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A VOLCANO is a rupture in the crust of a planetary-mass object , such as Earth
Earth
, that allows hot lava , volcanic ash , and gases to escape from a magma chamber below the surface.

Earth's volcanoes occur because its crust is broken into 17 major, rigid tectonic plates that float on a hotter, softer layer in its mantle. Therefore, on Earth, volcanoes are generally found where tectonic plates are diverging or converging , and most are found underwater. For example, a mid-oceanic ridge , such as the Mid-Atlantic Ridge , has volcanoes caused by divergent tectonic plates whereas the Pacific Ring of Fire
Pacific Ring of Fire
has volcanoes caused by convergent tectonic plates. Volcanoes can also form where there is stretching and thinning of the crust's plates, e.g., in the East African Rift and the Wells Gray-Clearwater volcanic field and Rio Grande Rift in North America. This type of volcanism falls under the umbrella of "plate hypothesis" volcanism. 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. Volcanoes are usually not created where two tectonic plates slide past one another.

Erupting volcanoes can pose many hazards, not only in the immediate vicinity of the eruption. One such hazard is that 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 ; the melted particles then adhere to the turbine blades and alter their shape, disrupting the operation of the turbine. 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 from the Earth, thereby warming the upper atmosphere (or stratosphere ). Historically, volcanic winters have caused catastrophic famines .

CONTENTS

* 1 Etymology

* 2 Plate tectonics
Plate tectonics

* 2.1 Divergent plate boundaries * 2.2 Convergent plate boundaries * 2.3 Hotspots

* 3 Volcanic features

* 3.1 Fissure vents * 3.2 Shield volcanoes
Shield volcanoes
* 3.3 Lava
Lava
domes * 3.4 Cryptodomes * 3.5 Volcanic cones (cinder cones) * 3.6 Stratovolcanoes (composite volcanoes) * 3.7 Supervolcanoes * 3.8 Underwater volcanoes * 3.9 Subglacial volcanoes * 3.10 Mud volcanoes

* 4 Erupted material

* 4.1 Lava
Lava
composition * 4.2 Lava
Lava
texture

* 5 Volcanic activity

* 5.1 Popular classification of volcanoes

* 5.1.1 Active * 5.1.2 Extinct * 5.1.3 Dormant and reactivated

* 5.2 Technical classification of volcanoes

* 5.2.1 Volcanic-alert level * 5.2.2 Volcano warning schemes of the United States

* 6 Decade volcanoes

* 7 Effects of volcanoes

* 7.1 Volcanic gases

* 7.2 Significant consequences

* 7.2.1 Prehistory * 7.2.2 Historical

* 7.3 Acid rain * 7.4 Hazards

* 8 Volcanoes on other celestial bodies * 9 Traditional beliefs about volcanoes * 10 See also * 11 References * 12 Further reading * 13 External links

ETYMOLOGY

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

PLATE TECTONICS

Map showing the divergent plate boundaries (oceanic spreading ridges) and recent sub-aerial volcanoes Main article: Plate tectonics

DIVERGENT PLATE BOUNDARIES

Main article: Divergent boundary

At the mid-oceanic ridges , two tectonic plates diverge from one another as new oceanic crust is formed by the cooling and solidifying of hot molten rock. Because the crust is very thin at these ridges due to the pull of the tectonic plates, the release of pressure leads to adiabatic expansion (without transfer of heat or matter) 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 on the Earth
Earth
is submarine, forming new seafloor. Black smokers (also known as deep sea vents) are evidence of this kind of volcanic activity. Where the mid-oceanic ridge is above sea-level, volcanic islands are formed; for example, Iceland
Iceland
.

CONVERGENT PLATE BOUNDARIES

Main article: Convergent boundary

Subduction
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. In a process called flux melting , water released from the subducting plate lowers the melting temperature of the overlying mantle wedge, thus creating magma . This magma tends to be extremely viscous because of its high silica content, so it often does not attain the surface but cools and solidifies at depth. When it does reach the surface, however, a volcano is formed. Typical examples are Mount Etna and the volcanoes in the Pacific Ring of Fire
Pacific Ring of Fire
.

HOTSPOTS

Main article: Hotspot (geology)

Hotspots are volcanic areas believed to be formed by mantle plumes , which are hypothesized to be columns of hot material rising from the core-mantle boundary in a fixed space that causes large-volume melting. Because tectonic plates move across them, each volcano becomes dormant and is eventually re-formed as the plate advances over the postulated plume. The Hawaiian Islands
Hawaiian Islands
are said to have been formed in such a manner; so has the Snake River Plain , with the Yellowstone Caldera
Yellowstone Caldera
being the part of the North American plate above the hot spot. This theory, however, has been doubted.

