Why do volcanic eruptions occur?

There is a lot more to volcanic eruptions than meets the eye – they are complex, diverse and caused by a multitude of events.

Directly underneath the Earth’s crust there are layers of molten rock, which, because of the high temperatures at the core, are melted to form magma.

Magma contains various gases – predominantly carbon dioxide, water vapour and sulphur dioxide. It is stored in magma chambers that are sealed beneath a lid of solid rock.

Due to its liquefied state, the magma is less dense than the rock above it and begins to rise. Simultaneously water vapour escapes as bubbles, increasing the relative density of the carbon dioxide and sulphur dioxide.

Over time these gases expand to 1000 times their original size, increasing the pressure on the chamber lid to such an extent that an eruption occurs. This is similar to when you shake a soft drink and it explodes when opening the lid – by shaking it you are separating carbon dioxide molecules, causing a build-up of gas and pressure.

When magma is released – through the ruptures in Earth’s crust we call volcanoes – it is known as lava, which is hot as 1100 degrees Celsius. Typically, it will also form pyroclastic clouds: cascades of hot ash, gas and molten rock fragments, which are around 1000 degrees Celsius and travel around 700 kilometres per hour.

What causes a volcanic eruption?

The fundamental concept of an eruption is that an increase in pressure on the chamber lid causes the magma to be released from beneath it. However, there are variances in the cause of this magma movement and the type of eruption generated.

Volcanoes are usually found near the boundaries of Earth’s tectonic plates. These can either spread apart and leave a gap in the surface, or they can push underneath one another – a process called subduction.

When the plates separate, magma rises slowly in order to fill the gap through a gentle explosion of thin basaltic lava, which is at temperatures from 800 to 1200 degrees Celsius.

However, when one plate pushes underneath the other, this forces molten rock, sediment and seawater down into the magma chamber. The rock and sediment are melted into fresh magma, and eventually overfill the chamber until it erupts, releasing sticky and thick andesitic lava, at temperatures from 800 to 1000 degrees Celsius.

Plate tectonics is, however, not the only cause of eruptions.

Decreasing temperatures can cause old magma to crystalise and sink to the bottom of the chamber, forcing fresh liquefied magma up and out – similar to what happens when a brick is dropped in a bucket of water.

A decrease in external pressure on the magma chamber may also allow for an eruption by minimising its ability to hold back increasing pressures from the inside. This is often caused by natural events, such as typhoons, that decrease rock density, or by glacial melting on top of the chamber lid, which alters molten rock composition. Glacial melting is believed to have been one of the causes of the 2010 Eyjafjallajökull eruption in Iceland.

So-called ‘hot-spot’ volcanoes are ones that form away from tectonic plate boundaries. They are created as plates move and expose hot uprisings from Earth’s mantle, known as plumes. The volcanoes found in the Hawaiian islands are of this sort.

What are the different types of volcano?

There are three main types of volcano.

  • A shield volcano has a flat dome-like appearance and releases basaltic lava in a gentle manner that is often slow and easy for humans to outrun.
  • A stratovolcano has the classic cone shape and releases andesitic magma. It produces violent large eruptions and often leads to pyroclastic flows and mudflows. The active Mount Agung volcano in Bali falls under this category.
  • A caldera volcano has a circular basin-shaped appearance and releases thick rhyolotic lava which is between 650 to 800 degrees Celsius. Its shape is due to the magnitude of its eruptions, which cause the magma chamber walls to collapse. This occurs as the entire chamber empties, leaving it unsupported and liable to cave in. This process is cyclical, and does not mean the volcano is dormant.
Cross-sections of 3 types of volcano. Vertical scale is somewhat exaggerated.
Cross-sections of 3 types of volcano. Vertical scale is somewhat exaggerated.
Credit: UIG / Getty

What determines the size of an eruption?

Ultimately, the size of an eruption will depend on the thickness of the magma, the density of gases it contains and the amount of new magma being pushed into the magma chamber.

Basaltic lava allows gas to escape easily, resulting in smaller eruptions, while andesitic and rhyolotic lava makes it harder for the gas to escape, leading to larger eruptions.

Typical volcanic hazards

Lava is often thought to be the main danger of a volcanic eruption, but this is not the case. Numerous hazards result from eruptions and they can have a range of consequences. The most dangerous are the pyroclastic clouds, which destroy anything in their path. Other hazards include ash clouds, ash rain, mudslides, earthquakes, tsunamis, odd weather patterns and glacial flooding.

Related reading: Earth’s most volcanic places

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