Volcano
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Smoking Bromo and Semeru (background) volcanoes on Java in Indonesia. |
A
volcano is a
geological landform usually generated by the eruption through a vent in a planet's surface of
magma, molten rock welling up from the planet's interior. Volcanoes of various types are found on other planets and their moons as well as on earth. Roughly defined, a volcano consists of a
magma chamber, pipes and vents. The magma chamber is where magma from deep within the planet pools, while pipes are channels that lead to surface vents, openings in the volcano's surface through which
lava is ejected during an eruption.
Though the common perception of a volcano as
a mountain spewing lava and poisonous gases from a crater in its top is not wrong per se, the features of volcanoes are much more complicated and vary from volcano to volcano depending on a number of factors. Some volcanoes even 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 broken 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
Puu Ōō on a flank of
Hawaii's
Kīlauea.
Other types of volcanoes include
ice volcanoes (particularly on some moons of
Jupiter,
Saturn and
Neptune) and
mud volcanoes.
Mud volcanoes 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. On
Earth, volcanoes tend to occur near the boundaries of
crustal plates. Important exceptions exist in
hotspot volcanoes, which occur at locations far from plate boundaries; hotspot volcanoes are also found elsewhere in the
solar system, especially on its rocky planets and moons.
A popular way of classifying magmatic volcanoes goes 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. More significant ones refer to a particular volcano's formative and eruptive processes and resulting shapes; these and other details are explained below.
Volcano is thought to derive from
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. The study of volcanoes is called
volcanology, sometimes spelled
vulcanology.
The Roman name for the island
Vulcano has contributed the word for
volcano in most modern European languages.
Erupted material
Lava composition
One 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 (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 silicious 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).
*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 typically less viscous than rhyolitic lavas, depending on their eruption
temperature. These 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.
*If the erupted magma contains <=45% silica, the lava is called
ultramafic. Ultramafic flows are very rare and are thought to be even more fluid than common mafic lavas.
Lava texture
Two types of lava are erupted according to the surface texture: Aa (pronounced
IPA ) and
pāhoehoe (pronounced ), both words having
Hawaiian origins. Aa is characterized by a rough, clinkery surface and is what most viscous and hot lava flows look like. However, even basaltic or mafic flows can be erupted as aa flows, particularly if the eruption rate is high and the slope is steep.
Pāhoehoe is characterized by its smooth and often ropy 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 makeup to allow them to flow at a higher fluidity.
Shape
Shield volcanoes
Main article: Shield volcano
Hawaii and
Iceland are examples of places where volcanoes extrude huge quantities of basaltic
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, rises over 9,000 m from the ocean floor, is 120 km in diameter and forms part of the Big Island of Hawaii.
Olympus Mons is the largest shield volcano on
Mars, and is the tallest mountain in the known
solar system. Smaller versions of shield volcanoes include
lava cones, and
lava mounds.
Quiet eruptions spread out basaltic lava in flat layers. The buildup of these layers form a broad volcano with gently sloping sides called a shield volcano. Examples of shield volcanoes are the Hawaiian Islands.
Cinder cones
Volcanic cones or
cinder cones result from eruptions that throw out 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 m high. Most cinder cones erupt only once. Cinder cones may form as flank vents on larger volcanoes, or occur on their own.
Paricutín in
Mexico and
Sunset Crater in
Arizona are examples of cinder cones.
Stratovolcanoes
These 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. Classic examples include
Mt. Fuji in Japan,
Mount Mayon in the Philippines, and
Mount Vesuvius and
Stromboli in Italy.
Supervolcanoes
Supervolcano is the popular term for large volcanoes that usually have 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 can be the most dangerous type of volcano. Examples include
Yellowstone Caldera in
Yellowstone National Park,
Lake Taupo in
New Zealand and
Lake Toba in
Sumatra,
Indonesia. Supervolcanoes are hard to identify given their enormous areas covered.
Large igneous provinces are also considered supervolcanoes because of the vast amount of
basalt lava erupted.
Submarine volcanoes
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 due to volcanic gases. Even large submarine eruptions may not disturb the ocean surface. Submarine volcanoes often form rather steep pillars and in due time, may break the ocean surface as new islands.
Pillow lava is a common eruptive product of submarine volcanoes.
Origin
Up until July 2006, there were three
tectonic settings identified in which a volcano could be formed. The first setting is a divergent plate boundary. The
Earth crust is thin and magma can easily flow out. A well-known example of this type of volcano is the
Hekla in
Iceland, located on the
Mid-Atlantic Ridge. The second setting is on a convergent plate boundary, where one tectonic plate is submerged under another. Due to the high friction, the rock melts and magma is formed.
Mount Etna and
Mount Vesuvius are typical for this category. The last setting was thought to be caused by large vertical movements of the
Earth mantle, the so-called
mantle plumes. These places are called
hotspots. Because the tectonic plates move whereas the mantle plume remains in the same place, each volcano becomes extinct after a while and a new volcano is then being formed. The
Hawaiian Islands are thought to be formed in such a manner.
In July 2006, a new type of volcano was discovered
[N.Hirano et al., "Volcanism in Response to Plate Flexure", Science, July 27, 2006.] which was called a
petitspot, to accentuate the difference with volcanoes formed by mantle plumes. The newly discovered volcanoes do not fit in one of the above-mentioned categories, since they are located far from the plate boundary, but are too small to be the result of a mantle plume. The new theory suggests that submergence of tectonic plates causes stress all over the plate, which causes the plate to crack in some places. However, other scientists believe the mantle plume theory to be incorrect, and consider this discovery a confirmation of their ideas
[M.K. McNutt, "Another Nail in the Plume Coffin?", Science, July 27, 2006.].
