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What Causes A Volcanic Eruption?

author

Robin Andrews

Science & Policy Writer

clockJan 6 2016, 21:16 UTC
461 What Causes A Volcanic Eruption?
What causes the world's explosive volcanic eruptions, like the 1980s blast at Mount St. Helens, to occur? USGS

A team of volcanologists led by the University of Liverpool have released a perhaps controversial Nature study on the causes of volcanic eruptions. Going against the current consensus, they have suggested that it isn’t huge pressure differences that trigger volcanic blasts, but a strange behavior of magma called “frictional heating.”

Volcanic eruptions, despite being studied for several thousands of years in one form or another, are still relatively poorly understood phenomena. Although volcanologists have attempted to categorize eruptions as best they can, observing their underlying physical processes is impossible, and can only be interpreted after the act. The arguable “holy grail” of volcanology is to determine why exactly an eruption, particularly an explosive one, occurs, in order to aid our ability to predict when the next one will happen.

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Volcanic eruptions are largely thought to occur when there is a huge pressure difference (or “gradient”) between the broiling magma within the chamber and the outside world. When this gradient becomes too large for the encasing rock to keep it in, it fractures, allowing the magma to violently decompress onto the surface.

This chamber pressure is largely controlled by the gas content of the magma, which itself is variably gloopy, or “viscous.” As the magma initially begins to decompress as it rises from the depths of the Earth, gas bubbles form from the magma in a process known as vesiculation, which increases the internal pressure of the magma chamber. The more viscous and gassy the magma is, the greater the pressure gradient will be, and the more explosive the subsequent eruption.

Is temperature or pressure more important when it comes to triggering explosive eruptions? Credit: mik ulyannikov / Shutterstock

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This new study, led by Yan Lavallée, professor of volcanology at the University of Liverpool, has concluded that temperature, not pressure, is the controlling mechanism for vesiculation. Laboratory experiments were set up to melt various types of igneous rocks in various ways. The team looked carefully at how each melting technique produced varying degrees of vesiculation, comparing their results with fieldwork on Santiaguito volcano.

Their experiments show that magma and partially molten rocks moving up through a tube or “conduit” heat up as they do so. This temperature increase is caused by the “drag” of the magma against both the walls of the conduit and the internal currents within the magma itself.

“A good analogy to this is peanut butter,” Lavallée said in a statement. “When it is too cold and viscous, we plunge a knife into it and stir to warm it up and make it runnier.”

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This “frictional heating” caused substantial temperature increases in the laboratory, which had several effects: Primarily, the formation of bubbles is easier when the magma is hotter, or more energetically excitable. The more fluid, less confining magma also permits the more efficient growth of bubbles.

In addition, this temperature increase induced the melting of solid crystals within the magma, depositing a huge amount of chemical compounds into the molten phase of the magma. This so-called “supersaturation” causes a chemical imbalance within the magma, which releases these compounds as gassy bubbles in order to redress this.

These findings, if corroborated by other independent studies, have the potential to rewrite a key component of volcanological science, potentially transforming how we determine when, and indeed how, the most dangerous volcanoes on Earth erupt.


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