Why do volcanoes erupt?

Deep under our feet, the Earth is so hot that rock melts. Melted rock underground is called magma.

Magma is lighter than the solid rock around it, so it slowly rises, collecting in large pockets beneath the surface. Gases dissolved in the magma — mostly steam and carbon dioxide — build up pressure, like the gas in a shaken bottle of soda.

When the pressure becomes too great, the magma breaks through a crack in the Earth’s crust. Once it reaches the surface it is called lava. An eruption can be a slow red river or a violent explosion, depending on how thick the magma is and how much gas it holds.

India has few active volcanoes — Barren Island in the Andamans is the only one — because the country sits in the middle of a plate, not on an edge where plates meet.