A
volcano is an opening, or
rupture, in a planet's surface or
crust, which allows hot
magma,
volcanic ash and gases to escape from below the surface.
Volcanoes are generally found where
tectonic plates are
diverging or
converging. A
mid-oceanic ridge, for example the
Mid-Atlantic Ridge, has examples of volcanoes caused by
divergent tectonic plates pulling apart; the
Pacific Ring of Fire has examples of volcanoes caused by
convergent tectonic plates coming together. By contrast, volcanoes are usually not created where two tectonic plates slide past one another. Volcanoes can also form where there is stretching and thinning of the
Earth's crust in the interiors of plates, e.g., in the
East African Rift, the
Wells Gray-Clearwater volcanic field and the
Rio Grande Rift in North America. This type of volcanism falls under the umbrella of "Plate hypothesis" volcanism.
[1] Volcanism away from plate boundaries has also been explained as
mantle plumes. These so-called "
hotspots", for example
Hawaii, are postulated to arise from upwelling
diapirs with magma from the
core-mantle boundary, 3,000 km deep in the Earth.
Erupting volcanoes can pose many hazards, not only in the immediate vicinity of the eruption. Volcanic ash can be a threat to
aircraft, in particular those with
jet engines where ash particles can be melted by the high operating temperature. Large eruptions can affect temperature as ash and droplets of
sulfuric acid obscure the
sun and cool the Earth's lower atmosphere or
troposphere; however, they also absorb heat radiated up from the Earth, thereby warming the
stratosphere. Historically, so-called
volcanic winters have caused catastrophic
famines.
Etymology
Plate tectonics
Map showing the divergent plate boundaries (OSR – Oceanic Spreading Ridges) and recent sub aerial volcanoes.
Divergent plate boundaries
At the
mid-oceanic ridges, two
tectonic plates diverge from one another. New
oceanic crust is being formed by hot molten rock slowly cooling and solidifying. The crust is very thin at mid-oceanic ridges due to the pull of the tectonic plates. The release of pressure due to the thinning of the crust leads to
adiabatic expansion, and the partial melting of the
mantle causing volcanism and creating new oceanic crust. Most
divergent plate boundaries are at the bottom of the oceans, therefore most volcanic activity is submarine, forming new seafloor.
Black smokers or deep sea vents are an example of this kind of volcanic activity. Where the mid-oceanic ridge is above sea-level, volcanic islands are formed, for example,
Iceland.
Convergent plate boundaries
Subduction zones are places where two plates, usually an oceanic plate and a continental plate, collide. In this case, the oceanic plate subducts, or submerges under the continental plate forming a deep ocean trench just offshore. Water released from the subducting plate lowers the melting temperature of the overlying mantle wedge, creating
magma. This magma tends to be very
viscous due to its high
silica content, so often does not reach the surface and cools at depth. When it does reach the surface, a volcano is formed. Typical examples for this kind of volcano are
Mount Etna and the volcanoes in the
Pacific Ring of Fire.
"Hotspots"
"
Hotspots" is the name given to volcanic provinces postulated to be formed by
mantle plumes. These are postulated to comprise columns of hot material that rise from the core-mantle boundary. They are suggested to be hot, causing large-volume melting, and to be fixed in space. Because the tectonic plates move across them, each volcano becomes dormant after a while and a new volcano is then formed as the plate shifts over the postulated plume. The
Hawaiian Islands have been suggested to have been formed in such a manner, as well as the
Snake River Plain, with the
Yellowstone Calderabeing the part of the North American plate currently above the hot spot. This theory is currently under criticism, however.
[1]Volcanic features
Lakagigar fissure vent in
Iceland, source of the major world climate alteration of 1783–84.
The most common perception of a volcano is of a
conical mountain, spewing
lava and poisonous
gases from a
crater at its summit. This describes just one of many types of volcano, and the features of volcanoes are much more complicated. The structure and behavior of volcanoes depends on a number of factors. Some volcanoes have rugged peaks formed by
lava domes rather than a summit crater, whereas others present
landscape features such as massive
plateaus. Vents that issue volcanic material (lava, which is what magma is called once it has escaped to the surface, and
ash) and gases (mainly
steam and magmatic gases) can be located anywhere on the
landform. Many of these vents give rise to smaller cones such as
Puʻu ʻŌʻō on a flank of
Hawaii's
Kīlauea. Other types of volcano include
cryovolcanoes (or ice volcanoes), particularly on some moons of
Jupiter,
Saturn and
Neptune; and
mud volcanoes, which are formations often not associated with known magmatic activity. Active mud volcanoes tend to involve temperatures much lower than those of
igneousvolcanoes, except when a mud volcano is actually a vent of an igneous volcano.
