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New clues to predicting volcanic eruptions

Mount Hood

A study of the Mount Hood volcano has provided a new insight into the early prediction of eruptions (Source: Erik Klemetti)

Predicting eruptions Volcanoes are more likely to erupt when their magma is hot, liquid and runny, according to a new study.

The research in the journal Nature, means the detection of large amounts of liquid magma in a volcano, implies an eruption may be imminent, according to one of the study's authors, Dr Adam Kent of Oregon State University.

"Until now, people monitor volcanoes looking for changes in the temperature of gases or water, and the number of earthquakes, to indicate the magma is ready to erupt," says Kent.

"Our work gives some extra insight into how to interoperate results from geophysical and seismic studies which identify large bodies of liquid magma."

Kent and co-author, Dr Kari Cooper of the University of California, base their findings on a study of the thermal history of the magma reservoir of Mount Hood, a 3429-metre high active volcano in Oregon's Cascade Mountains.

"We've been trying to understand how hot magma is when it's stored," says Kent.

"Magma needs to be at temperatures of over 750°C to have sufficiently low viscosity to erupt."

The conditions under which magma is stored have an important influence over the processes that lead to volcanic eruptions.

"Most volcanoes aren't erupting all the time, many have magma or the cooled remains of magma, stored underground for long periods, sometimes hundreds of thousands of years," says Kent.

"At Mount Hood you have cool and rigid magma about five kilometres below the volcano. All of a sudden some new, hotter magma rises and mixes with it, heating the whole thing up, making it easier to move up fractures to the surface."

Kent and Cooper wanted to know how long magma was stored underground, and how much of that time it would have been sufficiently liquid to erupt.

Past eruptions

The authors examined solidified magma from two Mount Hood eruptions, an event known as Old Maid which occurred 220 years ago, and the Timberline eruption 1500 years ago.

"The main mineral we worked on is plagioclase, a calcium aluminium silicate, which is the most common mineral to erupt from Mt Hood." says Kent.

Using radioactive decay dating techniques, Kent and Cooper determined the crystals that erupted in the Mount Hood magma were 100,000 years old.

Based on the size of the crystals and known growth rates, the authors were able to determine how long the crystals were growing. And the temperature could be calculated by their chemical composition.

Uncommon state

"We found that Mount Hood magma storage at temperatures high enough to erupt were rare, occurring at most 12 per cent, and more likely less than one per cent of the total time the magma is in storage," says Kent.

Because these data are specific to Mount Hood, Kent and Cooper want to see if the same holds true for other volcanoes.

"We've done a little preliminary work by looking at data from other volcanoes, and it looks like the pattern is pretty common, so we can infer that other volcanoes work in a similar way most of the time," says Kent.

Tags: geology