, and in the continental lithosphere by rift valleys such as the famous East African Great Rift Valley. Divergent boundaries can create massive fault zones in the oceanic ridge system. Spreading is generally not uniform, so where spreading rates of adjacent ridge blocks are different massive transform faults occur. These are the fracture zones, many bearing names, that are a major source of submarine earthquakes. A sea floor map will show a rather strange pattern of blocky structures that are separated by [http://pubs.usgs.gov/publications/text/baseball.html linear features] perpendicular to the ridge axis. If one views the sea floor between the fracture zones as conveyor belts carrying the ridge on each side of the rift away from the spreading center the action becomes clear. Crest depths of the old ridges, parallel to the current spreading center, will be older and deeper (due to thermal contraction and subsidence). It is at mid-ocean ridges that one of the key pieces of evidence forcing acceptance of the sea-floor spreading hypothesis was found. Airborne geomagnetic surveys showed a strange pattern of symmetrical magnetic reversals on opposite sides of ridge centres. The pattern was far too regular to be coincidental as the widths of the opposing bands were too closely matched. Scientists had been studying polar reversals and the link was made. The magnetic banding directly corresponds with the Earth's polar reversals. This was confirmed by measuring the ages of the rocks within each band. The banding furnishes a map in time and space of both spreading rate and polar reversals.

Convergent (destructive) boundaries

The nature of a convergent boundary depends on the type of lithosphere in the plates that are colliding. Where a dense oceanic plate collides with a less-dense continental plate, the oceanic plate is typically thrust underneath, forming a subduction zone. At the surface, the topographic expression is commonly an oceanic trench on the ocean side and a mountain range on the continental side. An example of a continental-oceanic subduction zone is the area along the western coast of South America where the oceanic Nazca Plate is being subducted beneath the continental South American Plate. As the subducting plate descends, its temperature rises driving off volatiles (most importantly water). As this water rises into the mantle of the overriding plate, it lowers its melting temperature, resulting in the formation of magma with large amounts of dissolved gases. This can erupt to the surface, forming long chains of volcanoes inland from the continental shelf and parallel to it. The continental spine of South America is dense with this type of volcano. In North America the Cascade mountain range, extending north from California's Sierra Nevada, is also of this type. Such volcanoes are characterized by alternating periods of quiet and episodic eruptions that start with explosive gas expulsion with fine particles of glassy volcanic ash and spongy cinders, followed by a rebuilding phase with hot magma. The entire Pacific ocean boundary is surrounded by long stretches of volcanoes and is known collectively as The Ring of Fire. Where two continental plates collide the plates either crumple and compress or one plate burrows under or (potentially) overrides the other. Either action will create extensive mountain ranges. The most dramatic effect seen is where the northern margins of the Indian subcontinental plate is being thrust under a portion of the Eurasian plate, lifting it and creating the Himalaya. When two oceanic plates converge they form an island arc as one oceanic plate is subducted below the other. A good example of this type of plate convergence would be Japan.

Sources of plate motion

As noted above, the plates are able to move because of the relative weakness of the asthenosphere. Dissipation of heat from the mantle is acknowledged to be the source of energy driving plate tectonics. Somehow, this energy must be converted into force in order for the plates to move. There are essentially two forces that could be driving plate motion: friction and gravity. These are further subdivided below.

Friction

;Mantle drag : Convection currents in the mantle are transmitted through the asthenosphere; motion is driven by friction between the asthenosphere and the lithosphere. ;Trench suction : Local convection currents exert a downward frictional pull on plates in subduction zones at ocean trenches.

Gravity

;Ridge-push : Plate motion is driven by the higher elevation of plates at mid-ocean ridges. Essentially stuff slides downhill. The higher elevation is caused by the relatively low density of hot material upwelling in the mantle. The real motion producing force is the upwelling and the energy source that runs it. This is a misnomer as nothing is pushing and tensional features are dominant along ridges. Also, it is difficult to explain continental break-up with this. ;Slab-pull : Plate motion is driven by the weight of cold, dense plates sinking into the mantle at trenches. There is considerable evidence that convection is occurring in the mantle at some scale. The upwelling of material at mid-ocean ridges is almost certainly part of this convection. Some early models of plate tectonics envisioned the plates riding on top of convection cells like conveyor belts. However, most scientists working today believe that the asthenosphere is not strong enough to directly cause motion by friction. Slab pull is widely believed to be the strongest force directly operating on plates. Recent models indicate that trench suction plays an important role as well. The over-all driving force and its energy source are still debatable subjects of on-going research.

Major plates

Convection The main plates are
- African Plate, covering Africa
- Antarctic Plate, covering Antarctica
- Australian Plate, covering Australia
- Eurasian Plate covering Eurasia
- North American Plate covering North America and north-east Siberia
- South American Plate covering South America
- Pacific Plate, covering the Pacific Ocean Notable minor plates include the Indian Plate and the Arabian Plate. The movement of plates has caused the formation and breakup of continents over time, including occasional formation of a supercontinent that contains most or all of the continents. The supercontinent Rodinia is thought to have formed about 1000 million years ago and to have embodied most or all of Earth's continents, and broken up into eight continents around 600 million years ago. The eight continents later re-assembled into another supercontinent called Pangaea; Pangea eventually broke up into Laurasia (which became North America and Eurasia) and Gondwana (which became the remaining continents). ;Related article
- List of tectonic plates

