Volcanoes: Mountains of Fire

A volcano is an opening in the Earth's surface where hot melted rock, gas, and ash can escape. Deep underground, it is so hot that rock melts into a thick, glowing liquid called magma. The magma is lighter than the solid rock around it, so it slowly rises towards the surface, gathering in a space called a magma chamber.

Pressure builds up under the ground, a bit like shaking a fizzy drink before you open it. Gases trapped inside the magma push harder and harder. When the pressure gets too strong, the volcano erupts and lava bursts or flows out. Some eruptions are gentle, with runny lava that oozes slowly downhill. Others are explosive, blasting ash and rock high into the sky.

Why does runny lava behave so differently from sticky lava? It is all about how thick the magma is. Thin, runny magma lets gas escape easily, so it flows gently like syrup. Thick, sticky magma traps the gas until it explodes all at once — that is what makes some volcanoes so dangerous.