Stromboli Volcano Stromboli Volcano, Sicily. © Tomasso Checci.

By Royal Society University Research Fellow Jo Gottsmann
University of Bristol

What is a volcano?

A volcano occurs when magma (hot molten rock that exists beneath the Earth’s surface) is expelled through an opening in the Earth’s crust. Magma is composed of crystals, melt (hot liquid) and gas. Volcanoes are not unique to Earth, they are also found on the surface of other bodies in our solar system - such as Venus and Io (one of Jupiter’s moons).

What causes volcanoes to erupt?

Magma is formed by the melting of the Earth’s interior. As molten rock is lighter than solid rock it is buoyant, so the magma rises. Whether or not molten rock eventually erupts as a volcano depends on what happens to the rising magma - it may cool and crystallise to eventually create new rock; however, magma may also break through to the surface to erupt. The Eruptions may last from a few minutes to many years - Stromboli volcano in Italy has erupted almost continuously for the last 1500 years.

Can we predict their activity?

It is very difficult to predict when a volcano will erupt in the distant future, but we have made progress in understanding what makes volcanoes reawaken. Some information from geophysical observations can be used to help forecast activity in the short-term.

What was the worst ever eruption?

The largest known eruptions rated 8 on the Volcanic Explosivity Index, but there hasn’t been one of this size for 26,500 years. The worst in recent history was the eruption of Mount Tambora in 1815 (rated 7) - it was the first eruption of this size for over 1600 years. The eruption caused a year-long change in global climate and was responsible for the death of at least 71,000 people.