A volcano is an opening in Earth’s crust through which molten rock, ash and gases escape from deep below. Volcanoes can build mountains, create new islands and also cause destruction.

Magma and lava

Deep underground, rock melts into a hot liquid called magma. Because magma is lighter than the surrounding rock, it rises and collects in a magma chamber. When it erupts onto the surface, it is called lava.

Why do volcanoes erupt?

Magma contains dissolved gases such as water vapour and carbon dioxide. As magma rises, pressure drops and the gases form bubbles, just like opening a shaken soda bottle. When pressure becomes too great, the magma bursts out.

• Runny magma lets gas escape easily, causing gentle lava flows (as in Hawaii).
• Thick, sticky magma traps gas, leading to violent explosive eruptions with ash clouds.

Where do volcanoes form?

• Where plates collide: one plate sinks under another and melts, as in Japan and Indonesia.
• Where plates pull apart: magma rises to fill the gap, as in Iceland.
• Hotspots: plumes of heat in the middle of a plate, as in Hawaii.

About three-quarters of the world’s active volcanoes lie along the Pacific Ring of Fire.

Types of volcanoes

• Shield volcanoes: broad, gentle slopes built by runny lava.
• Composite (strato) volcanoes: tall cones built from layers of lava and ash, such as Mount Fuji.
• Cinder cones: small, steep hills of volcanic fragments.

Volcanoes in India

Barren Island in the Andaman Sea is India’s only confirmed active volcano. India’s huge Deccan Traps were formed by massive volcanic eruptions about 66 million years ago, and their weathered basalt created the fertile black soil used for growing cotton.

Benefits of volcanoes

Volcanic soils are very fertile, volcanic heat provides geothermal energy, and eruptions have built entire islands.