Scientists gathering seismic data in West Antarctica recently discovered a simmering volcano about a kilometer under the ice. If it erupts, the volcano will melt a lot of ice.
“We weren’t expecting to find anything like this,” says Doug Wiens, professor of earth and planetary sciences at Washington University in St. Louis.
In January 2010, Wiens and colleagues set up two crossing lines of seismographs across Marie Byrd Land in West Antarctica. It was the first time they had deployed so many instruments in the interior of the continent that could operate year-round even in the coldest parts of Antarctica.
Like a giant CT machine, the seismograph array used disturbances created by distant earthquakes to make images of the ice and rock deep within West Antarctica.
The goal, says Wiens, was essentially to weigh the ice sheet to help reconstruct Antarctica’s climate history.
BURDEN OF ICE
But to do this accurately, 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. The seismic data would allow them to map the mantle’s properties.
In the meantime, automated-event-detection software was put to work to comb the data for anything unusual. When it found two bursts of seismic events between January 2010 and March 2011, Amanda Lough, a PhD student working with Wiens, looked more closely to see what was rattling the continent’s bones.
Was it rock grinding on rock, ice groaning over ice, or, perhaps, hot gases and liquid rock forcing their way through cracks in a volcanic complex? Uncertain at first, the more researchers looked, the more convinced they became that a new volcano was forming a kilometer beneath the ice.
Their findings on the as-yet-unnamed volcano are published in an advance online issue ofNature Geoscience.
NOT JUST A COINCIDENCE
The teams that install seismographs in Antarctica are given first crack at the data. Lough has traveled to East Antarctica three times to install or remove stations.
In 2010, many of the instruments were moved to West Antarctica, and Wiens asked Lough to look at the seismic data coming in, the first large-scale dataset from this part of the continent.
“I started seeing events that kept occurring at the same location, which was odd,” Lough says. “Then I realized they were close to some mountains, but not right on top of them. 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 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.
WAY TOO DEEP
But glacial processes can generate low-frequency events. If the events weren’t tectonic, could they be glacial?
To probe further, Lough 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 kilometers (15 to 25 miles below the surface). This is extraordinarily deep—deep enough to be near the boundary between the earth’s crust and mantle, called the Moho, which more or less rules out a glacial origin—and also casts doubt on a tectonic one.
“A tectonic event might have a hypocenter 10 to 15 kilometers (6 to 9 miles) deep, but at 25 to 40 kilometers, these were way too deep,” Lough says.
A colleague suggested that the event waveforms looked like Deep Long Period earthquakes, or DPLs, which occur in volcanic areas, have the same frequency characteristics, and are as deep. “Everything matches up,” Lough says.
ASH UNDER ICE
The seismologists also talked to scientists Duncan Young and Don Blankenship, of the University of Texas at Austin, who fly airborne radar over Antarctica to produce topographic maps of the bedrock.
“In these maps, you can see that there’s elevation in the bed topography at the same location as the seismic events,” Lough says.
The radar images also showed a layer of ash buried under the ice. “They see this layer all around our group of earthquakes and only in this area,” Lough says.
“Their best guess is that it came from Mount Waesche, an existing volcano near Mount Sidley. But that is also interesting because scientists had no idea when Mount Waesche was last active, and the ash layer sets the age of the eruption at 8,000 years ago.”
Researchers say a case for a volcanic origin has been made but they still aren’t sure what’s causing the seismic activity.
HIDDEN HOT SPOT
“Most mountains in Antarctica are not volcanic,” Wiens says, “but most in this area are. Is it because East and West Antarctica are slowly rifting apart? We don’t know exactly. But we think there is probably a hot spot in the mantle here producing magma far beneath the surface.”
“People aren’t really sure what causes DPLs,” Lough says. “It seems to vary by volcanic complex, but most people think it’s the movement of magma and other fluids that leads to pressure-induced vibrations in cracks within volcanic and hydrothermal systems.”
The new volcano will definitely erupt, researchers say. “In fact, because the radar shows a mountain beneath the ice, I think it has erupted in the past, before the rumblings we recorded.”
The scientists calculated that an enormous eruption, one that released 1,000 times more energy than the typical eruption, would be necessary to breach the 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,” Wiens says. The water will rush beneath the ice toward 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.
By lubricating the bedrock, it will speed the flow of the overlying ice, perhaps increasing the rate of ice-mass loss in West Antarctica, Wiens says.
The National Science Foundation, Division of Polar Programs funded the work.
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