History and impact

Continental drift

Continental drift was one of many ideas about tectonics proposed in the late 19th and early 20th century. The theory has been superseded by and the concepts and data have been incorporated within plate tectonics. By 1915 Alfred Wegener was making serious arguments for the idea with the first edition of The Origin of Continents and Oceans. In that book he noted how the east coast of South America and the west coast of Africa looked as if they were once attached. Wegener wasn't the first to note this (Francis Bacon, Benjamin Franklin and Snider-Pellegrini preceded him), but he was the first to marshal significant fossil and paleo-topographical and climatological evidence to support this simple observation. However, his ideas were not taken seriously by many geologists, who pointed out that there was no apparent mechanism for continental drift. Specifically they did not see how continental rock could plow through the much denser rock that makes up oceanic crust. In the early 1940s, Maurice Ewing seismically tested the Atlantic edge of the North American continental shelf, and found a granitic layer dropped down to the basaltic ocean floor. If the continent had been torn from Europe and was plowing through the ocean bottom, the edge of the continental shelf should have marked the end of granitic rocks. Later studies aboard the Atlantis found that ocean bottom was not smooth, which suggested it was much stronger than if continents could push it aside. Beginning in the 1950s, scientists, using magnetic instruments (magnetometers) adapted from airborne devices developed during World War II to detect submarines, began recognizing odd magnetic variations across the ocean floor. This finding, though unexpected, was not entirely surprising because it was known that basalt -- the iron-rich, volcanic rock making up the ocean floor-- contains a strongly magnetic mineral (magnetite) and can locally distort compass readings. This distortion was recognized by Icelandic mariners as early as the late 18th century. More important, because the presence of magnetite gives the basalt measurable magnetic properties, these newly discovered magnetic variations provided another means to study the deep ocean floor. When newly formed rock cools, such magnetic materials recorded the Earth's magnetic field at the time. As more and more of the seafloor was mapped during the 1950s, the magnetic variations turned out not to be random or isolated occurrences, but instead revealed recognizable patterns. When these magnetic patterns were mapped over a wide region, the ocean floor showed a zebra-like pattern. Alternating stripes of magnetically different rock were laid out in rows on either side of the mid-ocean ridge: one stripe with normal polarity and the adjoining stripe with reversed polarity. The overall pattern, defined by these alternating bands of normally and reversely polarized rock, became known as magnetic striping. When the rock strata of the tips of separate continents are very similar it suggests that these rocks were formed in the same way implying that they were joined initially. For instance, some parts of Scotland contain rocks very similar to those found in eastern North America. Furthermore, the Caledonian Mountains of Europe and parts of the Appalachian Mountains of North America are very similar in structure and lithology.

Floating continents

The prevailing concept was that there were static shells of strata under the continents. It was early observed that although granite existed on continents, seafloor seemed to be composed of denser basalt. It was apparent that a layer of basalt underlies continental rocks. However, based upon abnormalities in plumb line deflection by the Andes in Peru, Pierre Bouguer deduced that less-dense mountains must have a downward projection into the denser layer underneath. The concept that mountains had "roots" was confirmed by George B. Airy a hundred years later during study of Himalayan gravitation, and seismic studies detected corresponding density variations. By the mid-1950s the question remained unresolved of whether mountain roots were clenched in surrounding basalt or were floating like an iceberg.

Plate tectonic theory

Significant progress was made in the 1960s, and was prompted by a number of discoveries, most notably the Mid-Atlantic ridge. The most notable was the 1962 publication of a paper by American geologist Harry Hess. Hess suggested that instead of continents moving through oceanic crust (as was suggested by continental drift) that an ocean basin and its adjoining continent moved together on the same crustal unit, or plate. In 1967, Jason Morgan proposed that the Earth's surface consists of 12 rigid plates that move relative to each other. Two months later, in 1968, Xavier Le Pichon published a complete model based on 6 major plates with their relative motions.

Explanation of magnetic striping

Xavier Le Pichon The discovery of magnetic striping and the stripes being symmetrical around the crests of the mid-ocean ridges suggested a relationship. In 1961, scientists began to theorize that mid-ocean ridges mark structurally weak zones where the ocean floor was being ripped in two lengthwise along the ridge crest. New magma from deep within the Earth rises easily through these weak zones and eventually erupts along the crest of the ridges to create new oceanic crust. This process, later called seafloor spreading, operating over many millions of years has built the 50,000 km-long system of mid-ocean ridges. This hypothesis was supported by several lines of evidence: # at or near the crest of the ridge, the rocks are very young, and they become progressively older away from the ridge crest; # the youngest rocks at the ridge crest always have present-day (normal) polarity; # stripes of rock parallel to the ridge crest alternated in magnetic polarity (normal-reversed-normal, etc.), suggesting that the Earth's magnetic field has flip-flopped many times. By explaining both the zebralike magnetic striping and the construction of the mid-ocean ridge system, the seafloor spreading hypothesis quickly gained converts and represented another major advance in the development of the plate-tectonics theory. Furthermore, the oceanic crust now came to be appreciated as a natural "tape recording" of the history of the reversals in the Earth's magnetic field.

