U.S. seismologists have made a surprising discovery near Mount Sidley in Marie Byrd Land, Antarctica – an active volcano smoldering under 1.2 km thick ice.

In 2010, the seismologists had set up two crossing lines of seismographs across Marie Byrd Land in West Antarctica. It was the first time scientists had deployed many instruments in the interior of the continent that could operate year-round even in the coldest parts of Antarctica.

The goal was essentially to weigh the ice sheet to help reconstruct Antarctica’s climate history. But to do this accurately the scientists had to know how the Earth’s mantle would respond to an ice burden, and that depended on whether it was hot and fluid or cool and viscous.

In the meantime, automated-event-detection software was put to work to comb the data for anything unusual.

In January 2010 and March 2011, the seismic network recorded two unusual bursts of seismic activity beneath Antarctica’s ice sheet.

“I started seeing events that kept occurring at the same location, which was odd. Then I realized they were close to some mountains, but not right on top of them,” explained PhD student Amanda Lough from Washington University in St. Louis, who is the lead author of the paper appearing in the journal Nature Geoscience.

“My first thought was, ‘OK, maybe it’s just coincidence.’ But then I looked more closely and realized that the mountains were actually volcanoes and there was an age progression to the range. The volcanoes closest to the seismic events were the youngest ones.”

The seismic events were weak and very low frequency, which strongly suggested they weren’t tectonic in origin.

While low-magnitude seismic events of tectonic origin typically have frequencies of 10 to 20 cycles per second, this shaking was dominated by frequencies of 2 to 4 cycles per second.

Ms Lough with colleagues used a global computer model of seismic velocities to relocate the hypocenters of the events to account for the known seismic velocities along different paths through the Earth. This procedure collapsed the swarm clusters to a third their original size. It also showed that almost all of the events had occurred at depths of 25 to 40 km.

“A tectonic event might have a hypocenter 10 to 15 km deep, but at 25 to 40 km, these were way too deep,” Ms Lough said.

The scientists suggested that the event waveforms looked like Deep Long Period earthquakes (DPLs), which occur in volcanic areas, have the same frequency characteristics and are as deep.

They then used airborne radar to create topographic maps of the bedrock and identify a layer of ash in the ice overlying the seismic swarm.

The ash layer is located at about 1.4 km depth. It is 8,000 years old and was probably sourced from the nearby Mount Waesche volcano.

“In fact because of the radar shows a mountain beneath the ice I think it has erupted in the past, before the rumblings we recorded,” Ms Lough said.

Ms Lough with colleagues calculated that an enormous eruption, one that released a thousand times more energy than the typical eruption, would be necessary to breach the 1.2 km-thick ice above the volcano.

On the other hand a subglacial eruption and the accompanying heat flow will melt a lot of ice.

“The volcano will create millions of gallons of water beneath the ice – many lakes full. This water will rush beneath the ice towards the sea and feed into the hydrological catchment of the MacAyeal Ice Stream, one of several major ice streams draining ice from Marie Byrd Land into the Ross Ice Shelf,” explained study co-author Prof Doug Wiens, also from Washington University in St. Louis.

“By lubricating the bedrock, it will speed the flow of the overlying ice, perhaps increasing the rate of ice-mass loss in West Antarctica.”

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Bibliographic information: Lough AC et al. Seismic detection of an active subglacial magmatic complex in Marie Byrd Land, Antarctica. Nature Geoscience, published online November 17, 2013; doi: 10.1038/ngeo1992