Why do volcanoes erupt?

Magma is buoyant underground

Hot rock can partially melt deep inside Earth. The resulting magma is often less dense than surrounding solid rock, so it rises through fractures and weak zones. Magma can also be forced upward by pressure from new material entering a chamber.

As magma rises, the pressure around it drops. Gases dissolved at depth become bubbles, just as carbon dioxide comes out of a fizzy drink when opened.

Viscosity controls the release

Runny basaltic magma lets bubbles escape more easily and can produce lava flows. Silica-rich magma is more viscous; gas can become trapped, building pressure until the eruption is explosive. Water and ice can add steam and fragment magma.

The same volcano can produce different behaviour over time if magma composition, supply rate or the surrounding water changes.

An eruption is a process, not one moment

Magma may intrude underground, deform the ground, release gases and cause small earthquakes before reaching the surface. Monitoring these signals helps scientists assess changing hazard, but no single signal guarantees an eruption date.

Volcanoes are openings in a dynamic planet. Their eruptions reflect the interaction of heat, buoyancy, gas, rock strength and surface conditions.

Why water can make an eruption more violent

When magma meets shallow groundwater or an ice cap, liquid water can flash into expanding steam. That rapid expansion can fragment magma and widen the hazard beyond a simple lava flow. Scientists therefore consider the surrounding terrain and water supply, not only the chemistry of the magma itself.