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. While it is now known that 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 so-called hotspot volcanoes.
The study of volcanoes is called vulcanology (or volcanology in some spellings).
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, and it is not infrequently so described. The truth is that a volcano seldom emits either smoke or flame. What is mistaken for smoke consists of vast volumes of fine dust, mingled with steam and other vapours—chiefly sulphurous. 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.
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Perhaps the most conspicuous part of a volcano is the crater, a basin, roughly of a 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.
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 30,000 feet high (it sits on the sea floor) and 75 miles in diameter. Olympus Mons is a shield volcano on Mars, and the tallest mountain in the solar system.
Smaller versions of the "lava shield" include the lava dome (tholoid), lava cone, and lava mound.
If the magma contains a high percentage (>65%) of silica the lava is called acidic and tends to be very viscous (not very fluid) and is pushed up in a blob which will then solidify, Lassen Peak in California is an example. This type of volcano has a tendency to explode because it easily plugs. Mt. Pelée 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 basic, and it will be very fluid, capable of flowing like water for long distances. A good example of this is the Great þjórsárhraun (Thjórsárhraun) lava flow which was produced by an eruptive fissure almost in the geographical center of Iceland roughly 8.000 years ago, and it flowed all the way down to the sea, a distance of 130 kilometers, and covered an area of 800 square km.
Volcanic 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 100 to 1000 feet 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 are a class of volcanoes that 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 very hard to define given that there is no minimum requirement to be categorized as a supervolcano.
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.
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.
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.