Fissure vents
Main article:
Fissure ventVolcanic
fissure vents are flat, linear cracks through which
lava emerges.
Shield volcanoes
Main article:
Shield volcanoShield volcanoes, so named for their broad, shield-like profiles, are formed by the eruption of low-viscosity lava that can flow a great distance from a vent, but not generally explode catastrophically. Since low-viscosity magma is typically low in silica, shield volcanoes are more common in oceanic than continental settings. The
Hawaiian volcanic chain is a series of shield cones, and they are common in
Iceland, as well.
Lava domes
Lava domes are built by slow eruptions of highly viscous lavas. They are sometimes formed within the crater of a previous volcanic eruption (as in
Mount Saint Helens), but can also form independently, as in the case of
Lassen Peak. Like stratovolcanoes, they can produce violent, explosive eruptions, but their lavas generally do not flow far from the originating vent.
Cryptodomes
Cryptodomes are formed when viscous lava forces its way up and causes a bulge. The
1980 eruption of Mount St. Helens was an example. Lava was under great pressure and forced a bulge in the mountain, which was unstable and slid down the north side.
Volcanic cones (cinder cones)
Volcanic cones or
cinder cones are the result from eruptions that erupt mostly small pieces of
scoria and
pyroclastics (both resemble cinders, hence the name of this volcano type) that build up around the vent. These can be relatively short-lived eruptions that produce a cone-shaped hill perhaps 30 to 400 meters high. Most cinder cones erupt only
once. Cinder cones may form as
flank ventson larger volcanoes, or occur on their own.
Parícutin in
Mexico and
Sunset Crater in
Arizona are examples of cinder cones. In
New Mexico,
Caja del Rio is a
volcanic field of over 60 cinder cones.
Stratovolcanoes (composite volcanoes)
| Cross-section through a stratovolcano (vertical scale is exaggerated): |
|---|
1. Large magma chamber
2. Bedrock
3. Conduit (pipe)
4. Base
5. Sill
6. Dike
7. Layers of ash emitted by the volcano
8. Flank | 9. Layers of lava emitted by the volcano
10. Throat
11. Parasitic cone
12. Lava flow
13. Vent
14. Crater
15. Ash cloud |
Main article:
StratovolcanoStratovolcanoes or
composite volcanoes are tall conical mountains composed of lava flows and other ejecta in alternate layers, the
stratathat give rise to the name. Stratovolcanoes are also known as composite volcanoes, created from several structures during different kinds of eruptions. Strato/composite volcanoes are made of cinders, ash and lava. Cinders and ash pile on top of each other, lava flows on top of the ash, where it cools and hardens, and then the process begins again. Classic examples include
Mt. Fuji in Japan,
Mayon Volcano in the Philippines, and
Mount Vesuvius and
Stromboli in Italy.
In recorded history, explosive eruptions by stratovolcanoes have posed the greatest hazard to civilizations, as
ash is produced by an
explosive eruption. No supervolcano erupted in recorded history. Shield volcanoes have not an enormous pressure build up from the lava flow. Fissure vents and
monogenetic volcanic fields (volcanic cones) have not powerful explosive eruptions, as they are many times under
extension. Stratovolcanoes (30–35°) are steeper than shield volcanoes (generally 5–10°), their loose
tephra are material for dangerous
lahars.
[3]Supervolcanoes
Main article:
SupervolcanoSubmarine 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 because of
volcanic gases.
Pumice rafts may also appear. Even large submarine eruptions may not disturb the ocean surface. Because of the rapid cooling effect of water as compared to air, and increased buoyancy, submarine volcanoes often form rather steep pillars over their volcanic vents as compared to above-surface volcanoes. They may become so large that they break the ocean surface as new islands.
Pillow lava is a common eruptive product of submarine volcanoes.
Hydrothermal vents are common near these volcanoes, and
some support peculiar ecosystems based on dissolved minerals.
