November 17, 2013
By Diana Lutz
It wasn’t what they were looking for — but that only made the discovery all the more exciting.
In January 2010, a team of scientists had set up two crossing lines
of seismographs across Marie Byrd Land in West Antarctica. It was the
first time the scientists had deployed many instruments in the interior
of the continent that could operate year-round even in the coldest parts
of Antarctica.
Mount Sidley, at the leading edge of the Executive Committee Range in
Marie Byrd Land, is the last volcano in the chain that rises above the
surface of the ice. But a group of seismologists has detected new
volcanic activity under the ice about 30 miles ahead of Mount Sidley in
the direction of the range’s migration. The new finding suggests that
the source of magma is moving beyond the chain beneath the crust and the
Antarctic Ice Sheet.
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.
There were big questions to be asked and answered. The goal, said
Doug Wiens, PhD, was essentially to weigh the ice sheet to help
reconstruct Antarctica’s climate history. (Wiens is a professor of earth
and planetary sciences in Arts & Sciences at Washington University
in St. Louis and one of the project’s principal investigators.) 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. 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, Wiens’ PhD student Amanda Lough 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 Lough and her colleagues looked, the
more convinced they became that a new volcano was forming a kilometer
beneath the ice.
The discovery of the new as-yet-unnamed volcano is announced in the Nov. 17 advance online issue of
Nature Geoscience.
Following the trail of cluesThe teams that
install seismographs in Antarctica are given first crack at the data.
Lough had done her bit as part of the WUSTL team, traveling 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 said. “Then I realized they were close to some
mountains, but not right on top of them.”
doug Wiens
Washington University in St. Louis
PhD student Amanda Lough bundles up on a particularly cold day during
one of her trips to tend seismographs in East Antarctica.
“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.
Ruling out iceBut 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, and more or less rules out a
glacial origin.
It 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 said.
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 said.
An ash layer encased in iceThe seismologists
also talked to scientists Duncan Young, PhD, and Don Blankenship, PhD,
of the University of Texas, 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 said.
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 said.
“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.”
What’s up down there?
The case for volcanic origin has been made. But what exactly is causing the seismic activity?
“Most mountains in Antarctica are not volcanic,” Wiens said, “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 said. “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.”
Will the new volcano erupt?
“Definitely,” Lough said. “In fact, because the radar shows a mountain
beneath the ice, I think it has erupted in the past, before the
rumblings we recorded.”
Will the eruptions punch through a kilometer or more of ice above it?
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.
Earth Observatory/NASA
Melt water from the new volcano will
drain into the MacAyeal Ice Stream, labeled above as ice stream E, its
original designation. This radar image of West Antarctica (see box on
the inset at bottom right for location) has been color-coded to indicate
the speed at which the ice is moving. Red marks the fast-moving centers
of the ice streams and black lines outline each stream’s catchment
area. By greasing the skids with water, the new volcano might increase
the rate of ice loss from the MacAyeal Ice Stream.
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 said.
This 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.
“We weren’t expecting to find anything like this,” Wiens said.
The research was funded by the National Science Foundation, Division of Polar Programs.
Source:
http://news.wustl.edu/news/Pages/25611.aspx
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