Volcano

:This article is about volcanoes. For the action movie, see Volcano (movie). For other meanings of the word eruption, see eruption (disambiguation)

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 or "Acidic" 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 Pelée on the island of Martinique is another example.

Related Topics:
Silica - Felsic - Viscous - Lassen Peak - California - Mount Pelée - Martinique

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If, on the other hand, the magma contains relatively small amounts (<52%) of silica, the lava is called mafic or "Basic" 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 Þjórsárhraun (Thjórsárhraun) 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.

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• 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 tall (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 are tall conical mountains composed of both lava flows and ejected material, which form the strata which give rise to the name. Classic examples include Mt. Fuji in Japan and Mount Mayon in the Philippines.
• 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.
• 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.
• 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 hotspots. Volcanoes may be either dormant (having no activity) or active (near constant expulsion and occasional eruptions), and change state unpredictably.

Related Topics:
Tectonic plates - Hotspot

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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.

Related Topics:
Cone - Cinder cone - Water - Pillar - Ocean - Island

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Behavior of volcanoes

There are many different kinds of volcanic activity and eruptions:

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• 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.

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Volcanic activity is often accompanied by earthquakes, hot springs, fumaroles, mud pots and geysers. Low-magnitude earthquakes often precede eruptions.

Related Topics:
Earthquake - Hot spring - Fumarole - Mud pot - Geyser

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Active, Dormant, or Extinct?

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?

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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.

Related Topics:
Mediterranean - Hawaii

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Dormant volcanoes are those that are not currently active (as defined above), but could become restless or erupt again.

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Extinct volcanoes are those that scientists consider unlikely to erupt again. Whether a volcano is truly extinct is often difficult to determine. 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.

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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.

Related Topics:
Yellowstone Caldera - Supervolcano - Yellowstone National Park

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Volcanoes on Earth

:Main article: List of volcanoes

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• Mount Baker (Washington, USA)
• Cold Bay Volcano (Alaska, USA)
• El Chichon/El Chichonal, (Chiapas, Mexico)
• Citlaltépetl/Pico de Orizaba, (Veracruz/Puebla, Mexico)
• Cotopaxi (Ecuador)
• Mount Fuji (Honshu, Japan)
• Mount Hood (Oregon, USA)
• Mount Erebus (Ross Island, Antarctica)
• Etna (Sicily, Italy)
• Krafla (Iceland)
• Hekla (Iceland)
• Kick-'em-Jenny, (Grenada)
• Kilauea (Hawaii, USA)
• Kluchevskaya (Kamchatka, Russia)
• Krakatoa (Rakata, Indonesia)
• Mauna Kea (Hawaii, USA)
• Mauna Loa (Hawaii, USA)
• El Misti (Arequipa, Peru)
• Novarupta (Alaska, USA)
• Paricutín (Michoacán, Mexico)
• Mount Pinatubo (Luzon Island, Philippines)
• Popocatépetl (Mexico-Puebla state line, Mexico)
• Santorini (Santorini islands, Greece)
• Soufriere Hills volcano, (Montserrat)
• 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 Earth's Moon has no large volcanoes, but does have many volcanic features such as rilles and domes.

Related Topics:
Moon - Rille - Domes

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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.

Related Topics:
Venus - Basalt

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There are several extinct volcanoes on Mars, four of which are vast shield volcanoes far bigger than any on Earth:

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• 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.

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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 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.

Related Topics:
Jupiter - Moon - Io - Tidal - Sulfur - Sulfur dioxide - Silicate - 2001 - List of geological features on Io

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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

Related Topics:
Cassini-Huygens - Saturnian - Titan

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Many 'ice volcanoes' have been found on Triton, a moon of Neptune; they are believed to eject liquid nitrogen, dust, or methane compounds.

Related Topics:
Ice volcanoes - Triton - Moon - Neptune - Nitrogen - Methane

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The Kuiper Belt Object Quaoar is also suspected of having ice volcanoes.

Related Topics:
Kuiper Belt Object - Quaoar

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