Volcanoes are holes or cracks in the Earth's crust through which molten rock erupts.

They usually occur at structural weaknesses in the crust, often in regions of geological instability, such as the edges of crustal plates.

Volcanoes are important to us as humans because they provide information about the Earth's interior, and because volcanically-formed soils are highly fertile and good for growing crops. Volcanoes are also nature's pressure valves, without them the Earth would quite literally explode.

Violent eruptions, however, can devastate huge areas, and accurate techniques for predicting eruptions are essential if major disasters are to be avoided.

Volcano Creation

Scientists do not fully understand the process by which volcanoes are formed. It seems that at points where the Earth's mantle (the layer imme­diately beneath the crust) is particularly hot (hot spots) or where part of the crust is being forced down into the mantle (e.g. where two crustal plates meet and one is forced down under the other) the heat causes the lower part of the crust or upper part of the mantle to melt.

The molten rock - called magma - is under pressure as more magma forms and, being less dense than the sur­rounding rock, it rises, often along lines of weak­ness such as faults or joints in the crust. As the magma rises it melts a channel for itself in the rock and accumulates, together with gases released from the melting rock, in a magma chamber a few kilometers below the Earth's surface.

Eventually the pressure from the magma and gas builds up to such an extent that an eruption occurs, blasting a vent through the surface rocks.

Volcano LavaLava (magma after emission) piles up around the vent to form a volcanic mountain or, if the erup­tion is from a fissure, a lava plateau.

The volcano then undergoes periodic eruptions of gases, lava and rock fragments. It is termed active, dormant or extinct, according to the frequency with which it erupts.

Vents associated with declining volcanic activity and the cooling of lava periodically emit steam or hot water, and are often valuable sources of energy or minerals: solfataras (which are rich in sulfur) and fumaroles give out steam and gas; geysers are hot springs that eject jets of hot water or steam at regular intervals as underground water is heated to boiling point by the magma.

The characteristics of eruptions vary greatly from volcano to volcano, and those typical of any one volcano change over the years.

Eruptions are classified according to their explosiveness, which depends on the composition (especially the gas content) and viscosity of the magma involved, which in turn depends largely on the depth at which the rock becomes molten.

Relatively vis­cous magma causes explosive eruptions; sticky magma often forms a plug in the neck of the volcano, blocking further eruptions until enough magma and gas have accumulated for their pres­sure to blast away the plug and allow the emission of gas, lava and fragmented magma (tephra). This accumulation may take several decades, or even centuries.

Some explosive eruptions are quite small, but others (those in which large amounts of gas are trapped in the magma) are so violent that they blast away a large part of a mountain or a whole island.

Volcano Lava SprayVolcanoes formed mainly of rock fragments are generally steep-sided cones (with slopes of between 20° and 40° to the horizontal), because any fragments blasted into the air fall back near the vent.

Those formed chiefly of viscous lava are usually highly convex domes (typically about 150m high and 400m across), because the lava is too thick to flow far before solidifying. Exception­ally viscous lava may solidify in the vent. The solid mass may then be forced slowly upwards, forming a spine that rises several hundred meters above the summit. This movement usually precedes a particularly violent eruption, caused by the sud­den release of the accumulated pressure of the magma and gas. In 1902, such an event accom­panied the destructive eruption of Mont Pelee on the island of Martinique in the West Indies.

At the other extreme, relatively fluid magma is extruded quite freely and quietly, with small erup­tions that occur at frequent intervals or even con­tinuously. The lava flows for long distances before it solidifies, and therefore forms a low, broad dome, or shield volcano (usually with slopes of less than about 10°), such as Mauna Loa on Hawaii; the island rises about 10,000m from a sea-floor base 110km in diameter and is the largest volcano on Earth. Mauna Loa is also one of the most active and is thought to have been erupting for the last 700,000 years at least.

Underwater Volcanoes

Submarine volcanoes are particularly common near oceanic ridges, where magma is constantly extruded as the continental plates drift apart. Many also form over hot spots. As the crust moves, the volcano also moves away from the hot spot and becomes extinct; a new volcano forms directly over the original hot spot, and a chain of volcanoes gradually forms.

In oceanic ridges and hot spots the lava is formed from mantle material that is forced up by deep convection currents. This lava is dense but fluid, unlike the silica-rich lava produced by melt­ing crustal material, found in continental areas and offshore island chains.

