How Do Volcanoes Erupt?

What happens leading up to an eruption?

In the weeks to months before an eruption, magma begins rising from the magma chamber toward the surface. As it rises, pressure decreases, allowing dissolved gases (mostly water vapor, with carbon dioxide and sulfur dioxide) to start coming out of solution and forming bubbles. Volcanoes typically show warning signs during this phase: small earthquakes from rock fracturing as magma moves, ground deformation as the volcano swells, gas emissions changing in composition, and ground temperatures rising. Modern monitoring catches most major eruptions before they happen.

What drives the eruption?

The driving force of a volcanic eruption is the gas pressure inside the rising magma. As magma approaches the surface, dissolved gases form bubbles that grow rapidly. If the magma is thin (low viscosity), the bubbles escape easily and the eruption is gentle, with lava flowing out. If the magma is thick (high viscosity), bubbles can't escape and pressure builds until the magma explodes violently, fragmenting into ash and rocky debris. The amount of dissolved gas and the magma's viscosity together determine how explosive the eruption will be.

What comes out during an eruption?

Volcanic eruptions produce three main types of material. Lava is molten rock that flows or sprays from the vent and cools into solid volcanic rock. Pyroclastic material includes ash (tiny rock fragments), lapilli (pebble-sized pieces), bombs (large blocks), and pyroclastic flows of fast-moving hot gas and rock. Volcanic gases include water vapor (the largest amount), carbon dioxide, sulfur dioxide, hydrogen sulfide, and others. The mix of these three depends on the volcano's chemistry: shield volcanoes mostly produce lava; stratovolcanoes produce explosive ash and pyroclastic flows.

How long does an eruption last?

Eruption durations vary enormously. Some eruptions last hours or days, like the 1980 Mount St. Helens main eruption (9 hours of major activity). Others last weeks or months, like many Hawaiian eruptions. Some continue for years or decades, like Mount Etna, which has been intermittently erupting for over 500,000 years with frequent activity. Major caldera-forming super-eruptions are extremely rare but devastating, releasing more material than thousands of typical eruptions combined. Most volcanoes have intermittent activity patterns lasting hundreds to thousands of years between major eruptive phases.