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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.
Smoking Bromo and Semeru volcanoes on Java in Indonesia.
Smoking Bromo and Semeru volcanoes on Java in Indonesia.
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.

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Volcano classification
Notable Volcanoes
Effects of volcanoes
Past beliefs

Volcano classification - Contents

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 or Granitic. Felsic lava tends to be highly viscous (not very fluid) and is pushed up in a blob that solidifies relatively quickly. Viscous lavas tend to form stratovolcanoes. Lassen Peak in California is an example of a stratovolcano formed from felsic lava. This type of volcano has a tendency to explode when erupting, because the viscous lava traps volatiles (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 or basaltic and will be very fluid as it erupts, capable of flowing for long distances. 'Mafic' is a word referring to the chemical composition of the lava -- it contains higher percentages of magnesium (Mg) and iron (Fe), and correspondingly lower percentages of silica. Due to low viscosity, volatiles are able to escape. A good example of a mafic lava flow is the Great flow produced by an eruptive fissure near 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. The shield volcanoes forming the islands of Hawaii also produce low-viscosity, mafic lavas. Lavas (and rocks) with particularly high proportion of iron and/or magnesium are called 'ultra-mafic'. A third type of lava that eruptsfrom volcanoes is andesitic, this lava has moderate amounts of silica and a moderate temperature.


Shield volcanoes

Toes of a pāhoehoe advance across a road in Kalapana on the east rift zone of Kīlauea Volcano in Hawai‘i.
Toes of a pāhoehoe advance across a road in Kalapana on the east rift zone of Kīlauea Volcano in Hawai‘i.
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 Hawaii. 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'.

Cinder cones

Volcanic cones or cinder cones result from eruptions that throw out mostly small pieces of cinder 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. Cinder cones may be associated with other types of volcanoes, or occur on their own.

Stratovolcanoes or composite volcanoes

These are tall conical mountains composed of both lava flows and ejected material, being layered alternatively, 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.


" 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 the Yellowstone Caldera in Yellowstone National Park, the Long Valley Caldera near Mammoth Lakes, California, and Lake Toba, but are hard to identify given that there is no formal definition of the term.

Submarine volcanoes

Pillow lava (NOAA)
Pillow lava (NOAA)
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. Pillow lava is the typical eruptive manifestation of submarine volcanoes.

Active, dormant, or extinct?
A volcanic eruption can be devastating for the local wildlife, as well as the human population.
A volcanic eruption can be devastating for the local wildlife, as well as the human population.
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 (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 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. 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. Another common 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 is created, however, 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 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 (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 - Contents

Volcanoes on Earth
Mount St. Helens erupting in 1980
Mount St. Helens erupting in 1980

Volcanoes elsewhere in the solar system
Olympus Mons (Latin, "Mount Olympus") is the tallest known mountain in our solar system, located on the planet Mars.
Olympus Mons ( Latin, "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 [1]. See the list of geological features on Io for a list of named volcanoes on the moon. Europa, the smallest of Jupiter's moons, also appears to have an active volcanic system. Its surface appears to be a young 60 million years old, suggesting a constant lava flow.
Ice volcanoes on Enceladus
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 [2]. 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. [3] It is theorized that cryovolcanism may also be present on the Kuiper Belt Object Quaoar.

Volcanology - Contents

Volcano formation
Diagram of a destructive margin causing earthquakes and a volcanic eruption
Diagram of a destructive margin causing earthquakes and a volcanic eruption
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. As the oceanic crust 'subducts', it descends into the mantle where temperatures are generally higher than near the surface of the planet. Increases in temperature and pressure with depth cause water trapped in the descending oceanic crust to escape from minerals the crust. This process is called dehydration, commonly occurs at depths of about 100km (62 miles) and can also be a source of very deep earthquakes due to an associated change in volume of the dehydrating rock mass(such as the 2001 Nisqually Earthquake in Washington State, USA). The water that escapes from the dehydrating oceanic crust migrates into the surrounding mantle which has a different composition than the descending crust. At ambient conditions in the mantly at 100km depth, water will induce partial melting of the mantle. This melt is less dense than the surrounding mantle and will consequently rise though the mantle to the overlying crust. As the magma (melt) rises through the crust it may melt and assimilate some of the surrounding crust, it may cool and begin to grow crystals, and it may exsolve gas. The relative importance of these processes depends on the composition, amount and ascent rate of the magma. If the magma reaches the surface, it will generate a volcanic eruption. The style of the eruption will depend on the composition and gas content of the magma. The type of volcano will depend on the type of magma that usually erupts at that location over a long period of time, and the viscosity of the magma. High concentrations of silica are associated with high viscosity (thicker, goopier magma) and will form steep sided volcanoes. Volcanic arcs forming near subduction zones, on the edges of continental plates, usually form high-silica melt which create steep sided stratovolcanoes due to the high viscosity of the melt. For example, Mount St. Helens is found inland from the margin between the oceanic Juan de Fuca Plate and the continental North American Plate. Other examples of chains of stratovolcanoes include the South American Andes, the Cascade range, and the Aleutian Islands.
Shiprock, New Mexico a volcanic neck in the distance, with radiating dike on its south side. Photo credit: USGS Digital Data Series
Shiprock, New Mexico a volcanic neck in the distance, with radiating dike on its south side. Photo credit: USGS Digital Data Series
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 (called volcanic ash), 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 shape, formed by 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.
Hotspot and types of plate boundaries.
Hotspot and types of plate boundaries.
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 volcanoes reaching to the surface - Saint Helena and Tristan da Cunha in the Atlantic Ocean, and the Galapagos Islands in the Pacific Ocean are examples - allowing them to be studied in some detail. But most activity takes place at 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 (subducts) 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
1984 Eruption at Krafla, Iceland
1984 Eruption at Krafla, Iceland
Hotspots were originally a catch-all for volcanoes that didn't fit into one of the above two categories, but today 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 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. Additionally, 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 may 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.
Mount St. Helens erupted explosively on May 18, 1980 at 8:32 a.m. PDT
Mount St. Helens erupted explosively on May 18, 1980 at 8:32 a.m. PDT
Volcanologists monitor the following phenomena to help forecast eruptions:


Seismic activity (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 a house'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 a good indicator of increasing risk of eruption, 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, exactly when predicted, the volcano erupted spectacularly. It was Popocatépetl's largest eruption for a thousand years, and yet no one was hurt.

Gas emissions

The eruption of Vesuvius in Discovery Channel's Pompeii.
The eruption of Vesuvius in Discovery Channel's Pompeii.
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. A planet or moon with a much thicker crust, such as Mars, can support a heavier volcano. See for example Olympus Mons, the tallest known mountain in our solar system, which sits on the Tharsis bulge.

Effects of volcanoes - Contents

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.
Volcanic "injection"
Volcanic "injection"
Solar radiation reduction due to volcanic eruptions
Solar radiation reduction due to volcanic eruptions
Sulfur dioxide emissions by volcanoes.
Sulfur dioxide emissions by volcanoes.
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 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 (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 - Contents

Before it was understood that most of the Earth's interior is molten, various explanations existed for volcano behaviour. 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.
Kircher's model of the Earth's internal fires, from Mundus Subterraneus
Kircher's model of the Earth's internal fires, from Mundus Subterraneus
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