Subglacial volcanoes
Subglacial volcanoes develop underneath icecaps. They are made up of flat lava flows atop extensive pillow lavas and palagonite. When the icecap melts, the lavas on the top collapse leaving a flat-topped mountain. Then, the pillow lavas also collapse, giving an angle of 37.5 degrees. Very good examples of this can be seen in Iceland. These volcanoes are also called table volcanoes or mobergs.
Classifying volcanic activity
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A volcanic eruption can be devastating for the local wildlife and human population |
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Mount St. Helens shortly after the eruption of May 18, 1980 |
Volcanoes are usually situated either near the boundaries between tectonic plates or over geologically active
hotspots. Volcanoes may be either dormant (having no activity) or active (currently erupting) or extinct (no longer active at all).
Surprisingly, 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 our lifespans, however, they are not. Complicating the definition are volcanoes that become restless (producing earthquakes, venting gasses, or other non-eruptive activities) but do not actually erupt.
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. The Smithsonian Global Volcanism Program's definition of 'active' is having erupted within the last 10,000 years.
Dormant volcanoes are those that are not currently active (as defined above), but could become restless or erupt again. Confusion however, can arise because many volcanoes which scientists consider to be
active are referred to as
dormant by laypersons or in the media.
Extinct volcanoes are those that scientists consider unlikely to erupt again. 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.
For example, the
Yellowstone Caldera 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 relatively recently, with hydrothermal eruptions less than 10,000 years ago and lava flows about 70,000 years ago. For this reason, scientists do not consider the Yellowstone Caldera 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 an active volcano.
Volcanoes on Earth
Main article: List of volcanoes
The 16 current
Decade Volcanoes are::
*Avachinsky-Koryaksky, Kamchatka, Russia
*Colima, Mexico
*Mount Etna, Italy
*Galeras, Colombia
*Mauna Loa, Hawaii, USA
*Merapi, Indonesia
*Nyiragongo, Democratic Republic of the Congo
*Mount Rainier, Washington, USA |
*Sakurajima, Japan
*Santamaria/Santiaguito, Guatemala
*Santorini, Greece
*Taal Volcano, Philippines
*Teide, Canary Islands, Spain
*Ulawun, Papua New Guinea
*Mount Unzen, Japan
*Vesuvius, Italy |
Volcanoes elsewhere in the solar system
The Earth's
Moon has no large volcanoes, but 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, 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.
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, 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 volcanically active object in the solar system, due to
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
[Exceptionally Bright Eruption on lo Rivals Largest in Solar System, Nov. 13, 2002].
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.
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Ice volcanoes on Enceladus |
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 [PPARC, Cassini Finds an Atmosphere on Saturn's Moon Enceladus]. 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.
[NewScientist, Hydrocarbon volcano discovered on Titan, 8th June 2005 ] It is theorized that cryovolcanism may also be present on the
Kuiper Belt Object Quaoar.
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Solar radiation reduction due to volcanic eruptions |
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Sulfur dioxide emissions by volcanoes. |
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Average concentration of sulfur dioxide over the Sierra Negra Volcano (Galapagos Islands) from October 23-November 1, 2005 |
There are many different kinds of volcanic activity and eruptions:
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. 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.
Large, explosive volcanic eruptions inject water vapor (H
2O), carbon dioxide (CO
2), sulfur dioxide (SO
2), 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 (H
2SO
4), 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. 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 (O
3). 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 releases about 130 to 230 teragrams (145 million to 255 million
short tons) of
carbon dioxide each year. 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 dries on the water.
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Kircher's model of the Earth's internal fires, from Mundus Subterraneus |
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. Many ancient accounts claim that
divine intervention was the actual cause of volcanic eruptions.
One early idea counter to this, however, was
Jesuit Athanasius Kircher (1602-1680), who 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.
*
Volcanology (The science of volcanoes)
*
History of Volcanology*
Plinian eruption*
:Category:Volcanic eruption types*
Predicting Volcanoes*
Geomorphology*
Earth science *
Volcanic gas*
Magma*
Lava*
Tsunami*
LaharLists*
List of volcanoes*
List of famous volcanic eruption deaths*
Volcanic Explosivity Index (includes list of large eruptions)
*
List of deadliest natural disastersSpecific locations*
Iceland hotspot*
Pacific Ring of Fire*
Io (moon)*
Triton (moon)People*
Haroun Tazieff (famous volcanologist)
* 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).
*
Haraldur Sigurðsson, ed. (1999)
Encyclopedia of Volcanoes. Academic Press. ISBN 012643140X. This is a reference aimed at geologists, but many articles are accessible to non-professionals.
* Cas, R.A.F. and J.V. Wright, 1987.
Volcanic Successions. Unwin Hyman Inc. 528p. ISBN 0-04-552022-4
*
Platial map of 253 volcanoes--takes a minute to load all the markers
*
Glossary of Volcanic Terms from USGS*
Volcanic and Geologic Terms from
Volcano World *
Television program (BBC) on the prediction of Popocatepetl's 2000 eruption*
Smithsonian Global Volcanism Program*
Explore the geologic causes of an eruption*
How Volcanoes Work by Tom Harris*
How Volcanoes Work - Educational resource on the science and processes behind volcanoes, intended for university students of geology, volcanology and teachers of earth science.
*
Volcano Cam Geonet's live pictures of 4 of New Zealand's volcanoes*
Indonesian Volcanoes Discover some of the larger Indonesian volcanos.
*
Volcanic Materials Identification*
Erupting Volcano - Pyroclastic Flow - Video
*
Natural Disasters - Volcano Great research site for kids.]
*
Volcano Live - John Seach*
Plants predict where rumbling volcanoes will blow*
Google Video: Erupting Volcano
