A volcano is an opening in the Earth. Melted rock and gas can come out. Melted rock under the ground is called magma. When it comes out onto the surface, it is called lava. Magma rises because it is lighter than the rock around it.

Some volcanoes burst with a bang and send ash and gas high into the air, while others let hot lava ooze out slowly. An eruption can be explosive or effusive. Which one happens depends largely on how much gas is in the magma.

Thick, sticky magma holds gas in. It tends to erupt more explosively than runny magma. Gas trapped in magma can make a volcano burst out, a bit like a shaken bottle of soda when it is opened.

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

Explosive eruptions can produce pyroclastic flows. They are fast-moving mixtures of hot gas, ash and rock that sweep down valleys. A lahar is a mudflow of water and volcanic debris. It rushes down the slopes and valleys of a volcano.

The 1815 eruption of Mount Tambora in Indonesia was the largest volcanic explosion in recorded history. Ash and aerosols from it helped cause 1816 to become known as the Year Without a Summer. After the 1883 eruption of Krakatau, sunsets and the sky took on unusual colors around the world.

Composite volcanoes are also called stratovolcanoes. They get their name from the layers of lava and ash that build them.

Words to know

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