Subglacial volcanoes
Subglacial volcanoes develop underneath
icecaps. They are made up of flat lava which flows at the top of extensive pillow lavas and
palagonite. When the icecap melts, the lavas on the top collapse, leaving a flat-topped mountain. These volcanoes are also called
table mountains,
tuyas or (uncommonly) mobergs. Very good examples of this type of volcano can be seen in Iceland, however, there are also tuyas in
British Columbia. The origin of the term comes from
Tuya Butte, which is one of the several tuyas in the area of the
Tuya River and
Tuya Range in northern British Columbia. Tuya Butte was the first such
landform analyzed and so its name has entered the geological literature for this kind of volcanic formation. The
Tuya Mountains Provincial Park was recently established to protect this unusual landscape, which lies north of
Tuya Lake and south of the
Jennings River near the boundary with the
Yukon Territory.
Mud volcanoes
Main article:
Mud volcanoMud volcanoes or mud domes are formations created by geo-excreted liquids and gases, although there are several processes which may cause such activity. The largest structures are 10 kilometers in diameter and reach 700 meters high.
Erupted material
Lava composition
Another way of classifying volcanoes is by the composition of material erupted (lava), since this affects the shape of the volcano. Lava can be broadly classified into 4 different compositions (Cas & Wright, 1987):
- If the erupted magma contains a high percentage (>63%) of silica, the lava is called felsic.
- Felsic lavas (dacites or rhyolites) tend to be highly viscous (not very fluid) and are erupted as domes or short, stubby flows. Viscous lavas tend to formstratovolcanoes or lava domes. Lassen Peak in California is an example of a volcano formed from felsic lava and is actually a large lava dome.
- Because siliceous magmas are so viscous, they tend to trap volatiles (gases) that are present, which cause the magma to erupt catastrophically, eventually forming stratovolcanoes. Pyroclastic flows (ignimbrites) are highly hazardous products of such volcanoes, since they are composed of molten volcanic ash too heavy to go up into the atmosphere, so they hug the volcano's slopes and travel far from their vents during large eruptions. Temperatures as high as 1,200 °C are known to occur in pyroclastic flows, which will incinerate everything flammable in their path and thick layers of hot pyroclastic flow deposits can be laid down, often up to many meters thick. Alaska's Valley of Ten Thousand Smokes, formed by the eruption ofNovarupta 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.
- If the erupted magma contains <52% and >45% silica, the lava is called mafic (because it contains higher percentages of magnesium (Mg) and iron (Fe)) or basaltic. These lavas are usually much less viscous than rhyolitic lavas, depending on their eruption temperature; they also tend to be hotter than felsic lavas. Mafic lavas occur in a wide range of settings:
- Some erupted magmas contain <=45% silica and produce ultramafic lava. Ultramafic flows, also known as komatiites, are very rare; indeed, very few have been erupted at the Earth's surface since the Proterozoic, when the planet's heat flow was higher. They are (or were) the hottest lavas, and probably more fluid than common mafic lavas.
Lava texture
Two types of lava are named according to the
surface texture:
ʻA
ʻa (pronounced
[ˈʔaʔa]) and
pāhoehoe (
[paːˈho.eˈho.e]), both
Hawaiian words.
ʻA
ʻa is characterized by a rough, clinkery surface and is the typical texture of viscous lava flows. However, even basaltic or mafic flows can be erupted as
ʻa
ʻa flows, particularly if the eruption rate is high and the slope is steep.
Pāhoehoe is characterized by its smooth and often ropey or wrinkly surface and is generally formed from more fluid lava flows. Usually, only mafic flows will erupt as pāhoehoe, since they often erupt at higher temperatures or have the proper chemical make-up to allow them to flow with greater fluidity.
Volcanic activity
Popular classification of volcanoes
Active
A popular way of classifying magmatic volcanoes is by their frequency of
eruption, with those that erupt regularly called
active, those that have erupted in historical
times but are now quiet called
dormant, and those that have not erupted in historical times called
extinct. However, these popular classifications—extinct in particular—are practically meaningless to scientists. They use classifications which refer to a particular volcano's formative and eruptive processes and resulting shapes, which was explained above.
There is no real consensus among volcanologists on how to define an "active" volcano. The lifespan of a volcano can vary from months to several million years, making such a distinction sometimes meaningless when compared to the lifespans of humans or even civilizations. For example, many of Earth's volcanoes have erupted dozens of times in the past few thousand years but are not currently showing signs of eruption. Given the long lifespan of such volcanoes, they are very active. By human lifespans, however, they are not.