Subduction discovered

A profound consequence of seafloor spreading is that new crust was, and is now, being continually created along the oceanic ridges. This idea found great favor with some scientists who claimed that the shifting of the continents can be simply explained by a large increase in size of the Earth since its formation. However, this so-called "expanding Earth" hypothesis was unsatisfactory because its supporters could offer no convincing geologic mechanism to produce such a huge, sudden expansion. Most geologists believe that the Earth has changed little, if at all, in size since its formation 4.6 billion years ago, raising a key question: how can new crust be continuously added along the oceanic ridges without increasing the size of the Earth? This question particularly intrigued Harry Hess, a Princeton University geologist and a Naval Reserve Rear Admiral, and Robert S. Dietz, a scientist with the U.S. Coast and Geodetic Survey who first coined the term seafloor spreading. Dietz and Hess were among the small handful who really understood the broad implications of sea floor spreading. If the Earth's crust was expanding along the oceanic ridges, Hess reasoned, it must be shrinking elsewhere. He suggested that new oceanic crust continuously spread away from the ridges in a conveyor belt-like motion. Many millions of years later, the oceanic crust eventually descends into the oceanic trenches -- very deep, narrow canyons along the rim of the Pacific Ocean basin. According to Hess, the Atlantic Ocean was expanding while the Pacific Ocean was shrinking. As old oceanic crust was consumed in the trenches, new magma rose and erupted along the spreading ridges to form new crust. In effect, the ocean basins were perpetually being "recycled," with the creation of new crust and the destruction of old oceanic lithosphere occurring simultaneously. Thus, Hess' ideas neatly explained why the Earth does not get bigger with sea floor spreading, why there is so little sediment accumulation on the ocean floor, and why oceanic rocks are much younger than continental rocks.

Mapping with earthquakes

During the 20th century, improvements in seismic instrumentation and greater use of earthquake-recording instruments (seismographs) worldwide enabled scientists to learn that earthquakes tend to be concentrated in certain areas, most notably along the oceanic trenches and spreading ridges. By the late 1920s, seismologists were beginning to identify several prominent earthquake zones parallel to the trenches that typically were inclined 40-60° from the horizontal and extended several hundred kilometers into the Earth. These zones later became known as Wadati-Benioff zones, or simply Benioff zones, in honor of the seismologists who first recognized them, Kiyoo Wadati of Japan and Hugo Benioff of the United States. The study of global seismicity greatly advanced in the 1960s with the establishment of the Worldwide Standardized Seismograph Network (WWSSN) to monitor the compliance of the 1963 treaty banning above-ground testing of nuclear weapons. The much-improved data from the WWSSN instruments allowed seismologists to map precisely the zones of earthquake concentration worldwide.

Geological paradigm shift

The acceptance of the theories of continental drift and sea floor spreading (the two key elements of plate tectonics) can be compared to the Copernican revolution in astronomy (see Nicolaus Copernicus). Within a matter of only several years geophysics and geology in particular were revolutionized. The parallel is striking: just as pre-Copernican astronomy was highly descriptive but still unable to provide explanations for the motions of celestial objects, pre-tectonic plate geological theories described what was observed but struggled to provide any fundamental mechanisms. The problem lay in the question "How?". Before acceptance of plate tectonics, geology in particular was trapped in a "pre-Copernican" box. However, by comparison to astronomy the geological revolution was much more sudden. What had been rejected for decades by any respectable scientific journal was eagerly accepted within a few short years in the 1960s and 1970s. Any geological description before this had been highly descriptive. All the rocks were described and assorted reasons, sometimes in excruciating detail, were given for why they were where they are. The descriptions are still valid. The reasons, however, today sound much like pre-Copernican astronomy. One simply has to read the pre-plate descriptions of why the Alps or Himalaya exist to see the difference. In an attempt to answer "how" questions like "How can rocks that are clearly marine in origin exist thousands of meters above sea-level in the Dolomites?", or "How did the convex and concave margins of the Alpine chain form?", any true insight was hidden by complexity that boiled down to technical jargon without much fundamental insight as to the underlying mechanics. With plate tectonics answers quickly fell into place or a path to the answer became clear. Collisions of converging plates had the force to lift sea floor into thin atmospheres. The cause of marine trenches oddly placed just off island arcs or continents and their associated volcanoes became clear when the processes of subduction at converging plates were understood. Mysteries were no longer mysteries. Forests of complex and obtuse answers were swept away. Why were there striking parallels in the geology of parts of Africa and South America? Why did Africa and South America look strangely like two pieces that should fit to anyone having done a jigsaw puzzle? Look at some pre-tectonics explanations for complexity. For simplicity and one that explained a great deal more look at plate tectonics. A great rift, similar to the Great Rift Valley in northeastern Africa, had split apart a single continent, eventually forming the Atlantic Ocean, and the forces were still at work in the Mid-Atlantic Ridge. We have inherited some of the old terminology, but the underlying concept is as radical and simple as "The Earth moves" was in astronomy.

Plate tectonics on Mars

As a result of 1999 observations of the magnetic fields on Mars by the Mars Global Surveyor spacecraft, it has been proposed that the mechanisms of plate tectonics may once have been active on the planet - see Geology of Mars.

References

- Earth System History, Steven M. Stanley, (W.H. Freeman and Company; 1999) pages 211-228 ISBN 0-7167-2882-6
- Geographica: The complete illustrated Atlas of the world, Editors of James Mills-Hicks (Barnes and Noble Books; New York; 2004) ISBN 0-7607-5974-X
- Plate Tectonics : An Insider's History of the Modern Theory of the Earth, Oreskes, Naomi ed., Westview Press, 2003, ISBN 0813341329
- Krakatoa: The Day the World Exploded: August 27, 1883, Simon Winchester, (HarperCollins; 2003) Part 2-4 ISBN 0-0662-1285-5

