What causes a volcano to erupt, how they formed and different types of volcano revealed

THERE are around 1,500 volcanoes on land which have been active in the last 10,000 years spewing ash into the atmosphere periodically.

As about 50 to 60 volcanic eruptions happen each month we’ve brought together all the facts that you should know about the spectacular and potentially devastating natural events.

What makes a volcano erupt?

There are several different causes for a volcano to erupt which all fundamentally come down to a  pressure change within the volcano which forces the magma to overflow the chamber it is held in.

The most common type of eruption is caused by the movement of tectonic plates.

When one is pushed under the other the magma, sediment and seawater is forced into the chamber which eventually overflows and the volcano erupts spewing lava into the sky.

This kind of eruption produces sticky, thick lava at temperatures from 800 to 1,000C.

The second type of eruption caused by tectonic plates is  when the plates move away from each other allowing magma to rise and fill the the gap, which can cause a gentle explosion of thin lava of temperatures between 800 to 1,200C.

Decreasing temperatures can cause old magma to crystallise and sink to the bottom of the chamber and this movement can force fresh liquid magma up and out – similar to dropping a brick in a bucket of water.

Finally a decrease in external pressure can trigger an eruption as it may minimise the volcano’s ability to hold back by increasing the pressures inside the magma chamber.

This kind of eruption can be caused by natural events such as typhoons, which decrease rock density, and glacial melting on the top of the volcano which alters the molten rock composition.

Glacial melting is believed to be one cause behind the 2010 Eyjafjallajökull eruption in Iceland.

How are volcanoes formed?

Volcanoes are formed by eruptions of lava and ash when magma rises through cracks or weak-spots in the Earth’s crust.

A build up of pressure in the earth is released, by things such as a plate movement which forces molten rock to exploded into the air causing a volcanic eruption.

The lava from the eruption then cools to form a new crust.

Over time, generally several eruptions, the rock builds up and forms a volcano.

What are the different types of Volcano?

There are three different types of volcano and each produces and releases a different type of lava:

• Shield volcano These volcanoes have a flat dome-like appearance and release lava in a gentle manner that is generally slow and easy for humans and animals to out run. They produce Basaltic lava, which is the most common type found on earth. Shield volcanoes tend to erupt frequently but are relatively gentle eruptions. Kilauea volcano in Hawaii is a shield volcano. They tend to erupt more frequently and on a smaller scale than other volcanoes.
• Stratovolcano This type has the classic cone shape associated with volcanoes. These volcanoes produce large violent eruptions which often lead to mud-slides and produce fast moving clouds of hot gas and volcanic matter called pyroclastic flow. Stratovolacnoes produce andesitic magma, which is molten rock formed at plate boundaries which typically cools and hardens before spreading too far but can reach up to temperatures of 400C. Krakatoa, Vesuvius and Mount Agung are examples of this type of volcano.

• Caldera volcano These volcanoes have a circular basin-shaped appearance and release a thick lava which is between 650 to 800C. After an eruption, which is normally cause by a pressure build up, the crust above the chamber collapses inwards. This creates a depression in the surface and is sometimes referred to as a crater, but it is actually a type of sink hole. Caldera Volcanoes are a common type of Super Volcanoes, which have large eruptions of effecting at least 1,000km3, such as Yellowstone in America. Other caldera volcanoes include Mont Elgon in Ugnada/Kenya, Mount Fogo in Cape Verde, and Mount Meru in Tanzania.