Volcanoes… To ooze or to boom?

By Leanne Gunn

Volcanoes come in lots of shapes and sizes. It is all down to the style of the eruptions that they produce. Some have steep slopes and erupt with big explosions, throwing vast quantities of rock and ash high into the atmosphere. Others have gentle slopes indicative of their less dramatic, but mesmerizing, volcanic eruptions.

A peaceful photo of Nakano-jima volcano, Japan
All rights reserved Leanne Gunn

All volcanic eruptions start the same, with molten rock (magma) rising up through the Earth and breaking through the Earth’s crust. However, this is where the similarities end. Sometimes magma spews out of the volcanic vent as red hot lava flows, but sometimes it doesn’t. Instead it clogs up inside the vent forming a plug or lava dome. These differences are all to do with how sticky or runny the lava is, which also holds to key to why some volcanoes erupt explosively while others do not.

A matter of viscosity

Honey is thicker than water

Honey is thicker than water

Viscosity is a measure of how thick a liquid is, and therefore how easily it flows. For example, water pours more easily than honey and therefore has a lower viscosity.

Magma is much more viscous than honey. Although it is still technically a fluid, it has an internal structure made from silicon-oxygen tetrahedrons (silicon atoms surrounded by 4 oxygen atoms). The strength of this structure, and therefore how viscous, or thick, the magma is depends on its chemical composition.

Lava domes in the crater of Mt. St. Helens (Washington, USA) formed by high viscosity lava. CC-BY-Seattle Skier

Lava domes in the crater of Mt. St. Helens (Washington, USA) formed by the eruption of high viscosity lava
CC-BY-Seattle Skier


If the Magma contains a lot of silica the silicon-oxygen tetrahedrons will start to share oxygen atoms, and as a result a chain of tetrahedrons is formed. The bonds holding these together are really strong and it requires a lot of energy to break them. This means that the magma doesn’t deform easily, in other words, it has a very high viscosity and barely flows at all. Volcanoes that erupt high silica magma have thick, high viscosity lava flows. Instead of flowing out of the volcanic vent these lavas barely move, forming a plug or dome which gradually grows bigger and bigger as more and more lava is erupted.

Low viscosity, flowing lava in Hawaii CC-By-Mbz1

Low viscosity, flowing lava in Hawaii

In complete contrast to this, magma with lower silica contents do not need to form silicon-oxygen tetrahedron chains. As a result they have a weaker internal structure allowing them to flow much more easily. These form the red, flowing rivers of lava that most people are used to seeing in books and documentaries. Kilauea in Hawaii has been erupting lavas like this almost continuously for over 30 years.



 What does this have to do with explosions?

A small explosion at Soufriere Hills Volcano, Montserrat.  All rights reserved Leanne Gunn

A small explosion at Soufriere Hills Volcano, Montserrat.
All rights reserved Leanne Gunn

Magma contains lots of volcanic gasses, like carbon dioxide, sulphur dioxide and water vapour.

Deep within the Earth, where pressures are extremely high, these gasses are dissolved within the magma. As the magma rises up through the Earth the pressure on the magma decreases and these gasses start to form bubbles.

These bubbles want to expand as the magma rises towards the surface. If the magma has a low viscosity they are able to do this easily but if the magma has a high viscosity these gasses struggle to expand and instead pressure begins to build.

Eventually this pressure becomes too much and gas explosions take place, blasting rock and lava fragments high into the air. The smallest rock fragments are known as volcanic ash, which is easily carried in the wind and deposited miles away from the volcano itself.

What else affects volcanic eruptions?

Explosive volcanism due to overlying ice during the 2010 Eyjafjallajökull eruption, Iceland.  CC-BY-Rémih

Explosive volcanism due to overlying ice during the 2010 Eyjafjallajökull eruption, Iceland.

The silica content of the magma is is not the only thing controlling lava viscosity. Things like the temperature and water content of the magma, or even the number of crystals within it also play an important role.

Sometimes even eruptions of low viscosity, runny lava can become explosive under certain conditions. For example, when hot lava mixes with cold water, the water become super-heated, and can cause steam explosions.

This is particularly common when volcanic islands are born or when volcanoes erupt under ice, such as the 2010 Eyjafjallajökull eruption of South Iceland which led to the closure of European air-space for nearly a week.


Now you know more about volcanic eruptions, perhaps brush up on these guidelines of what to do if you find yourself near an erupting volcano!


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Posted in Chemistry, Exploring Science, Geology