Volcano
A volcano is a geologic structure formed where magma comes close to the surface of a planet. This magma can break the surface and flow as lava (rare), cause rocks above it to melt and flow as lava, or its presence can cause the ejection of cinders, volcanic glass, steam, ash and gases of various temperatures.
One way of classifying volcanoes is by the type of material erupted, which also affects the shape of the volcano:
- The Lava Shield: Hawaii and Iceland contain examples of volcanoes which extrude huge quantities of lava which gradually builds up to form a tall wide mountain. Their lava is very fluid and solidifies as basalt. 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 huge shield volcano on Mars, and the tallest mountain in the solar system.
- Smaller versions of the lava shield include the Lava Dome, Cone, and Mound.
- If the magma contains a lot (>65%) of silica the lava is called acidicand is 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 gets plugged up. Mt. Pelée on the island of Martinique for example.
- If, on the other hand the magma contains relatively small (<52%) amounts 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 lava flow which was produced by 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 sq.kms.
- Cinder Cones result from eruptions that throw out pieces of rock that build up around the vent. These can be relatively short lived and produce a hill perhaps 1000 feet high.
- Strato-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 and rocks.
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. Under water, volcanoes often form rather steep pillars and in due time break the ocean surface in new islands.
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. Volcanic activity is often accompanied by earthquakes, hot springs, fumaroles, solfatares 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's 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 hasn't 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.
Famous volcanoes include Etna, Hekla, Krakatoa, Vesuvius, Mt. Fuji, and Mount Saint Helens.
See also Surtsey, Santorini, Io, Triton (moon), and Mount Baker.
See Volcanic Explosivity Index.
Public domain picture from NASA of Tambora, Indonesia.
External link and reference
Volcanos by Cliff Ollier, Published by Basil Blackwell, Oxford, UK, 1988, ISBN: 0-631-15664-X (hardback), ISBN: 0-631-15977-0 (paperback).