VOLCANIC FEATURES

Lakagigar fissure vent in Iceland
Iceland
, source of the major world climate alteration of 1783–84 Skjaldbreiður , a shield volcano whose name means "broad shield"

The most common perception of a volcano is of a conical mountain, spewing lava and poisonous gases from a crater at its summit; however, this describes just one of the many types of volcano. The features of volcanoes are much more complicated and their structure and behavior depends on a number of factors. Some volcanoes have rugged peaks formed by lava domes rather than a summit crater while others have landscape features such as massive plateaus . Vents that issue volcanic material (including lava and ash ) and gases (mainly steam and magmatic gases ) can develop anywhere on the landform and may 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
Jupiter
, Saturn
Saturn
, and Neptune
Neptune
; and mud volcanoes , which are formations often not associated with known magmatic activity. Active mud volcanoes tend to involve temperatures much lower than those of igneous volcanoes except when the mud volcano is actually a vent of an igneous volcano.

FISSURE VENTS

Main article: Fissure vent

Volcanic FISSURE VENTS are flat, linear fractures through which lava emerges.

SHIELD VOLCANOES

Main article: Shield volcano

SHIELD VOLCANOES, so named for their broad, shield-like profiles, are formed by the eruption of low-viscosity lava that can flow a great distance from a vent. They generally do not 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
Iceland
, as well.

LAVA DOMES

Main article: Lava
Lava
dome

LAVA DOMES are built by slow eruptions of highly viscous lava. They are sometimes formed within the crater of a previous volcanic eruption, as in the case of 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 lava generally does not flow far from the originating vent.

CRYPTODOMES

CRYPTODOMES are formed when viscous lava is forced upward causing the surface to bulge. The 1980 eruption of Mount St. Helens was an example; lava beneath the surface of the mountain created an upward bulge which slid down the north side of the mountain.

VOLCANIC CONES (CINDER CONES)

Main articles: volcanic cone and Cinder cone Izalco (volcano) , located in the Cordillera de Apaneca volcanic range complex in El Salvador. Only a few generations old, Izalco is the youngest and best known cone volcano. Izalco erupted almost continuously from 1770 (when it formed) to 1958, earning it the nickname of "Lighthouse of the Pacific".

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

Based on satellite images it was suggested that cinder cones might occur on other terrestrial bodies in the Solar system
Solar system
too; on the surface of Mars
Mars
and the Moon.

STRATOVOLCANOES (COMPOSITE VOLCANOES)

CROSS-SECTION THROUGH A STRATOVOLCANO (VERTICAL SCALE IS EXAGGERATED):

* Large magma chamber * Bedrock * Conduit (pipe) * Base * Sill * Dike * Layers of ash emitted by the volcano * Flank * Layers of lava emitted by the volcano * Throat * Parasitic cone * Lava
Lava
flow * Vent * Crater * Ash cloud

Main article: Stratovolcano

STRATOVOLCANOES or COMPOSITE VOLCANOES are tall conical mountains composed of lava flows and other ejecta in alternate layers, the strata that gives rise to the name. Stratovolcanoes are also known as composite volcanoes because they are created from multiple 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 repeats. Classic examples include Mount Fuji
Mount Fuji
in Japan, Mayon Volcano in the Philippines, and Mount Vesuvius
Mount Vesuvius
and Stromboli in Italy.

Throughout recorded history , ash produced by the explosive eruption of stratovolcanoes has posed the greatest volcanic hazard to civilizations. Not only do stratovolcanoes have greater pressure build up from the underlying lava flow than shield volcanoes, but their fissure vents and monogenetic volcanic fields (volcanic cones) have more powerful eruptions, as they are many times under extension . They are also steeper than shield volcanoes, with slopes of 30–35° compared to slopes of generally 5–10°, and their loose tephra are material for dangerous lahars . Large pieces of tephra are called volcanic bombs . Big bombs can measure more than 4 feet(1.2 meters) across and weigh several tons.

SUPERVOLCANOES

Main article: Supervolcano See also: List of largest volcanic eruptions

A SUPERVOLCANO usually has a large caldera and can produce devastation on an enormous, sometimes continental, scale. Such volcanoes are able to severely cool global temperatures for many years after the eruption due to the huge volumes of sulfur and ash released into the atmosphere. They are the most dangerous type of volcano. Examples include: Yellowstone Caldera
Yellowstone Caldera
in Yellowstone National Park and Valles Caldera
Caldera
in New Mexico
New Mexico
(both western United States); Lake Taupo in New Zealand; Lake Toba in Sumatra
Sumatra
, Indonesia; and Ngorongoro Crater in Tanzania. Because of the enormous area they may cover, supervolcanoes are hard to identify centuries after an eruption. Similarly, large igneous provinces are also considered supervolcanoes because of the vast amount of basalt lava erupted (even though the lava flow is non-explosive ).

