From Academic Kids
A volcano is a geological landform (usually a mountain) where magma (rock of the earth's interior made molten or liquid by high pressure and temperature) erupts through the surface of the 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.
Types of volcanoes
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 and tends to be very viscous (not very fluid) and is pushed up in a blob that will solidify relatively quickly. Lassen Peak in California is an example. This type of volcano has a tendency to explode because it easily plugs. [[Mount Pel饝] on the island of Martinique is another example.
If, on the other hand, the magma contains relatively small amounts (<52%) of silica, the lava is called mafic and will be very fluid as it erupts, capable of flowing for long distances. A good example of a mafic lava flow is the Great ުrhraun (Thjrhraun) flow produced by an eruptive fissure almost in the geographical center of Iceland roughly 8,000 years ago; it flowed all the way to the sea, a distance of 130 kilometers, and covered an area of 800 square km. Note that the terms felsic and mafic are sometimes substituted by the older chemistry terms "acidic" and "basic", respectively. The latter were thought to be a little misleading, however, and are slowly falling into disuse.
- Shield volcano: 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 high (it sits on the sea floor), 120 km in diameter and forms part of the Island of Hawai'i. Olympus Mons is a shield volcano on Mars, and the tallest mountain in the known solar system.
- Smaller versions of the "lava shield" include the lava dome (tholoid), lava cone, and lava mound.
- Volcanic cones or cinder cones result from eruptions that throw out mostly small pieces of rock that build up around the vent. These can be relatively short-lived eruptions that produce a cone-shaped hill perhaps 30 to 300 m high.
- Stratovolcanoes or composite volcanoes such as Mt. Fuji in Japan, Vesuvius in Italy, Mount Erebus in Antarctica, and Mount Rainier in the northwestern United States are tall conical mountains composed of both lava flows and ejected material.
- Supervolcanoes is the popular term for large volcanoes that usually have a large caldera and can potentially produce devastation on a continental scale and cause major global weather pattern changes. Potential candidates include Yellowstone National Park and Lake Toba, but are hard to identify given that there is no formal definition of the term.
- 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.
Volcanoes are usually situated either at the boundaries between tectonic plates or over hot spots. Volcanoes may be either dormant (having no activity) or active (near constant expulsion and occasional eruptions), and change state unpredictably.
Volcanoes on land often take the form of flat cones, as the expulsions build up over the years, or in short-lived cinder cones. Under water, volcanoes often form rather steep pillars and in due time break the ocean surface in new islands.
Behavior of volcanoes
There are many different kinds of volcanic activity and eruptions:
- phreatic eruptions (steam)
- explosive eruption of high-silica lava (e.g., rhyolite)
- effusive eruption of low-silica lava (e.g., basalt)
- pyroclastic flows
- lahars (debris flow)
- carbon dioxide emission
All of these activities can pose a hazard to humans.
Surprisingly, there is no consensus among volcanologists on how to define an "active" volcano. The lifespan of a volcano can vary from months to several million years, making such a distinction sometimes meaningless when compared to the lifespans of humans or even civilizations. For example, many of Earth's volcanoes have erupted dozens of times in the past few thousand years but are not currently showing signs of activity. Given the long lifespan of such volcanoes, they are very active. By our lifespans, however, they are not. Complicating the definition are volcanoes that become restless but do not actually erupt. Are these volcanoes active?
Scientists usually consider a volcano active if it is currently erupting or showing signs of unrest, such as unusual earthquake activity or significant new gas emissions. Many scientists also consider a volcano active if it has erupted in historic time. It is important to note that the span of recorded history differs from region to region; in the Mediterranean, recorded history reaches back more than 3,000 years but in the Pacific Northwest of the United States, it reaches back less than 300 years, and in Hawaii, little more than 200 years.
Dormant volcanoes are those that are not currently active (as defined above), but could become restless or erupt again.
Extinct volcanoes are those that scientists consider unlikely to erupt again. Whether a volcano is truly extinct is often difficult to determine. For example, since calderas have lifespans sometimes measured in millions of years, a caldera that has not produced an eruption in tens of thousands of years is likely to be considered dormant instead of extinct. Yellowstone caldera in Yellowstone National Park is at least 2 million years old and hasn't erupted for 70,000 years, yet scientists do not consider Yellowstone as extinct. In fact, because the caldera has frequent earthquakes, a very active geothermal system, and rapid rates of ground uplift, many scientists consider it to be a very active volcano.
