What causes volcanoes?

Sarychev Volcano seen from space


In some special places under the Earth’s crust, there is red-hot molten rock, called magma.

Volcanoes happen when magma rises to the surface of the earth, which causes bubbles of gas to appear in it.

This gas can cause pressure to build up in the mountain, and it eventually explodes. When the magma bursts out of the earth, it is called lava.


Are there different types of volcanoes?

Mount Etna

The type of magma in the earth can create different volcanoes.

If the magma is quite thin, the gas can escape easily and there will not be an explosion. The magma just comes out of the mountain and flows down the sides, like Volcanoes in Hawaii and Mount Etna.

But, if the magma is thick and sticky, the gas cannot escape, so it builds up and up until it explodes.

This can cause landslides and sends out huge clouds of burning rock and gas, which devastate everything around them, like the famous eruptions at Mount St Helens and Montserrat.

Can we predict when a volcano is going to erupt?

The Anak Krakatau volcano

Yes and no.

Scientists who specialise in volcanoes are called volcanologists. They are growing more and more confident at predicting when volcanoes will erupt in the short-term.

If a volcano was going to erupt in one hour they’d have a good idea it was going to happen. If it was going to blow in a week they’d be less sure, and in six months even less so.

The further a volcano is from erupting, the harder it is to predict. Working out if a volcano will erupt in future years is still impossible.

Volcanologists combine several techniques to predict what will happen.

They use monitors to detect movement in the rocks that make up the volcano and in the earth’s crust. They also measure the gases that come out of the volcanic mountains, and even the angle of the slopes.

If an eruption is likely to happen very soon the the behaviour of animals in the area can be a clue.

Animals often seem to be able to ‘detect’ when an eruption is coming, and they become agitated and worried.

And volcanologists are always trying to find new ways to detect eruptions. Some are now using satellites to try to understand how and when they may blow.


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