Section through volcano

Composite Volcanoes (or Stratovolcanoes)


Composite, or stratovolcanoes, are typically steep sided and may be created over many thousands of years - possibly hundreds of thousands of years - and dozens of eruptions. They are slowly built up by the repeated eruption of lava flows, pyroclastic flows and ash, forming a cone shape with steep sides and a crater at the top. (A pyroclastic flow is a very high temperature (over 400° C) mixture of steam, ash, rock and dust that travels at very high speeds.)

Of the 1,511 volcanoes that are known to have erupted in the past 10,000 years, 699 of them are stratovolcanoes.

When composite volcanoes erupt we are usually treated to a violent and very explosive event. In fact, only the composite volcanoes that produce basalt have reasonably gentle eruptions. Mount St Helens, in the Cascades range (USA), is a stratovolcano that has erupted at least 35 times in the last 3,500 years. In 1980 it erupted with a blast equivalent to 500 times the power of the atomic bomb that flattened Hiroshima.

Popocatapetl, in Mexico, is another well known stratovolcano.

A composite volcano typically has several separate vents, with secondary cinder cones and craters. These are the typical volcanoes seen in paintings and films; triangular, steep, high and violently explosive.


Structure of a Composite Volcano


Main Vent

The Main vent is the path taken by the liquid rock from the Magma Chamber to the surface. It is like a pipe up which the lava can flow. Sometimes the main vent has branches which, if they reach the surface, may form Secondary Cones or Fumeroles.
When a volcano erupts, lava, gasses and fragments of rock travel right up the main vent and are thrown out through the crater. When the eruption finishes the lava may drop back down the pipe or form a lava lake in the crater.

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Lava Flow

When magma reaches the surface it is called lava. ( pronounced 'larver'). If the level of lava in the crater overflows the rim, it will begin to flow down the sides of the volcano. Different types of lava are more runny than others and the most runny types can flow down the volcano at over 30 miles per hour.
Although the rock is liquid, is is very dense. If you stood on it, you wouldn't sink, but you would burn. The rock surface may seem to have cooled and set, but below the very thin crust the lava can remain liquid for a long time, at temperatures of 500°C or more.

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Ash and Lava Strata

The volcano in the diagram is made of layers, coloured light and dark grey. These are the layers of ash and lava which have been thrown out of the volcano during its life. Ash consists of small fragments of rock, some as fine as tiny dust particles, other chunks being bigger than your fist. The layers of lava are old lava flows which cooled and set as they flowed down the volcano. The volcanic ash is usually thrown out of the volcano before the lava. It settles to the ground and forms a steep sided pile. You can see the same effect by pouring salt onto a flat surface. It will form a pyramid, the steepness of which is determined by the size and shape of the individual salt grains. The ash is soft and is easily worn away by wind and rain. The layers of lava which flow over the ash protect it from erosion and thus allow the volcano to keep growing upwards.

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Secondary Cone

When the Main Vent develops branches, the volcano may begin to grow secondary cones. The rock and ash layers which make up the volcano are often cracked and weakened by the explosions that occur during eruptions. If these cracks form a path from the main vent to the surface, magma is able to move up the new pipe and reach the surface. As it erupts the ash and lava is sprayed into the air like a fountain. Splashes of lava mix with the ash to form a new cone. Given time, a secondary cone may divert so much of the magma that it becomes the main vent and the original cone becomes less important.

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Magma Chamber

Deep below the Earth's surface, between 100 and 200km down, the rocks are semi liquid. In certain parts of the world there are 'hot spots', areas where the rocks are hotter than elsewhere. These areas are believed to be the sources of the magma which rises to the surface via volcanoes. Although diagrams often show the Magma Chamber as having well defined sides, it is very unlikely that they are like that. It is much more likely that the rocks slowly cool as distance from the hot spot increases, so the rock turns from liquid, to semi-molten, to thick sticky stuff and finally becomes solid.

Hot spots change with the passing of time. We know this because we can find extinct volcanoes in areas which have no volcanic activity today. Areas of Cornwall in the UK, such as Saltash, have rocks containing volcanic ash dating from the Devonian period, some 410 million to 360 million years ago, and in Scotland, Edinburgh Castle is built on the remains of a volcano. Neither area is now volcanically active.

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A fumerole is a crack in the surface through which steam and gas can escape. The magma below the surface heats water to the point where it turns to steam and is able to dissolve minerals from the surrounding rock. As the gas reaches the surface it is both hot and under pressure. It cools and expands, depositing the dissolved minerals around the vent. In some parts of the world local industries have built up collecting the sulphur deposited around fumerole vents.

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A volcanic crater is a funnel shaped hollow at the top of the vent. It is formed as lava, gas and ash are blasted upwards from the main vent. Material falls back down to earth around the vent and slowly piles up forming a rim around it. The inside of the crater is kept clear by the force of upward moving material constantly removing any debris which falls there.

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Ash and Gas Clouds

Gas escapes all the time from active volcanoes. It may be just steam, ( 90% of all volcanic gas is water and carbon dioxide ) but more often is water vapour containing dissolved minerals such as sulphur. During an eruption the volume of gas released increases considerably, tonnes of material being thrown into the atmosphere, forming a mixture of gas, ash and rock fragments. Along with the smaller particles there are often larger chunks of liquid rock, thrown high into the air by the force of the eruption. These pieces of rock cool as they spin through the air, forming rod shaped chunks which are called 'volcanic bombs'. Dangerous though the bombs are, they are not usually the cause of most casualties. The hot ash and poisonous gasses kill many more people.

Clouds of red hot ash can flow, almost like a liquid, down the side of the volcano at speeds of over 250 km/hour. Anything in the path of the cloud, called an ash flow or a nuée ardent, is burned or suffocated. Ash flows have been known to travel 400km from their origin.

Volcanoes also produce invisible gasses which are heavier than the surrounding air. Many people who have sought shelter from ash and bombs, by hiding in cellars, have been killed by these gasses which include carbon monoxide and sulphuric acid droplets. The gasses are highly toxic to both people and animals.

The famous Roman town of Pompeii, in Italy, was covered by a white hot cloud of ash and gas on 24th August AD79 when Mount Vesuvius erupted. In a day, the town and many of its inhabitants had been suffocated by sulphurous fumes and buried by the ash. Plaster casts of people have been recovered from the ash debris, their hands and clothing held over their faces in an attempt to protect themselves.

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