Where it appears above the water surface - in Iceland and Hawaii, for example - it forms flat lava plateau or shield volcanoes.

Marine volcanic activity may lead to the sud­den creation of islands (e.g. Surtsey, off Iceland, in 1963). Volcanic islands are subject to severe ero­sion by the sea, and may also subside when they move away from a ridge or hot spot and cease to be active.

There are more than 2,000 submerged - usually extinct - volcanoes (seamounts) in the world; those that have been eroded nearly to sea level and then subsequently submerged, which are known as guyots, are also common.

Volcano Prectiction

Prediction of eruptions is of great importance because of the extensive damage they can cause to surrounding areas, which are often fertile and densely-populated.

Volcanic activity used to be assessed in terms of temperature and pressure, measured by means of borings into the sides of the vent. Recently, however, geologists have come to rely more on seismography, on measurements of changes in emissions of gas and its sulfur diox­ide content, and on detecting activity inside the crater (monitored with mirrors).

Most of all, they look for changes in the angle of the mountainside (measured with tiltmeters): any expansion in one part of the mountain indicates that an eruption there is likely.

Further information is obtained from analyses of the mineral content of the local water, recordings of vertical ground swelling, and readings from geodimeters, which use lasers to measure minute swellings in the ground.

These techniques are, however, by no means perfect. They were in use on Mount St Helens in the State of Washington, USA, when it erupted in May 1980 but, despite the fact that scientists were aware that an eruption was imminent, they were not able to anticipate the time, force or exact direction of the blast.

The Mount St Helens eruption

Mount St Helens eruptionMount St Helens is one of a chain of continental volcanoes in the Cascade Range in the north­western United States. All the volcanoes in this mountain range are the result of the Pacific oceanic crustal plate being forced down into the mantle by the North American continental plate riding over it. The molten parts of the oceanic plate then rise through the crustal material, form­ing volcanoes.

Normally an eruptive phase involves several of the Cascade Range volcanoes. During the nineteenth century, for example, Mount St Helens erupted three times, simultane­ously with nearby Mount Baker. Because of these coincident eruptions, some scientists believe that the two volcanoes may have a common origin where, at a depth of about 200km below the sur­face, the Pacific crustal plate is being overridden by the North American plate.

After 123 years of dormancy, Mount St Helens erupted in May 1980 - one of the most violent (and closely-monitored) eruptions at the time. Volcanic activity was first noticed on March 20, when small tremors began and the mountain top started to bulge; about a week later fissures in the flank of the volcano emitted steam.

The first violent eruption occurred on May 18, when the slow accumulation of pressure within the volcano was released with explosive force. The north flank of the mountain collapsed and the contents of the vent were blasted out.

The abrupt release of pressure caused the gas dissolved in the magma to come out of solution suddenly, forming bubbles throughout the hot mass - rather like the sudden formation of bubbles in champagne when the bottle is uncorked. A white-hot cloud of gas and pulverized magma (called a nuee ardente) then swept over the surrounding countryside, engulf­ing everything within a distance of about 8km from the peak. (This phenomenon also occurred when Mont Pelee erupted in 1902; within a few minutes of the eruption the cloud had covered Saint-Pierre, then the capital of Martinique, killing its 30,000 inhabitants.) At the same time, a verti­cal column of dust and ash was blown upwards. These two major effects were accompanied by a blast of air caused by the sudden expansion of the freed gases; the blast was so powerful that it flattened all trees near the volcano and knocked down some as far as 25km away.

The nuee ardente and the vertical ash column produced cauliflower-shaped clouds 32km wide that eventually reached a height of 24km. The ash in this cloud consisted mainly of silica, a reflection of the high silica content of the material emitted by continental volcanoes.

The ash falling back to earth and the debris of the collapsed flank (which amounted to about 3.7 cubic kilometers) combined with the water of nearby rivers and the meltwater of the mountain snows to form a mudflow (called a labor).The mudflow plunged along the river valleys at speeds of up to about 80km/hr, destroying bridges and settlements as far as 20km downstream; in some places, the mud deposited by this flow was as much as 130m deep.

Although the May eruption is perhaps the best known, Mount St Helens erupted several times during the later part of the year. Each eruption was preceded by the growth of a dome of volcanic material in the crater left by the initial explosion, and the general pattern of the subsequent erup­tions resembled that of the first.