UNDERWATER VOLCANOES

Main article: Submarine volcano See also: Subaqueous volcano

SUBMARINE VOLCANOES are common features of the ocean floor. In shallow water, active volcanoes disclose their presence by blasting steam and rocky debris high above the ocean's surface. In the ocean's deep, the tremendous weight of the water above prevents the explosive release of steam and gases; however, they can be detected by hydrophones and discoloration of water because of volcanic gases . Pillow lava
Pillow lava
is a common eruptive product of submarine volcanoes and is characterized by thick sequences of discontinuous pillow-shaped masses which form under water. Even large submarine eruptions may not disturb the ocean surface due to the rapid cooling effect and increased buoyancy of water (as compared to air) which often causes volcanic vents to form steep pillars on the ocean floor. Hydrothermal vents are common near these volcanoes, and some support peculiar ecosystems based on dissolved minerals. Over time, the formations created by submarine volcanoes may become so large that they break the ocean surface as new islands or floating pumice rafts .

SUBGLACIAL VOLCANOES

Main article: Subglacial volcano

SUBGLACIAL VOLCANOES develop underneath icecaps . They are made up of flat lava which flows at the top of extensive pillow lavas and palagonite . When the icecap melts, the lava on top collapses, 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
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
Yukon Territory
.

MUD VOLCANOES

Main article: Mud volcano

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

ERUPTED MATERIAL

Pāhoehoe lava flow on Hawaii
Hawaii
. The picture shows overflows of a main lava channel . The Stromboli stratovolcano off the coast of Sicily
Sicily
has erupted continuously for thousands of years, giving rise to the term strombolian eruption . San Miguel (volcano) , El Salvador. On December 29, 2013, San Miguel volcano, also known as "Chaparrastique", erupted at 10:30 local time, spewing a large column of ash and smoke into the sky; the eruption, the first in 11 years, was seen from space and prompted the evacuation of thousands of people living in a 3 km radius around the volcano. Ash plume from San Miguel (volcano) "Chaparrastique", seen from a satellite, as it heads towards the Pacific Ocean from the El Salvador
El Salvador
Central America
Central America
coast, December 29, 2013

LAVA COMPOSITION

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

* If the erupted magma contains a high percentage (>63%) of silica , the lava is called felsic .

* Felsic lavas (dacites or rhyolites ) tend to be highly viscous (not very fluid) and are erupted as domes or short, stubby flows. Viscous lavas tend to form stratovolcanoes or lava domes. Lassen Peak in California is an example of a volcano formed from felsic lava and is actually a large lava dome. * Because siliceous magmas are so viscous, they tend to trap volatiles (gases) that are present, which cause the magma to erupt catastrophically, eventually forming stratovolcanoes. Pyroclastic flows (ignimbrites ) are highly hazardous products of such volcanoes, since they are composed of molten volcanic ash too heavy to go up into the atmosphere, so they hug the volcano's slopes and travel far from their vents during large eruptions. Temperatures as high as 1,200 °C are known to occur in pyroclastic flows, which will incinerate everything flammable in their path and thick layers of hot pyroclastic flow deposits can be laid down, often up to many meters thick. Alaska 's Valley of Ten Thousand Smokes , formed by the eruption of Novarupta near Katmai in 1912, is an example of a thick pyroclastic flow or ignimbrite deposit. Volcanic ash
Volcanic ash
that is light enough to be erupted high into the Earth\'s atmosphere may travel many kilometres before it falls back to ground as a tuff .

* If the erupted magma contains 52–63% silica, the lava is of intermediate composition.

* These "andesitic " volcanoes generally only occur above subduction zones (e.g. Mount Merapi in Indonesia).

* Andesitic lava is typically formed at convergent boundary margins of tectonic plates , by several processes:

* Hydration melting of peridotite and fractional crystallization Play media Sarychev Peak eruption, Matua Island , oblique satellite view * Melting of subducted slab containing sediments * Magma
Magma
mixing between felsic rhyolitic and mafic basaltic magmas in an intermediate reservoir prior to emplacement or lava flow.

* If the erupted magma contains 45% silica, the lava is called mafic (because it contains higher percentages of magnesium (Mg) and iron (Fe)) or basaltic . These lavas are usually much less viscous than rhyolitic lavas, depending on their eruption temperature; they also tend to be hotter than felsic lavas. Mafic lavas occur in a wide range of settings:

* At mid-ocean ridges , where two oceanic plates are pulling apart, basaltic lava erupts as pillows to fill the gap; * Shield volcanoes
Shield volcanoes
(e.g. the Hawaiian Islands
Hawaiian Islands
, including Mauna Loa and Kilauea ), on both oceanic and continental crust ; * As continental flood basalts .

* Some erupted magmas contain

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