1.2 Behavior of volcanoes

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, 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.

1.3 Volcanoes on Earth

Main article: List of volcanoes

• Etna ( Sicily, Italy)
• Hekla ( Iceland)
• Kilauea ( Hawaii, USA)
• Krakatoa ( Rakata, Indonesia)
• Mauna Loa ( Hawaii, USA)
• Mauna Kea ( Hawaii, USA)
• Mount Baker ( Washington, USA)
• Mount Erebus ( Ross Island, Antarctica)
• Mount Hood ( Oregon, USA)
• Mount Fuji ( Honshu, Japan)
• Mount Rainier ( Washington, USA)
• Mount Shasta ( California, USA)
• Mount St. Helens ( Washington, USA)
• Novarupta ( Alaska, USA)
• Popocatépetl ( Mexico- Puebla state line, Mexico)
• Surtsey (Surtsey island, Iceland)
• Santorini (Santorini island, Greece)
• Tambora ( Sumbawa, Indonesia)
• Teide ( Tenerife, Canary Islands, Spain)
• Mount Vesuvius (Bay of Naples, Italy)

1.4 Volcanoes in the universe

On Mars:

• Olympus Mons
• Arsia Mons
• Pavonis Mons
• Ascraeus Mons

Many volcanoes have been found on Io, a moon of the planet Jupiter; they are believed to eject sulfur or possibly sulfur dioxide.

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

2 Volcanology

2.1 Volcano formation

Diagram of a destructive margin causing earthquakes and a volcanic eruption 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, cinder s, 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. 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.

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

2.1.1 Tectonic environments of Volcanos

Volcanos can principally be found in one of 3 tectonic environments:

2.1.1.1 Constructive Plate Margins

These are by far the commonest volcanos 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 volcanos 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 on a ridge, but has different characteristics.

It could be argued that the volcanos of the Great Rift Valley system of East Africa are modified constructive margin volcanos. 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 volcanos.

2.1.1.2 Destructive Plate Margins

These are the most visible and well-known types of volcanos 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.

2.1.1.3 Hotspot situations

Hotspots were originally a catch-all for volcanos 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 centre that is not obviously connected with a plate margin. The classic example is the Hawaiian chain of volcanos 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 a third classical example, but complicated by the coincidence of a hotspot intersecting a 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.

2.2 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. 200px Mount St. Helens erupted explosively on May 18, 1980 at 8:32 a.m. PDT Volcanologists use the following to forecast eruptions.

2.2.1 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 within two days from Popocatépetl, on the outskirts of Mexico City. Their prediction used research 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.

2.2.2 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 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.

Gas emissions from volcanoes are a natural contributor to acid rain.

2.2.3 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.

3 See also

• Prehistoric volcano
• List of volcanoes
• List of famous volcanic eruption deaths
• Volcanic Explosivity Index
• Pacific Ring of Fire
• Geomorphology
• Earth science
• Io
• Triton (moon)
• Top 10 most deadly Volcanic Eruptions

4 Books

• Macdonald, Gordon A., and Agatin T. Abbott. (1970). Volcanoes in the Sea. University of Hawaii Press, Honolulu. 441 p.
• Ollier, Cliff. (1988). Volcanoes. Basil Blackwell, Oxford, UK, BooksEnthsiast.com (hardback), BooksEnthsiast.com (paperback).

5 External links

• Murni's in Bali: Balinese Origins, Volcanoes and Civilization
• Glossary of Volcanic Terms from USGS
• Volcanic and Geologic Terms from Volcano World
• Television program (BBC) on the prediction of Popocatepetl's 2000 eruption
• Smithsonian Global Volcanism Program
• Explore the geologic causes of an eruption
• How Volcanoes Work by Tom Harris

Landforms Natural hazards Plate tectonics Volcanology

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