Magma Crystals May Help Volcanologists Understand Eruptive Processes

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False colour image of zoned orthopyroxene crystal from Mt St Helen’s: Image courtesy of Dr. Kate Saunders, University of Bristol

Volcanoes are strange things.

Like earthquakes, they follow scientific principles to a sufficient extent to be generally understood, but their behaviour remains unpredictable.

To this end, volcanologists continue to gather information which might give them an understanding of the processes which lead to an eruption – and a new study from the University of Bristol, in the UK, hopes to make just such a contribution.

Understanding a Volcanic Eruption Through Geology: How Magma Chambers Work

Understanding the workings of a volcano is, at its simplest level, schoolboy stuff – rock melts and rises, collecting in a magma chamber and, eventually, forces its way to the surface where it appears as lava, the product of a volcanic eruption.

For most people, that’s all you need to know.

Inevitably, of course, the processes are much more complicated.

Over the lifetime of a volcano, the composition of a magma chamber (and of the lava which eventually erupts) changes as the magma goes through eruptive cycles, with new pulses of magma arriving in the chamber itself from the melted rock below.

Fractional Crystallisation of Magma

Magma is made up of different minerals, which melt and solidify at different temperatures. As a result, during the cooling process, the minerals which solidify at the highest temperature crystallise first, and settle out from the liquid – therefore changing the composition of both the remaining magma in the chamber, and, in consequence, the lava which is erupted (which may be further altered by the addition of a fresh influx of molten rock).

Such crystals often form over a considerable period of time, during which the chemical composition of the liquid magma around them may change several times. The outer layer of the crystal reflects the composition of the magma chamber at the period of formation and as the crystal grows the composition successive outer layers may change, leading to a feature known as zonation.

Volcano Studies: Learning From the Composition of Lava

Bristol University’s research, published in the journal Science, involved using a variety of different techniques to study the composition of individual crystals of the mineral orthopyroxene erupted over a period of time.

Their study assessed material ejected from Mount St. Helens between 1980 and 1986, and was able to identify key features in the way in which the composition of the magma chamber changed over time, by the addition of new pulses of magma.

Eruption of Mount St Helens 1980: Photo courtesy of USGS

Lead author Kate Saunders told Decoded Science that the most significant finding of the report was that it provided clues to identifying new magma inputs (and the low-level seismic activity which they generate) and linking these to potential eruptive activity.

This correlation,” she said, “strongly suggests that the time-series information locked up in zoned volcanic crystals can be used to provide insights into the nature and timescales of precursory activity at past eruptions.

Volcanic Magma Chambers: Hindsight is 20/20

Although this geology research is necessarily retrospective, meaning that changes in an individual magma chamber can only be understood after the event, Dr. Saunders explained that such information may in the future be used to help predict future volcanic activity.

The better we understand the behaviour of a volcano, the better we can anticipate future eruptions” she said.

 

Resources

Saunders, K. et al Linking Petrology and Seismology at an Active Volcano(2012). Science. Accessed May 28, 2012.

Blake, S., PhD., et al. Melting the Mantle. (1997). Open University Worldwide.

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