A volcano is a geological landform formed where magma comes close to the surface of a planet. On earth, this phenomenon tends to occur near the boundaries of the continental plates (see, however, hotspot volcanoes).
Like most of the interior of the Earth, the movements and dynamics of magma is poorly understood. However, it is known that the volcanic process begins when magma rises to near the surface deep beneath a volcano, occupying a magma chamber. Magma in the chamber is forced upwards and flows from a vent as lava, or can react with water in the surrounding landform and cause explosive discharges of steam, escaping gases from the magma, and ejection of rocks, cinders, volcanic glass, and volcanic ash.
The study of volcanos is called vulcanology (or volcanology in some spellings).
Most volcanoes are formed at destructive plate margins, where oceanic crust sinks below the continental crust because oceanic crust is denser than its continental counterpart. Friction will cause the oceanic crust to melt, and the reduced density will force the newly formed magma to rise. As the magma rises it pushes through the continental crust, erupting as 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.
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. Volcanos 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.
Volcano types and structural components
Shield volcano: Hawaii and Iceland are examples of volcanoes which extrude huge quantities of lava that gradually builds to form a wide mountain. Their lava is generally very fluid and solidifies in long flows 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 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 acidic and 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 plugs. Mt. Pelée on the island of Martinique is another 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 jórsárhraun 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 and rocks.
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.
Methods used in 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.
Volcanologists use the following to forecast eruptions.
Seismicity
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 witihin two days from Popocatépetl, on the outskirts of Mexico city. Their prediction used reserarch done by M. Chouet, a Swiss vulacanologist, 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 Popocatépetl's largest eruption for a thousand years and yet no one was hurt.
Gas emissions
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 tons 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 tons, 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.
Volcanic activity
There are many different kinds of volcanic activity and eruptions:
phreatic (steam) eruptions
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, solfatare 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.