External links

- U.S. Geological Survey Web Page Links
- [http://pubs.usgs.gov/publications/text/dynamic.html This Dynamic Earth] provides an excellent overview of the subject.
- [http://pubs.usgs.gov/publications/text/understanding.html Understanding plate motions]
- [http://pubs.usgs.gov/publications/text/slabs.html plate map]
- [http://pubs.usgs.gov/publications/text/Vigil.html Artist's cross section illustrating the main types of plate boundaries]
- [http://vulcan.wr.usgs.gov/Glossary/PlateTectonics/description_plate_tectonics.html "Ring of Fire", Plate Tectonics, Sea-Floor Spreading, Subduction Zones, "Hot Spots"]
- [http://pubs.usgs.gov/publications/text/Wilson.html J. Tuzo Wilson: Discovering transforms and hotspots]
- [http://vulcan.wr.usgs.gov/Glossary/PlateTectonics/Maps/map_plate_tectonics_world.html Active volcanoes]
- [http://sepwww.stanford.edu/oldsep/joe/fault_images/BayAreaSanAndreasFault.html San Andreas fault information]
- [http://www-sst.unil.ch/research/plate_tecto/links.htm Academic research] sites.
- [http://www.ucmp.berkeley.edu/geology/tecall1_4.mov Interactive movie] showing 750 myr (million years) of global tectonic activity.
- [http://www.ucmp.berkeley.edu/geology/tectonics.html More movies] over smaller regions and smaller time scales.
- [http://www.scotese.com/ The Paleomap Project:] numerous maps and movies.
- [http://www.uky.edu/ArtsSciences/Geology/webdogs/plates/reconstructions.html Web Dogs] tectonic reconstructions and interactive movies.
- [http://my.execpc.com/~acmelasr/mountains/wghnf.html Exceptionally detailed tectonic history] of Wisconsin.
- [http://www.windows.ucar.edu/tour/link=/earth/interior/how_plates_move.html Illustration of ridge-push and slab-pull].
- [http://www2.nature.nps.gov/geology/usgsnps/pltec/scplseqai.html Maps of the Earth back to 620 million years ago]
- [http://www.pbs.org/wgbh/aso/tryit/tectonics/ See what happens when you move tectonic plates] - An interactive guide
- [http://www.tectonic-forces.org The Origin and the Mechanics of the Forces Responsible for Tectonic Plate Movements]
- [http://www.sciencenews.org/pages/sn_arc99/5_1_99/bob2.htm Evidence (but not proof) for tectonics on Mars]
- [https://www3.imperial.ac.uk/portal/page?_pageid=46,73409&_dad=portallive&_schema=PORTALLIVE Plate Tectonics on Venus]"The mapping and interpretion of the regional tectonic features of Venus over the past ten years or so has led to a qualitative picture of buoyant plate tectonics."
- [http://www.platetectonics.com/ The Story of Plate Tectonics]
- [http://www.djburnette.com/projects/climate.html Plate Tectonics and Climate]
- [http://www.agiweb.org/earthcomm/resources/platetectonics.html Plate Tectonics...and Your Community] Category:Geophysics
- Category:Evolution Category:Geology ko:판구조론 ms:Plat tektonik ja:プレートテクトニクス

Tectonic plates

A Tectonic plate is a piece of the Earth's crust (or lithosphere). The surface of the Earth consists of seven major tectonic plates and many more minor ones. The plates are around 100 km (60 miles) thick and consist of two principal types of material: oceanic crust (also called sima from silicone and magnesium) and continental crust (sial from silicone and aluminium). Under both lies a relatively plastic layer of the Earth's mantle called the asthenosphere, which is in constant motion. This is in turn underlaid by a solid layer of mantle. The composition of the two types of crust differs markedly. Oceanic crust consists largely of basaltic rocks, while the continental crust consists principally of lower density granitic rocks rich in aluminium and silica. The two types of crust also differ in thickness, with continental crusts considerably thicker than oceanic. The churning of the asthenosphere carries the plates along in a process known as continental drift, which is explained by the theory of plate tectonics. Interaction between the plates creates mountains and volcanoes, as well as giving rise to earthquakes and other geological phenomena. The boundaries of the plates do not coincide with those of the continents. For instance, the North American Plate covers not only North America but also Greenland, far eastern Siberia and northern Japan. As far as is known, the Earth is the only planet in the Solar System to possess tectonic plates, although there have been suggestions that Mars may also have possessed plates in the past before the planet's crust froze in place.

External links

- [http://element.ess.ucla.edu/publications/2003_PB2002/2003_PB2002.htm Bird, P. (2003) An updated digital model of plate boundaries] also available as a large (13 mb) PDF file [http://element.ess.ucla.edu/publications/2003_PB2002/2001GC000252.pdf]
- [http://snobear.colorado.edu/Markw/Mountains/03/week3.html Map of tectonic plates] Category:Plate tectonics ja:プレート (地学)

Interplate earthquake

An interplate earthquake is an earthquake that occurs at the boundary between two tectonic plates. If one plate is trying to move past the other, they will be locked until sufficient stress builds up to cause the plates to slip relative to each other. The slipping process creates an earthquake with land deformations and resulting seismic waves which travel through the Earth and along the Earth's surface. Relative plate motion can be lateral as along a transform fault boundary or vertical if along a convergent subduction boundary or a rift at a divergent boundary. At a subduction boundary the motion is due to one plate slipping beneath the other plate resulting in an interplate thrust or megathrust earthquake. Some areas of the world that are particularly prone to such events include the west coast of North America (especially California and Alaska), the northeastern Mediterranean region (Greece, Italy, and Turkey in particular), Iran, New Zealand, Indonesia, Japan, and parts of China. The frequency (how often) of these large events may be controlled by the Gap Theory and Characteristic earthquakes.