Volcanoes on Earth
- Main article: List of volcanoes
- Cold Bay Volcano (Alaska, USA)
- Mount Baker (Washington, USA)
- Mount Fuji (Honshu, Japan)
- Mount Hood (Oregon, USA)
- Mount Erebus (Ross Island, Antarctica)
- Etna (Sicily, Italy)
- Krafla (Iceland)
- Hekla (Iceland)
- Kilauea (Hawaii, USA)
- Krakatoa (Rakata, Indonesia)
- Mauna Kea (Hawaii, USA)
- Mauna Loa (Hawaii, USA)
- El Misti (Arequipa, Peru)
- Novarupta (Alaska, USA)
- Mount Pinatubo (Luzon Island, Philippines)
- Popocat鰥tl (Mexico-Puebla state line, Mexico)
- Santorini (Santorini islands, Greece)
- Mount Rainier (Washington, USA)
- Mount Shasta (California, USA)
- Mount St. Helens (Washington, USA)
- Surtsey (Iceland)
- Tambora (Sumbawa, Indonesia)
- Teide (Tenerife, Canary Islands, Spain)
- White Island (Bay of Plenty, New Zealand)
- Mount Vesuvius (Bay of Naples, Italy)
Volcanoes elsewhere in the solar system
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:
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 1800K (1527C). In February 2001, the largest recorded volcanic eruptions in the solar system occurred on Io  (http://www2.keck.hawaii.edu/news/archive/eruption/). See the list of geological features on Io for a list of named volcanoes on the moon.
The Cassini-Huygens mission has found evidence of a methane-spewing cryovolcano on the Saturnian moon Titan. This volcanism is believed to be a significant source of the methane found in its atmosphere.  (http://www.newscientist.com/article.ns?id=dn7489)
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. While always displaying powerful forces, eruptions can vary from effusive to extremely explosive.
Most volcanoes on the land are formed at destructive plate margins: where oceanic crust is forced below the continental crust because oceanic crust is denser than continental crust. Friction between these moving plates will cause the oceanic crust to melt, and reduced density will force the newly formed magma to rise. As the magma rises it rises up through weak areas in the continental crust, eventually erupting as one or more volcanoes. For example, Mount St. Helens is found inland from the margin between the oceanic Juan de Fuca Plate and the continental North American Plate.
A volcano generally presents itself to the imagination as a mountain sending forth from its summit great clouds of smoke with vast sheets of flame. The truth is that a volcano seldom emits either smoke or flame, although various combinations of hydrogen, carbon, oxygen, and sulfur do sometimes ignite. What is mistaken for smoke consists of vast volumes of fine dust, mingled with steam and other vapors, chiefly sulfurous. Most of what appears to be flames is the glare from the erupting materials, glowing because of their high temperature; this glare reflects off the clouds of dust and steam, resembling fire.
Perhaps the most conspicuous part of a volcano is the crater, a basin of a roughly circular form within which occurs a vent (or vents) from which magma erupts as gases, lava, and ejecta. A crater can be of large dimensions, and sometimes of vast depth. Very large features of this sort are termed calderas. Some volcanoes consist of a crater alone, with scarcely any mountain at all; but in the majority of cases the crater is situated on top of a mountain (the volcano), which can tower to an enormous height. Volcanoes that terminate in a principal crater are usually of a conical form.
Volcanic cones are usually smaller features composed of loose ash and cinder, with occasional masses of stone which have been tossed violently into the air by the eruptive forces (and are thus called ejecta). Within the crater of a volcano there may be numerous cones from which vapours are continually issuing, with occasional volleys of ashes and stones. In some volcanoes these cones form lower down the mountain, along rift zones or fractures. When the cone is eroded these rifts or lava filled fractures remain as radial near vertical dikes of volcanic rock. For example the radiating dikes at Shiprock in NW New Mexico.
Tectonic environments of volcanoes
Volcanoes can principally be found in three tectonic environments.
Constructive plate margins
These are by far the most common volcanoes on the Earth. They are also the least frequently seen, because most of their activity takes place beneath the surface of the oceans. Along the whole of the oceanic ridge system are irregularly spaced surface eruptions, and more frequent sub-surface intrusions without surface expression. The large majority of these are only known about at surface because of earthquakes as part of the eruptions/ intrusions, or occasionally if passing shipping happens to notice unusually high water temperatures or chemical precipitates in the seawater. In a few places oceanic ridge activity has lead to the volcanoes coming up to the surface - Saint Helena and Tristan da Cunha in the Atlantic Ocean; the Galapagos Islands in the Pacific Ocean, allowing them to be studied in some detail. But most activity takes place in considerable water depths. Iceland is also on a ridge, but has different characteristics than a simple volcano.
It could be argued that the volcanoes of the Great Rift Valley system of East Africa are modified constructive margin volcanoes. However the modifications caused by the presence of thick continental crust are very substantial, and the magmas produced are very different from the typically very homogenous MORB (Mid-Ocean Ridge Basalt) that makes up the huge majority of constructive margin volcanoes.
Destructive plate margins
These are the most visible and well-known types of volcanoes on earth, forming above the subduction zones where (oceanic) plates dive into the Earth to their destruction. Their magmas are typically "calc-alkaline" as a result of their origins in the upper parts of altered ocean plate materials, mixed with sediments, and processed through variable thicknesses of more-or-less continental crust. Unsurprisingly, their compositions are much more varied than at constructive margins.
Hotspots were originally a catch-all for volcanoes that didn't fit into one of the above two categories, but these days this refers to a more specific circumstance - where an isolated plume of hot mantle material intersects the underside of crust (oceanic or continental), leading to a volcanic center that is not obviously connected with a plate margin. The classic example is the Hawaiian chain of volcanoes and seamounts; Yellowstone is cited as another classic example, in this case the intersection is with the underside of continental crust. Iceland is sometimes cited as yet a third classical example, but complicated by the coincidence of a hotspot intersecting an oceanic ridge constructive margin.
