Eruption styles

Discovering Geology – volcanoes

Volcanic eruptions can be explosive, sending ash, gas and magma high up into the atmosphere, or the magma can form lava flows, which we call effusive eruptions. Whether an eruption is explosive or effusive largely depends upon the amount of gas in the magma.

Explosive eruptions

If a magma has a lot of gas which becomes trapped in the magma, the pressure will build and build until eventually the magma erupts explosively out of the volcano. It’s a bit like a bottle of fizzy soda. Gas is trapped in the liquid, but if you shake the bottle the gas wants to escape. This builds pressure inside the bottle, and when you release the pressure by opening the bottle, the gas rushes out the top carrying some of the liquid with it.

Phreatomagmatic eruptions are a type of explosive eruption that results from the magma erupting through water. The second phase of the Eyjafjallajökull eruption in 2010 was phreatomagmatic as a result of magma erupting under the ice. Some submarine volcanoes are phreatomagmatic, if the magma is gas-rich. For example Surtsey in Iceland. This eruption formed a new island.

Explosive eruptions can form pyroclastic flows which sweep down valleys destroying everything in their path. They also send ash high into the atmosphere forming plumes.

Effusive eruptions

If a magma has low viscosity (it is runny), gas can escape easily, therefore, when the magma erupts at the surface, it forms lava flows. These eruptions are gentle effusive eruptions. If a magma rises very slowly within the conduit. or throat of the volcano, all the gas can escape. When the magma is viscous (or sticky), it can’t flow when it reaches the surface, so it builds up forming a lava dome.

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Plymouth, Montserrat. Mud flow deposits at clocktower.


There are more than 1500 active volcanoes on Earth. Around 50-70 volcanoes erupt every year. There are 82 volcanoes in Europe, 32 of these are in Iceland, one of the UK’s closest ‘volcanic neighbours’.

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Mt Fuji, Japan from space

Types of volcano

When magma erupts at the surface, as lava, it can form different types of volcanoes depending on the viscosity, or stickiness, of the magma, the amount of gas in the magma, and the way in which the magma reached the surface.

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Anak Krakatau volcano

Volcanic hazards

A volcanic hazard refers to any potentially dangerous volcanic process that puts human life, livelihoods and/or infrastructure at risk of harm.

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Iceland geothermal energy

Living with volcanoes

Why do people live alongside active volcanoes? On first thought, it may seem unwise to choose to live with such hazardous neighbours, but all of us live with risk every day, we just weigh the risks up against the benefits.

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