Scientists usually consider a volcano to be
erupting or
likely to erupt if it is currently erupting, or showing signs of unrest such as unusual earthquake activity or significant new gas emissions. Most scientists consider a volcano
active if it has erupted in
holocene times.
Historic times is another timeframe for
active.
[4] But it is important to note that the span of recorded history differs from region to region. In
China and the
Mediterranean, recorded history reaches back more than 3,000 years but in the Pacific Northwest of the
United States and
Canada, it reaches back less than 300 years, and in
Hawaii and
New Zealand, only around 200 years.
[5] The Smithsonian Global Volcanism Program's definition of
active is having erupted within the last 10,000 years (the 'holocene' period).
Presently there are about 500 active volcanoes in the world – the majority following along the Pacific '
Ring of Fire' – and around 50 of these erupt each year.
[6] The United States is home to 50 active volcanoes.
[7] There are more than 1,500 potentially active volcanoes.
[8] An estimated 500 million people live near active volcanoes.
[9]Extinct
Extinct volcanoes are those that scientists consider unlikely to erupt again, because the volcano no longer has a lava supply. Examples of extinct volcanoes are many volcanoes on the
Hawaiian – Emperor seamount chain in the
Pacific Ocean,
Hohentwiel,
Shiprock and the
Zuidwal volcano in the
Netherlands.
Edinburgh Castle in
Scotland is famously located atop an extinct volcano. Otherwise, whether a volcano is truly extinct is often difficult to determine. Since "supervolcano"
calderas can have eruptive lifespans sometimes measured in millions of years, a caldera that has not produced an eruption in tens of thousands of years is likely to be considered dormant instead of extinct.
Dormant
It is difficult to distinguish an extinct volcano from a
dormant one. Volcanoes are often considered to be extinct if there are no written records of its activity. Nevertheless, volcanoes may remain dormant for a long period of time. For example,
Yellowstone has a repose/recharge period of around 700
ka, and
Toba of around 380 ka.
[10] Vesuvius was described by Roman writers as having been covered with gardens and vineyards before its famous eruption of AD 79, which destroyed the towns of
Herculaneum and
Pompeii. Before its catastrophic eruption of 1991,
Pinatubo was an inconspicuous volcano, unknown to most people in the surrounding areas. Two other examples are the long-dormant
Soufrière Hills volcano on the island of
Montserrat, thought to be extinct before activity resumed in 1995 and
Fourpeaked Mountain in
Alaska, which, before its September 2006 eruption, had not erupted since before 8000 BC and had long been thought to be extinct.
Technical classification of volcanoes
Volcanic-alert level
The three common popular classifications of volcanoes can be subjective and some volcanoes thought to have been extinct have announced to the world they were just pretending.
[11] To help prevent citizens from falsely believing they are not at risk when living on or near a volcano, countries have adopted new classifications to describe the various levels and stages of volcanic activity.
[12] Some alert systems use different numbers or colors to designate the different stages. Other systems use colors and words. Some systems use a combination of both.
Volcano warning schemes of the United States
The United States Geological Survey (USGS) has adopted a common system nationwide for characterizing the level of unrest and eruptive activity at volcanoes. The new volcano alert-level system classifies volcanoes now as being in a normal, advisory, watch or warning stage. Additionally, colors are used to denote the amount of ash produced. Details of the US system can be found at
Volcano warning schemes of the United States.
Notable volcanoes
- Avachinsky-Koryaksky, Kamchatka, Russia
- Nevado de Colima, Jalisco and Colima, Mexico
- Mount Etna, Sicily, Italy
- Galeras, Nariño, Colombia
- Mauna Loa, Hawaii, USA
- Mount Merapi, Central Java, Indonesia
- Mount Nyiragongo, Democratic Republic of the Congo
- Mount Rainier, Washington, USA
| - Sakurajima, Kagoshima Prefecture, Japan
- Santa Maria/Santiaguito, Guatemala
- Santorini, Cyclades, Greece
- Taal Volcano, Luzon, Philippines
- Teide, Canary Islands, Spain
- Ulawun, New Britain, Papua New Guinea
- Mount Unzen, Nagasaki Prefecture, Japan
- Vesuvius, Naples, Italy
|
Effects of volcanoes
Schematic of volcano injection of aerosols and gases.
Solar radiation graph 1958-2008, showing how the radiation is reduced after major volcanic eruptions.