Intraplate earthquake

Although the theory of plate tectonics well describes the mechanisms for interplate earthquakes (earthquakes at plate boundaries), very large intraplate earthquakes (earthquake within plates) can inflict heavy damage on towns and cities. With plate tectonics the world is modeled as a collection of 'dinner plates' sliding past each other on the giant table of the earth, these are, in fact, cracked dinner plates, under high stress. Nearly all the relative motion takes place at the edges of the plates, but there are still the 'creaks and groans of an ancient crust'. At times, motions along these interior weak zones produce rather large earthquakes. A series of famous intraplate earthquakes occurred on the New Madrid fault zone in 1812 that were above magnitude 8 and were felt for hundreds of miles. A similar large earthquake devastated the region of Gujarat, India, in 2001, resulting in a large loss of life. Many cities in North America and elsewhere live with the seismic risk of a rare, large intraplate earthquake. Historic examples of this occurred in Boston in 1755 (the largest U.S. earthquake ever recorded east of the New Madrid fault zone; some estimates put its magnitude as high as 7.0), New York City in 1737 and 1884 (both quakes estimated at about 5.5 magnitude) and Charleston, SC earthquake in 1886 (estimated magnitude 6.5 to 7.3). The Charleston quake was particularly surprising because unlike Boston and New York the area had almost no history of even minor earthquakes (to put in perspective, in addition to the three northeastern U.S. events previously mentioned, a more moderate magnitude 4 earthquake was recorded just north of New York City in 1985). Nobody is exactly sure what causes these earthquakes. In many cases, the causative fault is deeply buried, and sometimes cannot even be found. Under these circumstances it is difficult to calculate the exact seismic hazard for a given city, especially if there was only one earthquake in historical times. An especially dangerous form of earthquake, which has been involved in many deaths is the blind thrust earthquake, although this is more associated with interplate earthquakes. Some progress is being made in understanding the fault mechanics driving these earthquakes. Scientists continue to search for the causes of these earthquakes, and especially for some indication of when they will strike next. The best success has come with detailed micro-seismic monitoring, involving dense arrays of seismometers. In this manner, very small earthquakes associated with a causative fault can be located with great accuracy, and in most cases these line up in patterns consistent with faulting.

Compare

- Interplate earthquake

External link

A very nice explanation of Intraplate earthquakes http://www2.bc.edu/~kafka/Why_Quakes/why_quakes.html Category:Seismology

Volcanic

:Eruption redirects here. For other meanings of the word eruption, see eruption (disambiguation) A volcano is a geological landform (usually a mountain) where a substance, usually magma (rock of the Earth's interior made molten or liquid by extremely high temperatures along with a reduction in pressure and/or the introduction of water or other volatiles) erupts through the surface of a planet. Although there are numerous volcanoes (some very active) on the solar system's rocky planets and moons, on Earth at least, this phenomenon tends to occur near the boundaries of the continental plates. However, important exceptions exist in hotspot volcanoes. hotspot volcanoes.]] The name "volcano" originates from the name of Vulcan, a god of fire in Roman mythology. The study of volcanoes is called vulcanology (or volcanology in some spellings). Mud volcanoes are formations which are 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. This article describes igneous volcanoes.

Volcano classification

Erupted material

One way of classifying volcanoes is by the type of material erupted, which affects the shape of the volcano. If the erupting magma contains a high percentage (65%) of silica the lava is called felsic or acidic and tends to be highly viscous (not very fluid) and is pushed up in a blob that will solidify relatively quickly. Lassen Peak in California is an example. This type of volcano has a tendency to explode because it retains the volatiles or gases and easily plugs. Mount Pelée on the island of Martinique is another example. If, on the other hand, the magma contains a relatively low percentage of silica, the lava is called mafic or basic and will be very fluid as it erupts, capable of flowing for long distances. Due to the low viscosity the volatiles are able to escape. A good example of a mafic lava flow is the Great flow produced by an eruptive fissure almost in the geographical center of Iceland roughly 8,000 years ago; it flowed to the sea, a distance of 130 kilometers, and covered an area of 800 square km.

Explosivity

The behaviour of volcanoes range from rare collossally explosive events to common cases of long term, gradual and gentle flow of magma. The Volcanic Explosivity Index is an attempt to categorise these into clear types, with low VEI values corresponding to gentle flows and high VEIs indicating a cataclysmic event with severe global consequences.

Shape

Shield volcanoes

Hawaii and Iceland are examples of places where volcanoes extrude huge quantities of lava that gradually build a wide mountain with a shield-like profile. Their lava flows are generally very hot and very fluid, contributing to long flows. The largest lava shield on Earth, Mauna Loa, is 9,000 m tall (it sits on the sea floor), 120 km in diameter and forms part of the Island of Hawai. Olympus Mons is a shield volcano on Mars, and the tallest mountain in the known solar system. Smaller versions of the "lava shield" include the 'lava dome' (tholoid), 'lava cone', and 'lava mound'. Volcanic cones or cinder cones result from eruptions that throw out mostly small pieces of rock that build up around the vent. These can be relatively short-lived eruptions that produce a cone-shaped hill perhaps 30 to 300 m high.

Stratovolcanoes or composite volcanoes

These are tall conical mountains composed of both lava flows and ejected material, which form the strata that give rise to the name. Classic examples include Mt. Fuji in Japan and Mount Mayon in the Philippines. Volcanoes on land often take the form of flat cones, as the expulsions build up over the years, or in short-lived volcanic cones, cinder cones.

Supervolcanoes

Supervolcano is the popular term for large volcanoes that usually have a large caldera and can potentially produce devastation on a continental scale and cause major global weather pattern changes. Potential candidates include Yellowstone National Park and Lake Toba, but are hard to identify given that there is no formal definition of the term.

Submarine volcanoes

Submarine volcanoes are common features on certain zones of the ocean floor. Some are active at the present time and, in shallow water, disclose their presence by blasting steam and rock-debris high above the surface of the sea. Many others lie at such great depths that the tremendous weight of the water above them results in high, confining pressure and prevents the formation and explosive release of steam and gases. Even very large, deepwater eruptions may not disturb the ocean surface. Under water, volcanoes often form rather steep pillars and in due time break the ocean surface in new islands.

