Why do Volcanoes Erupt?

On our planet, there are about 530 active volcanoes, which might erupt at any time.

Some of them lie under the sea. Each year, only about 25 volcanoes erupt.

Some, such as Stromboli in Italy, erupt frequently.

On the image below, look at the edges of the large sections, or plates, in the earth's crust.
The shaded areas show where most earthquakes happen. Most volcanoes also occur near the edges of these plates.

 

What happens inside the earth?

Underneath its surface, the earth is a mass of hot rocks and metals. In the mantle, the layer under the earth's crust, it is so hot that some rocks have melted and become molten rocks.
As the rocks melt, they produce gases. The mixture of gas and molten rock is called magma. 
Because it is mixed with - bubbles of gas, the molten rock weighs less than the solid rocks around it. The magma rises towards the earth's surface and collects in hollow spaces called chambers. Some magma chambers are only 1.8 miles (3 kilometers) underground.

Near the edges of the sections called tectonic plates, there are weak places called cracks, or fissures. Strong pressure underground forces the magma to carve a tunnel up through these cracks. When the magma bursts out, a volcano erupts. Gases from the magma become part of the earth's atmosphere, and a hot, fiery liquid, called lava, flows over the ground.

If the lava is quite thin, like soup, it spreads out and makes a wide, flat volcano. Mauna Loa, on Hawaii in the Pacific Ocean, is a volcano that was formed like this. When the lava is thick, like syrup, it makes a cone-shaped volcano with steep sides, such as Mount Fuji in Japan.

Sometimes, underground water is heated by very hot rocks to a temperature above the boiling point. The water changes to steam. Then jets of boiling water and steam, called geysers, burst out of the ground. The rocks that boil the water are sometimes heated by magma chambers.
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