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 16–32 kilometres (10–20 mi) above the Earth's surface. The most significant impacts from these injections come from the conversion of sulfur dioxide to
sulfuric acid (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 — sulfur dioxide from the eruption of
Huaynaputina probably caused the
Russian famine of 1601–1603.
[13]One proposed
volcanic winter happened c. 70,000 years ago following the
supereruption of
Lake Toba on Sumatra island in Indonesia.
[14] According to the
Toba catastrophe theory to which some anthropologists and archeologists subscribe, it had global consequences,
[15] killing most humans then alive and creating a
population bottleneck that affected the genetic inheritance of all humans today.
[16] The 1815 eruption of
Mount Tambora created global climate anomalies that became known as the "
Year Without a Summer" because of the effect on North American and European weather.
[17] Agricultural crops failed and livestock died in much of the Northern Hemisphere, resulting in one of the worst famines of the 19th century.
[18] The freezing winter of 1740–41, which led to widespread
famine in northern Europe, may also owe its origins to a volcanic eruption.
[19]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.
[22]Volcanic eruptions may inject
aerosols into the
Earth's atmosphere. Large injections may cause visual effects such as unusually colorful sunsets and affect global
climate mainly by cooling it. Volcanic eruptions also provide the benefit of adding nutrients to
soil through the
weathering process of volcanic rocks. These fertile soils assist the growth of plants and various crops. Volcanic eruptions can also create new islands, as the magma cools and solidifies upon contact with the water.
Volcanoes on other planetary bodies
The
Tvashtar volcano erupts a plume 330 km (205 mi) above the surface of
Jupiter's moon
Io.
The Earth's
Moon has no large volcanoes and no current volcanic activity, although recent evidence suggests it may still possess a partially molten core.
[23] However, the Moon does have many volcanic features such as
maria (the darker patches seen on the moon),
rilles and
domes.
The planet
Venus has a surface that is 90%
basalt, indicating that volcanism played a major role in shaping its surface. The planet may have had a major global resurfacing event about 500 million years ago,
[24] from what scientists can tell from the density of impact craters on the surface. Lava flows are widespread and forms of volcanism not present on Earth occur as well. Changes in the planet's atmosphere and observations of lightning have been attributed to ongoing volcanic eruptions, although there is no confirmation of whether or not Venus is still volcanically active. However, radar sounding by the Magellan probe revealed evidence for comparatively recent volcanic activity at Venus's highest volcano
Maat Mons, in the form of ash flows near the summit and on the northern flank.
Jupiter's
moon Io is the most volcanically active object in the solar system because of
tidal interaction with Jupiter. It is covered with volcanoes that erupt
sulfur,
sulfur dioxide and
silicate rock, and as a result,
Io is constantly being resurfaced. Its lavas are the hottest known anywhere in the solar system, with temperatures exceeding 1,800 K (1,500 °C). In February 2001, the largest recorded volcanic eruptions in the solar system occurred on Io.
[26] Europa, the smallest of Jupiter's
Galilean moons, also appears to have an active volcanic system, except that its volcanic activity is entirely in the form of water, which freezes into ice on the frigid surface. This process is known as
cryovolcanism, and is apparently most common on the moons of the outer planets of the
solar system.
A 2010 study of the
exoplanet COROT-7b, which was detected by
transit in 2009, studied that
tidal heating from the host star very close to the planet and neighboring planets could generate intense volcanic activity similar to Io.
[29]Traditional beliefs about volcanoes
Many ancient accounts ascribe volcanic eruptions to
supernatural causes, such as the actions of
gods or
demigods. To the ancient Greeks, volcanoes' capricious power could only be explained as acts of the gods, while 16th/17th-century German astronomer Johannes Kepler believed they were ducts for the Earth's tears.
[30] One early idea counter to this was proposed by
Jesuit Athanasius Kircher (1602–1680), who witnessed eruptions of
Mount Etna and
Stromboli, then visited the crater of
Vesuvius and published his view of an Earth with a central fire connected to numerous others caused by the burning of
sulfur,
bitumen and
coal.
Various explanations were proposed for volcano behavior before the modern understanding of the Earth's
mantle structure as a semisolid material was developed. For decades after awareness that compression and
radioactive materials may be heat sources, their contributions were specifically discounted. Volcanic action was often attributed to
chemical reactions and a thin layer of molten rock near the surface.
(
http://en.wikipedia.org/wiki/Volcano)
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