Active, Dormant, or Extinct?

Supervolcano Volcanoes are usually situated either at the boundaries between tectonic plates or over geology hotspots. Volcanoes may be either dormant (having no activity) or active (near constant expulsion and occasional eruptions), and change state unpredictably. Surprisingly, there is no 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 activity. Given the long lifespan of such volcanoes, they are very active. By our lifespans, however, they are not. Complicating the definition are volcanoes that become restless but do not actually erupt. Are these volcanoes active? Scientists usually consider a volcano active if it is currently erupting or showing signs of unrest, such as unusual earthquake activity or significant new gas emissions. Many scientists also consider a volcano active if it has erupted in historic time. It is important to note that the span of recorded history differs from region to region; in the Mediterranean, recorded history reaches back more than 3,000 years but in the Pacific Northwest of the United States, it reaches back less than 300 years, and in Hawaii, little more than 200 years. Dormant volcanoes are those that are not currently active (as defined above), but could become restless or erupt again. Extinct volcanoes are those that scientists consider unlikely to erupt again. Whether a volcano is truly extinct is often difficult to determine. Since calderas have 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. For example, the Yellowstone Caldera (considered a Supervolcano) in Yellowstone National Park is at least 2 million years old and hasn't erupted violently for approximately 640,000 years — although there has been some minor activity as relatively recent as 70,000 years ago. For this reason, scientists do not consider the Yellowstone Caldera as extinct. In fact, because the caldera has frequent earthquakes, a very active geothermal system (i.e., the entirety of the geothermal activity found in Yellowstone National Park), and rapid rates of ground uplift, many scientists consider it to be a very active volcano.

Notable Volcanoes

Volcanoes on Earth

:Main article: List of volcanoes List of volcanoes
- Mount Baker (Washington, USA)
- Cold Bay Volcano (Alaska, USA)
- El Chichon/El Chichonal, (Chiapas, Mexico)
- Citlaltépetl/Pico de Orizaba, (Veracruz/Puebla, Mexico)
- Cotopaxi (Ecuador)
- Mount Fuji (Honshu, Japan)
- Mount Hood (Oregon, USA)
- Mount Erebus (Ross Island, Antarctica)
- Etna (Sicily, Italy)
- Krafla (Iceland)
- Hekla (Iceland)
- Kick-'em-Jenny, (Grenada)
- Kilauea (Hawaii, USA)
- Kluchevskaya (Kamchatka, Russia)
- Krakatoa (Rakata, Indonesia)
- Mauna Kea (Hawaii, USA)
- Mauna Loa (Hawaii, USA)
- El Misti (Arequipa, Peru)
- Novarupta (Alaska, USA)
- Paricutín (Michoacán, Mexico)
- Mount Pinatubo (Luzon Island, Philippines)
- Popocatépetl (Mexico-Puebla state line, Mexico)
- Santorini (Santorini islands, Greece)
- Soufriere Hills volcano, (Montserrat)
- Stromboli (Aeolian Islands, Italy)
- Mount Rainier (Washington, USA)
- Mount Shasta (California, USA)
- Mount St. Helens (Washington, USA)
- Surtsey (Iceland)
- Tambora (Sumbawa, Indonesia)
- Teide (Tenerife, Canary Islands, Spain)
- White Island (Bay of Plenty, New Zealand)
- Mount Vesuvius (Bay of Naples, Italy)

Volcanoes elsewhere in the solar system

Italy, "Mount Olympus") is the tallest known mountain in our solar system, located on the planet Mars.]] The Earth's Moon has no large volcanoes, but does have many volcanic features such as rilles and domes. The planet Venus is believed to be volcanically active, and its surface is 90% basalt, indicating that volcanism plays a major role in shaping its surface. Lava flows are widespread and many of its surface features are attributed to exotic forms of volcanism not present on Earth. Other Venusian phenomena, such as changes in the planet's atmosphere and observations of lightning, have been attributed to ongoing volcanic eruptions. There are several extinct volcanoes on Mars, four of which are vast shield volcanoes far bigger than any on Earth:
- Arsia Mons
- Ascraeus Mons
- Hecates Tholus
- Olympus Mons
- Pavonis Mons These volcanoes have been extinct for many millions of years, but the European Mars Express spacecraft has found evidence that volcanic activity may have occurred on Mars in the recent past as well. Jupiter's moon Io is the most volcanic object in the solar system, due to tidal interaction with Jupiter. It is covered with volcanoes that erupt sulfur, sulfur dioxide and silicate rock, with the result that the moon is constantly being resurfaced. Its lavas are the hottest known anywhere in the solar system, with temperatures exceeding 1800 K (1500 °C). In February 2001, the largest recorded volcanic eruptions in the solar system occurred on Io [http://www2.keck.hawaii.edu/news/archive/eruption/]. See the list of geological features on Io for a list of named volcanoes on the moon. list of geological features on Io In 1989 the Voyager 2 spacecraft observed ice volcanoes (cryovolcanism) on Triton, a moon of Neptune and in 2005 the Cassini-Huygens probe photographed fountains of frozen particles erupting from Saturn's moon Enceladus. The ejecta are believed to consist of 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. [http://www.newscientist.com/article.ns?id=dn7489] It is theorized that cryovolcanism may also be present on the Kuiper Belt Object Quaoar.

