The magma reservoir beneath an active volcano turns out to decidedly cooler and more solid than previously thought, a finding that could help scientists predict eruptions more accurately.
By studying zircon crystals spewed forth by New Zealand’s Mount Tarawera volcano seven centuries ago, researchers led by University of California geoscientist Kari Cooper suggest that established textbook conceptualisations of what magma reservoir looks like need to to change.
Instead of the magma reservoir being seething mass of molten rock, Cooper says the picture emerging from their study is of something more like a snow cone – albeit a very hot one: of a mostly solid and crystalline composition, with a little superheated liquid seeping through it.
Since a volcano’s magma is very hard to study directly – temperatures generally of at least 700 °C and high pressures are sufficient to destroy most instruments – the zircon crystals acted something like a “black box” flight recorder.
“Instead of trying to piece together the wreckage, the crystals can tell us what was going on while they were below the surface, including the run-up to an eruption,” says Cooper.
By studying trace elements in zircon crystals, the scientists could determine when the crystals formed and how long they had been exposed to high heat (more than 700 °C). Of seven crystals from Mount Tarawera studied, six were found to have been at least tens of thousands of years old, spending less than 4% of that time exposed to molten magma. All seven crystals studied were found to have remained solid in the volcano’s magma reservoir throughout an eruption that about 25,000 years ago.
The research team, which includes scientists from US institutions, Nanyang Technical University in Singapore, and the University of Canterbury in Christchurch, suggest the solid or crystalline magma must interact with hotter liquid stored elsewhere in the reservoir in order to melt and mobilise to create the conditions for an eruption.
Developing ways to measure and track magma mobility could therefore lead to ways to identify volcanoes at high risk of erupting. The research was published in the journal Science.