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Volcanoes can look like small mountains or hills.

A volcano is an opening in the Earth's crust that allows magma, hot ash and gases to escape.

There are two main types of volcano:

• Composite volcanoes are the most common type of volcano. They can have violent eruptions and can grow bigger as layers of thick lava and ash harden on top of each other. Mount Etna in Sicily, Italy, is an example of a composite volcano.

• Shield volcanoes do not have such violent eruptions. These volcanoes tend to have gentle slopes and their runnier lava spreads and hardens over a wider area. Mauna Loa in Hawaii is an example of a shield volcano.

Most volcanic eruptions are caused by pieces of the Earth's crust, called tectonic plates, moving towards each other.

Some volcanoes, like Mauna Loa in Hawaii are caused by hot spots in the Earth’s crust. These do not erupt violently and lava usually flows slowly out of them.

Eruptions from volcanoes can be very dangerous. They can produce:

• pyroclastic flows - fast moving clouds of hot ash, gas and rock
• ash clouds - small pieces of rock and glass that can be carried in the air for many kilometres
• volcanic bombs - large bits of very hot rock blown out of a volcano

Magma, a mixture of hot, molten rock and gas, builds up deep beneath the surface of the Earth under enormous pressure. The magma rises, looking for weaknesses through rocks in the Earth’s crust.

Volcanoes are usually found along the boundaries of tectonic plates.

• Constructive plate boundaries are where tectonic plates are moving apart, and magma here can gradually rise and form new crust, usually without any violent eruptions.

• Destructive plate boundaries are where tectonic plate boundaries are colliding or pushing against each other. The intense pressure involved can create new magma which then rises to the surface through volcanic vents in explosive eruptions.

When magma erupts from a volcano it is called lava. This mixture of lava and gas flows out and down the sides of a volcano, some may cool and erupt as ash.