Could volcanoes be causing Antarctic ice loss?
by Staff WritersParis (AFP) Nov 17, 2013
Accelerating ice loss from the Antarctic icesheet could be due in part to active volcanoes under the frozen continent's eastern part, a study said on Sunday.
From 2002 to 2011, the average annual rate of Antarctic icesheet loss increased from about 30 billion tonnes to about 147 billion tonnes, the UN's panel of climate scientists reported in September.
The icesheet is a mass of glacial land ice -- one such sheet covers most of Greenland and the other Antarctica, and together they contain most of the freshwater on Earth.
The sheets are constantly moving, slowly flowing downhill and seawards under their own weight. Portions that extend out over the water are called ice shelf.
Previous research has blamed warmer seas swirling in a circular fashion around Antarctica for the quicker pace of icesheet loss from the southernmost continent.
These waters erode ice shelves, went the theory. And as more of the shelves disappeared, the quicker the sheet would flow and lose ice to the sea.
But in a new paper in the journal Nature Geoscience geologists led by Amanda Lough at Washington University in St. Louis, Missouri, suggested that, in West Antarctica, the faster flow may be also be due to volcanoes.
These heat the underside of the ice, causing melting that lubricates the flow, they suggested.
Evidence for this comes from recently deployed sensors that recorded two "swarms" of seismic activity under Mary Byrd Land, a highland region of West Antarctica, in 2010 and 2011.
Using ice-penetrating radar, the team found an intriguing elliptically-shaped deposit, measuring about 1,000 square kilometres (386 square miles) in the area, at a depth of 1,400 metres (4,550 feet).
The deposit is believed to be volcanic ash, spewed out by an enormous eruption some 8,000 years ago -- an estimate reached on the assumption it has since been covered by ice accumulating at the rate of 12.5 centimetres (five inches) a year.
"Together, these observations provide strong evidence for ongoing magmatic activity and demonstrate that volcanism continues to migrate southwards."
Several volcanoes were known to exist in West Antarctica, but none were thought to be active.
"Eruptions at this site are unlikely to penetrate the 1.2 to two-km (0.75-1.2-mile) -thick overlying ice, but would generate large volumes of melt water that could significantly affect ice stream flow," said the study.
from Science Daily:
Volcano Discovered Smoldering Under a Kilometer of Ice in West Antarctica: Heat May Increase Rate of Ice Loss
Nov. 17, 2013 — 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.
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, says Doug Wiens, professor of earth and planetary science at Washington University in St. Louis and one of the project's principle investigators, 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 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 advanced online issue of Nature Geoscience.
Following the trail of clues
The 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 East Antarctica.
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."
"My first thought was, 'Okay, maybe its 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 ice
But glacial processes can generate low-frequency events. If the events weren't tectonic could they be glacial?
To probe farther, 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 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.
An ash layer encased in ice
The seismologists also talked to Duncan Young and Don Blankenship 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 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 Mt Sidley. But that is also interesting because scientists had no idea when Mount Waesche was last active, and the ash layer is 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 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."
Will the new volcano erupt?
"Definitely," Lough says. "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.
Will the eruptions punch through a kilometer or more of ice above it?
The scientists calculated that an enormous eruption, one that released a thousand 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," says Wiens. 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.
By lubricating the bedrock, it will speed the flow of the overlying ice, perhaps increasing the rate of ice-mass loss in West Antarctica.
Washington University in St. Louis (2013, November 17). Volcano discovered smoldering under a kilometer of ice in West Antarctica: Heat may increase rate of ice loss. ScienceDaily. Retrieved November 17, 2013, from http://www.sciencedaily.com/releases/2013/11/131117155609.htm
from Nature Geoscience:
VOLCANOLOGY Mobile magma under Antarctic ice Volcanoes have been active under the West Antarctic Ice Sheet for millions of years, and there is evidence for recent activity. Now swarms of tiny earthquakes detected in 2010 and 2011 hint at current magma movement in the crust beneath the ice.
By John C. Behrendt
The West Antarctic Ice Sheet is losing
mass as the climate warms and the
surrounding floating ice shelves that
buttress the land-based ice are eaten away
at their base by warmer ocean waters1.
However, the ice can also melt from below
on land, where subglacial volcanic activity
causes a high flow of heat through the
crust2–5. Writing in Nature Geoscience,
Lough et al.6 use observations of hundreds
of small seismic events in the crust beneath
the West Antarctic Ice Sheet to infer
current magma movement in a volcanic
system beneath the ice, which may bring
heat up to the rock–ice interface and thus
affect ice flow.
Late Cenozoic volcanic activity
associated with the West Antarctic Riff
System7 extended over a wide area of West
Antarctica, including beneath the West
Antarctic Ice Sheet (WAIS) that flows
through it. In general, the volcanic activity
seems to have migrated southwards,
along north–south-oriented fractures,
away from the Marie Byrd Land dome8.
Active volcanism has also been reported
in a few other places in the West Antarctic
Riff System4,5, and aeromagnetic surveys
provide evidence for a number of volcanic
centres beneath the WAIS (ref. 2), but it has
been unclear whether magmatic activity
Lough et al.6 analysed seismic data
recorded by a deployment of 37 seismic
stations in Marie Byrd Land, a highland
region of West Antarctica (Fig. 1). Tey
identifed two swarms of earthquakes
about one year apart, in 2010 and 2011.
The swarms were comprised of hundreds
of small seismic events, with magnitudes
between 0.8 and 2.1. Te quakes occurred
at depths of about 25 to 40 km, close
to the boundary between the crust and
mantle beneath Marie Byrd Land, much
deeper than normal crustal earthquakes.
Tese characteristics, as well as the
observed wave frequencies, are typical for
deep long-period earthquakes that have
been associated with active volcanoes
worldwide9–12. Lough and colleagues therefore interpret the observed seismic
activity as a sign of magma movements
within an active subglacial magmatic
system, though it is unclear whether the
observed swarm activity presages an
Te earthquake swarms originated
beneath a subglacial mountain complex
with elevation of about 1,000 m above the
surrounding low-lying areas. Aeromagnetic
data show a 400 nf magnetic anomaly
at the high point, suggesting that rocks
in this region are highly magnetized.
Shallow-source magnetic anomalies from
rocks are ofen a sign of a volcanic origin,
so Lough et al. interpret the subglacial
mountain complex as a volcanic edifce.
Tey also identify a prominent 20 × 50 km
elliptical layer of ash in the ice above the
subglacial peak, about 400 to 1,400 m
below the ice surface. Given modern
ice accumulation rates of about 12.5 cm per year, the authors estimate that the
ash layer formed about 8,000 years ago
and was probably sourced from nearby
Radar ice-sounding data provide
measurements of ice thickness in West
Antarctica, but gaps in the data — the
aerogeophysical data lines are spaced about
15 km apart — lead to uncertainties. Deep
long-period earthquakes can occur up to
5 km away from active volcanic vents9–12,
and at this distance away from the source
of the earthquakes the ice is about 1,100 m
thick. Lough and colleagues show that
only an exceptionally large eruption could
breach the ice sheet in Marie Byrd Land
and vent to the surface.
Te earthquake swarms, magnetic
anomaly and ash layer are all located
about 55 km south of Mount Sidley in the
Executive Committee mountain range,
south of the area of Holocene volcanic ...