Volcanology

Volcano formation

Quaoar Like most of the interior of the earth, the movements and dynamics of magma are poorly understood. However, it is known that an eruption usually follows movement of magma upwards into the solid layer (the earth's crust) beneath a volcano and occupying a magma chamber. Eventually, magma in the chamber is forced upwards and flows out across the planet surface as lava, or the rising magma can heat water in the surrounding landform and cause explosive discharges of steam; either this or escaping gases from the magma can produce forceful ejections of rocks, cinders, volcanic glass, and/or volcanic ash also known as tephra. While always displaying powerful forces, eruptions can vary from effusive to extremely explosive. Most volcanoes on the land are formed at destructive plate margins: where oceanic crust is forced below the continental crust because oceanic crust is denser than continental crust. Friction between these moving plates will cause the oceanic crust to melt, and reduced density will force the newly formed magma to rise. As the magma rises through weak areas in the continental crust it may eventually erupt as one or more volcanoes. For example, Mount St. Helens is found inland from the margin between the oceanic Juan de Fuca Plate and the continental North American Plate. North American Plate A volcano generally presents itself to the imagination as a mountain sending forth from its summit great clouds of smoke with vast sheets of flame. The truth is that a volcano seldom emits either smoke or flame, although various combinations of hydrogen, carbon, oxygen, and sulfur do sometimes ignite. What is mistaken for smoke consists of vast volumes of fine dust, mingled with steam and other vapors, chiefly sulfurous. Most of what appears to be flames is the glare from the erupting materials, glowing because of their high temperature; this glare reflects off the clouds of dust and steam, resembling fire. Perhaps the most conspicuous part of a volcano is the crater, a basin of a roughly circular form within which occurs a vent (or vents) from which magma erupts as gases, lava, and ejecta. A crater can be of large dimensions, and sometimes of vast depth. Very large features of this sort are termed calderas. Some volcanoes consist of a crater alone, with scarcely any mountain at all; but in the majority of cases the crater is situated on top of a mountain (the volcano), which can tower to an enormous height. Volcanoes that terminate in a principal crater are usually of a conical form. Volcanic cones are usually smaller features composed of loose ash and cinder, with occasional masses of stone which have been tossed violently into the air by the eruptive forces (and are thus called ejecta). Within the crater of a volcano there may be numerous cones from which vapours are continually issuing, with occasional volleys of ashes and stones. In some volcanoes these cones form lower down the mountain, along rift zones or fractures. When the cone is eroded these rifts or lava filled fractures remain as radial near vertical dikes of volcanic rock. For example the radiating dikes at Shiprock in NW New Mexico.

Tectonic environments of volcanoes

Volcanoes can principally be found in three tectonic environments. New Mexico

Constructive plate margins

These are by far the most common volcanoes on the Earth. They are also the least frequently seen, because most of their activity takes place beneath the surface of the oceans. Along the whole of the oceanic ridge system are irregularly spaced surface eruptions, and more frequent sub-surface intrusions without surface expression. The large majority of these are only known about at surface because of earthquakes as part of the eruptions/ intrusions, or occasionally if passing shipping happens to notice unusually high water temperatures or chemical precipitates in the seawater. In a few places oceanic ridge activity has lead to the volcanoes coming up to the surface - Saint Helena and Tristan da Cunha in the Atlantic Ocean; the Galapagos Islands in the Pacific Ocean, allowing them to be studied in some detail. But most activity takes place in considerable water depths. Iceland is also on a ridge, but has different characteristics than a simple volcano. It could be argued that the volcanoes of the Great Rift Valley system of East Africa are modified constructive margin volcanoes. However the modifications caused by the presence of thick continental crust are very substantial, and the magmas produced are very different from the typically very homogenous MORB (Mid-Ocean Ridge Basalt) that makes up the huge majority of constructive margin volcanoes.

Destructive plate margins

These are the most visible and well-known types of volcanoes on earth, forming above the subduction zones where (oceanic) plates dive into the Earth to their destruction. Their magmas are typically "calc-alkaline" as a result of their origins in the upper parts of altered ocean plate materials, mixed with sediments, and processed through variable thicknesses of more-or-less continental crust. The heavier plate sinks under the lighter one and the friction from the melting plate causes magma to force it's way out through a crack in the crust. Unsurprisingly, their compositions are much more varied than at constructive margins.

Hotspot situations

subduction zones, Iceland]] Hotspots were originally a catch-all for volcanoes that didn't fit into one of the above two categories, but these days this refers to a more specific circumstance - where an isolated plume of hot mantle material intersects the underside of crust (oceanic or continental), leading to a volcanic center that is not obviously connected with a plate margin. The classic example is the Hawaiian chain of volcanoes and seamounts; Yellowstone is cited as another classic example, in this case the intersection is with the underside of continental crust. Iceland is sometimes cited as yet a third classical example, but complicated by the coincidence of a hotspot intersecting an oceanic ridge constructive margin. There are debates about the simple "hotspot" concept, since theorists cannot agree on whether the "hot mantle plumes" originate in the upper mantle or in the lower mantle. Meanwhile, field geologists and petrologists see considerable variation in the detailed chemistry of one hotspot's magmas versus a second hotspot's magmas. On the third hand, high-resolution seismology of different hotspots is yielding different pictures of the deep sub-structure of Hawaii versus Iceland. There is no detailed consensus about how to interpret these varied results, and it seems plausible that eventually several different sub-types of hotspots will be identified.

Predicting eruptions

Science has not yet been able to predict with absolute certainty when a volcanic eruption will take place, but significant progress in judging when one is probable has been made in recent time. Iceland, 1980 at 8:32 a.m. PDT]] Volcanologists use the following to forecast eruptions.

