Key facts

• A volcano is formed by eruptions of lava and ash.
• Volcanoes are usually cone shaped mountains or hills.
• When magma [magma: Magma is molten rock. ] reaches the Earth's surface it is called lava. When the lava cools, it forms rock.
• Volcanic eruptions can happen at destructive [destructive boundaries: The boundary of two plates which are moving towards each other. ] and constructive [constructive boundaries: Areas between two crustal plates that are moving away from each other, causing new crustal rocks to form. May also be referred to as divergent plate boundaries. ] boundaries, but not at conservative boundaries [conservative boundaries: Areas between two crustal plates that are moving past each other in opposite directions. ].
• Some volcanoes happen underwater, along the seabed or ocean floor.

The formation of volcanoes

Krakatoa in the Sunda Strait, Indonesia

1. Magma rises through cracks or weaknesses in the Earth's crust.
2. Pressure builds up inside the Earth.
3. When this pressure is released, eg as a result of plate movement, magma explodes to the surface causing a volcanic eruption.
4. The lava from the eruption cools to form new crust.
5. Over time, after several eruptions, the rock builds up and a volcano forms.