Massive 'lake' discovered under volcano in South America

Scientists have discovered a huge magmatic lake, 15 kilometres below a dormant volcano in South America, a finding that could unlock why and how volcanoes erupt.

The body of water - which is dissolved into partially molten rock at a temperature of almost 1,000 degrees Celsius - is the equivalent to what is found in some of the world's giant freshwater lakes, such as Lake Superior.

The finding by researchers from University of Bristol in the UK and colleagues has led scientists to consider if similar bodies of water may be 'hiding' under other volcanoes and could help explain why and how volcanoes erupt.

"The Bolivian Altiplano has been the site of extensive volcanism over past 10 million years, although there are no currently active volcanoes there," said Professor Jon Blundy, who was part of the project at Cerro Uturuncu volcano in the Bolivian Altiplano.

"The Altiplano is underlain by a large geophysical anomaly at depths of 15 km below the surface of Earth.

"This anomaly has a volume of one-and-a-half million cubic kilometres or more and is characterised by reduced seismic wave speeds and increased electrical conductivity.

This indicates the presence of molten rock," said Blundy.

"The rock is not fully molten, but partially molten. Only about 10 to 20 per cent of the rock is actually liquid; the rest is solid. The rock at these depths is at a temperature of about 970 degrees Celsius," he said.

In order to characterise the partially molten region the team performed high temperature and pressure experiments at the University of Orleans in France.

This measured the electrical conductivity of the molten rock in the 'anomalous' region and concluded that there must be about eight to ten per cent of water dissolved in the silicate melt.

"This is a large value. It agrees with estimates made for the volcanic rocks of Uturuncu using high temperature and pressure experiments to match the chemical composition of crystals," said Blundy.

"Silicate melt can only dissolve water at high pressure; at lower pressure this water comes out of the solution and forms bubbles. Crucially - these bubbles can drive volcanic eruptions.

"The eight to ten per cent of water dissolved in the massive anomaly region amounts to a total mass of water equivalent to what is found in some of the giant freshwater lakes of North America," Blundy said.

"Ten per cent by weight of dissolved water means that there is one molecule of water for every three molecules of silicate," said Professor Fabrice Gaillard at University of Orleans in the US.

"This is an extraordinarily large fraction of water, helping to explain why these silicate liquids are so electrically conductive," said Gaillard.

The researchers hope that better understanding of how water can trigger volcanic eruptions can improve predictions of when it is going to erupt.