Buoyant magma may trigger Earth's supervolcanoes

Geologists have revealed new insights into supervolcanoes, the brooding, enigmatic giants of the Earth's crust whose eruptions are as catastrophic as they are rare.

The buoyancy of molten lava is the key to understanding why these monsters explode, according to a Sunday report in Nature Geoscience.

Supervolcanoes include Yellowstone, in Wyoming, which spewed out more than 1,000 cubic kilometers of ash and rock when it last erupted about 600,000 years ago.

Events of this kind can chill the planet's surface by up to 10 C for a decade or more because the ash, which is carried into the stratosphere, reflects sunlight.

By comparison, the biggest volcanic eruption of the last quarter-century was that of the Philippines volcano Pinatubo in 1991, which discharged a relatively puny 10 cu km.

Seeking to understand why volcanoes can be so different, a team from Switzerland, France and Britain built a computer model of volcanic activity, basing the age of eruptions on a telltale mineral, zircon, found in volcanic rocks.

Separately, a team from the Swiss Federal Institute of Technology in Zurich used a high-tech X-ray facility to study the density of molten rock below supervolcanoes.

In conventional volcanoes, activity is determined by the size of the magma chamber, the rocky vault below the volcano, the investigations found.

Relatively small in volume, the chamber in conventional volcanoes is replenished regularly by bursts of upwelling magma, which is expelled in moderate amounts when the pressure becomes too much.

But in supervolcanoes, the magma chamber is simply too big to be pressurized by magma injections alone. In these leviathans, what happens is that a buoyant kind of magma steadily accumulates in the chamber.

Like a kettle, the chamber is initially strong enough to resist the pressure but eventually breaks apart in a cataclysmic discharge.

"Until now, nobody had measured the density of the magma that is present in the magma chambers of super-volcanoes," Wim Malfait from the institute in Zurich said in an e-mail.

"Because magma is less dense than solid rock, the magma in a magma chamber pushes on the roof of the chamber."

"This is similar to holding a football under water - the air in the football is lighter than the surrounding water, so the water pushes it against your hand. Overpressure is enough to start a supereruption if the magma chamber is thick enough," Malfait said.

The Swiss-French-British team calculated that the maximum volcanic eruption would entail a release of between 3,500 and 7,000 cu km of magma - the first time an upper limit has ever been established for a volcano.

The two studies should provide useful pointers as to the frequency of supervolcano events, the authors hope. Almost nothing is known about how swiftly these beasts recharge with magma and blow up. Only 23 such eruptions have occurred in the last 32 million years.

The work is useful for understanding the risks of Yellowstone, but far more work is needed, the authors said.

"We could potentially use estimates of the vertical extension of the magma chamber to infer if a volcanic system such as Yellowstone is in critical condition or not," said Luca Caricchi, an assistant professor of volcanology at the University of Geneva.

"However, the pressure for a volcanic eruption to occur is a function of many factors," he said.

"It would be premature to make any forecasting using our model."

A study from 2012 indicates that before Yellowstone erupts, the ground would lift by possibly hundreds of meters, said Malfait.

"But this would not help to distinguish directly if an eruption is going to happen next week, next year or next century," he said.

Agence France-Presse

(China Daily 01/07/2014 page11)