Seismicity

Seismic activity (small earthquakes and tremors) always occurs as volcanoes awaken and prepare to erupt. Some volcanoes normally have continuing low-level seismic activity, but an increase can signify an eruption. The types of earthquakes that occur and where they start and end are also key signs. Volcanic seismicity has three major forms: short-period earthquakes, long-period earthquakes, and harmonic tremor.
- Short-period earthquakes are like normal fault-related earthquakes. They are related to the fracturing of brittle rock as the magma forces its way upward. These short-period earthquakes signify the growth of a magma body near the surface.
- Long-period earthquakes are believed to indicate increased gas pressure in a volcano's "plumbing system." They are similar to the clanging sometimes heard in your home's plumbing system. These oscillations are the equivalent of acoustic vibrations in a chamber, in the context of magma chambers within the volcanic dome. Patterns of seismicity are complex and often difficult to interpret. However, increasing activity is very worrisome, especially if long-period events become dominant and episodes of harmonic tremor appear. In December 2000, scientists at the National Center for Prevention of Disasters in Mexico City predicted an eruption within two days from Popocatépetl, on the outskirts of Mexico City. Their prediction used research done by Dr. Bernard Chouet, a Swiss vulacanologist working at the United States Geological Survey, into increasing long-period oscillations as an indicator of an imminent eruption. The government evacuated tens of thousands of people. Forty eight hours later, bang on time, the volcano erupted spectacularly. It was Popocatépetl's largest eruption for a thousand years and yet no one was hurt.

Gas emissions

United States Geological Survey As magma nears the surface and its pressure decreases, gases escape. This process is much like what happens when you open a bottle of soda and carbon dioxide escapes. Sulfur dioxide is one of the main components of volcanic gases, and increasing amounts of it herald the arrival of more and more magma near the surface. For example, on May 13, 1991, 500 tonnes of sulfur dioxide were released from Mount Pinatubo in the Philippines. On May 28, just two weeks later, sulfur dioxide emissions had increased to 5,000 tonnes, ten times the earlier amount. Mount Pinatubo erupted on June 12, 1991. On several occasions, such as before the Mount Pinatubo eruption, sulfur dioxide emissions have dropped to low levels prior to eruptions. Most scientists believe that this drop in gas levels is caused by the sealing of gas passages by hardened magma. Such an event leads to increased pressure in the volcano's plumbing system and an increased chance of an explosive eruption.

Ground deformation

Swelling of the volcano signals that magma has accumulated near the surface. Scientists monitoring an active volcano will often measure the tilt of the slope and track changes in the rate of swelling. An increased rate of swelling, especially if accompanied by an increase in sulfur dioxide emissions and harmonic tremors is a high probability sign of an impending event.

Effects of volcanoes

There are many different kinds of volcanic activity and eruptions:
- phreatic eruptions (steam)
- explosive eruption of high-silica lava (e.g., rhyolite)
- effusive eruption of low-silica lava (e.g., basalt)
- pyroclastic flows
- lahars (debris flow)
- carbon dioxide emission All of these activities can pose a hazard to humans. Volcanic activity is often accompanied by earthquakes, hot springs, fumaroles, mud pots and geysers. Low-magnitude earthquakes often precede eruptions. 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, and volatile metal chlorides. carbon monoxide carbon monoxide carbon monoxide 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 10-20 miles 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 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. 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. Gas emissions from volcanoes are a natural contributor to acid rain. Volcanic activity now releases about 130 to 230 teragrams (145 million to 255 million short tons) of carbon dioxide each year. Volcanic eruptions may inject an aerosol of particles and chemicals in the Earth's atmosphere. Large injections may have visual effects and affect global climate through climate forcing.

Past beliefs

Before it was understood that most of the Earth's interior is molten, various explanations existed for volcano behavior. 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. Jesuit Athanasius Kircher (1602-1680), witnessed eruptions of Aetna 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. coal

References

- Macdonald, Gordon A., and Agatin T. Abbott. (1970). Volcanoes in the Sea. University of Hawaii Press, Honolulu. 441 p.
- Ollier, Cliff. (1988). Volcanoes. Basil Blackwell, Oxford, UK, ISBN 0-631-15664-X (hardback), ISBN 0-631-15977-0 (paperback).

External links

- [http://volcanoes.usgs.gov/Products/Pglossary/pglossary.html Glossary of Volcanic Terms from USGS]
- [http://volcano.und.nodak.edu/vwdocs/glossary.html Volcanic and Geologic Terms] from [http://volcano.und.nodak.edu/ Volcano World]
- [http://news.bbc.co.uk/1/hi/sci/tech/3183047.stm Television program (BBC) on the prediction of Popocatepetl's 2000 eruption]
- [http://www.volcano.si.edu Smithsonian Global Volcanism Program]
- [http://www.geology.sdsu.edu/how_volcanoes_work Explore the geologic causes of an eruption]
- [http://science.howstuffworks.com/volcano.htm/printable How Volcanoes Work by Tom Harris]
- [http://www.geology.sdsu.edu/how_volcanoes_work/ How Volcanoes Work] - Educational resource on the science and processes behind volcanoes, intended for university students of geology, volcanology and teachers of earth science.
- [http://www.geonet.org.nz/volcanocam.html Volcano Cam Geonet's live pictures of 4 of New Zealand's volcanoes]
- [http://facweb.bhc.edu/academics/science/harwoodr/GEOL101/Labs/VolcanicMaterials/ Volcanic Materials Identification] Category:Landforms Category:Plate tectonics Category:Volcanology
- Category:Geological hazards Category:Climate forcing agents als:Vulkanismus ms:Gunung berapi ja:火山 simple:Volcano th:ภูเขาไฟ

Humans

Humans or human beings define themselves in biological, social, and spiritual terms. Biologically, humans are classified as the species Homo sapiens (Latin for "wise man" or "thinking man"): a bipedal primate of the superfamily Hominoidea, together with the other apes: chimpanzees, gorillas, orangutans, and gibbons. Humans have an erect body carriage that frees their upper limbs for manipulating

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