There are debates about the simple "hotspot" concept, since theorists cannot agree on whether the "hot mantle plumes" originate in the upper mantle or in the lower mantle. Meanwhile, field geologists and petrologists see considerable variation in the detailed chemistry of one hotspot's magmas versus a second hotspot's magmas. On the third hand, high-resolution seismology of different hotspots is yielding different pictures of the deep sub-structure of Hawaii versus Iceland. There is no detailed consensus about how to interpret these varied results, and it seems plausible that eventually several different sub-types of hotspots will be identified.
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.
Volcanologists use the following to forecast eruptions.
Seismic activity (small earthquakes and tremors) always occurs as volcanoes awaken and prepare to erupt. Some volcanoes normally have continuing low-level seismic activity, but an increase can signify an eruption. The types of earthquakes that occur and where they start and end are also key signs. Volcanic seismicity has three major forms: short-period earthquakes, long-period earthquakes, and harmonic tremor.
- Short-period earthquakes are like normal fault-related earthquakes. They are related to the fracturing of brittle rock as the magma forces its way upward. These short-period earthquakes signify the growth of a magma body near the surface.
- Long-period earthquakes are believed to indicate increased gas pressure in a volcano's "plumbing system." They are similar to the clanging sometimes heard in your home's plumbing system. These oscillations are the equivalent of acoustic vibrations in a chamber, in the context of magma chambers within the volcanic dome.
- Harmonic tremor occurs when there is sustained movement of magma below the surface.
Patterns of seismicity are complex and often difficult to interpret. However, increasing activity is very worrisome, especially if long-period events become dominant and episodes of harmonic tremor appear.
In December 2000, scientists at the National Center for Prevention of Disasters in Mexico City predicted an eruption within two days from Popocat鰥tl, on the outskirts of Mexico City. Their prediction used research done by Dr. Bernard Chouet, a Swiss vulacanologist working at the United States Geological Survey, into increasing long-period oscillations as an indicator of an imminent eruption. The government evacuated tens of thousands of people. Forty eight hours later, bang on time, the volcano erupted spectacularly. It was Popocat鰥tl's largest eruption for a thousand years and yet no one was hurt.
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.
Swelling of the volcano signals that magma has accumulated near the surface. Scientists monitoring an active volcano will often measure the tilt of the slope and track changes in the rate of swelling. An increased rate of swelling, especially if accompanied by an increase in sulfur dioxide emissions and harmonic tremors, is a high probability sign of an impending event.
Effects of volcanoes
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 (H2O), carbon dioxide (CO2), sulfur dioxide (SO2), hydrogen chloride (HCl), hydrogen fluoride (HF) and ash (pulverized rock and pumice) into the stratosphere to heights of 10-20 miles above the Earth's surface. The most significant impacts from these injections come from the conversion of sulfur dioxide to sulfuric acid (H2SO4), which condenses rapidly in the stratosphere to form fine sulfate aerosols. The aerosols increase the reflection of radiation from the Sun back into space and thus cool the Earth's lower atmosphere or troposphere; however, they also absorb heat radiated up from the Earth, thereby warming the stratosphere. Several eruptions during the past century have caused a decline in the average temperature at the Earth's surface of up to half a degree (Fahrenheit scale) for periods of one to three years. The sulfate aerosols also promote complex chemical reactions on their surfaces that alter chlorine and nitrogen chemical species in the stratosphere. This effect, together with increased stratospheric chlorine levels from chlorofluorocarbon pollution, generates chlorine monoxide (ClO), which destroys ozone (O3). As the aerosols grow and coagulate, they settle down into the upper troposphere where they serve as nuclei for cirrus clouds and further modify the Earth's radiation balance. Most of the hydrogen chloride (HCl) and hydrogen fluoride (HF) are dissolved in water droplets in the eruption cloud and quickly fall to the ground as acid rain. The injected ash also falls rapidly from the stratosphere; most of it is removed within several days to a few weeks. Finally, explosive volcanic eruptions release the greenhouse gas carbon dioxide and thus provide a deep source of carbon for biogeochemical cycles.
Gas emissions from volcanoes are a natural contributor to acid rain.
Before it was understood that most of the Earth's interior is molten, various explanations existed for volcano behavior. For decades after awareness that compression and radioactive materials may be heat sources, their contributions were specifically discounted. Volcanic action was often attributed to chemical reactions and a thin layer of molten rock near the surface.
Jesuit Athanasius Kircher (1602-1680), witnessed eruptions of Aetna and Stromboli, then visited the crater of Vesuvius and published his view of an Earth with a central fire connected to numerous others caused by the burning of sulfur, bitumen and coal.
Pictures of Volcanoes
Volcano Pictures (http://classroomclipart.com/cgi-bin/kids/imageFolio.cgi?direct=Geography/Volcanoes)