A volcano is an opening in the Earth. Melted rock can come out. Gas can come out too.

Melted rock under the ground is called magma. Out on the surface, it is called lava. Lava flows out. It cools. It hardens into solid rock.

Some volcanoes burst with a bang. They send ash and gas high into the air. Others let hot lava ooze out slowly.

Gas is trapped in magma. It can make a volcano burst out. It is a bit like a shaken bottle of soda when it is opened.

Ash is made of tiny pieces of rock. Wind can carry ash hundreds of miles from the volcano.

Some volcanoes are steep, cone-shaped mountains. They are built up in layers.

Shield volcanoes are wide. They have gentle slopes. Their lava is runny. It flows far before it hardens.

Volcanoes also erupt under the sea. They can build whole chains of islands. The Hawaiian Islands are one.

Words to know

• magma: melted rock underground
• ash: tiny rock pieces