Source: www.topnews.net.nz | Original Post Date: June 27, 2015 –
When large chunks of ice break off of a glacier and plop with a giant splash into the chilly water, the result can be lots of thunderous shaking. These mysterious glacial quakes have increased seven-fold in Greenland in two decades, according to new research.
If Greenland goes, it is becoming clear that it won’t go quietly.
Scientists have already documented entire meltwater lakes vanishing in a matter of hours atop the vast Greenland ice sheet, as huge crevasses open beneath them. And now, they’ve cast light on the mechanisms behind another dramatic geophysical effect brought on by the rumbling and melting of this mass of often mile-thick ice.
Nearly half of Greenland’s mass loss occurs through iceberg calving. Calving at Greenland’s Helheim Glacier causes a minutes-long reversal of the glacier’s horizontal flow and a downward deflection of its terminus. The reverse motion results from the horizontal force caused by iceberg capsize and acceleration away from the glacier front. The downward motion results from a hydrodynamic pressure drop behind the capsizing berg, which also causes an upward force on the solid Earth. These forces are the source of glacial earthquakes.
Back in the summer of 2006, scientists studying the vast and in some places mile-thick Greenland ice sheet observed something that can only be called breathtaking.
Due to meltwater, lakes form atop the ice sheet in the summer – scientists call them “supraglacial lakes” — and they can grow to be quite large. And in July 2006, one large lake, over 2 square miles in area, suddenly vanished. It lost most of its water in under two hours – researchers calculated that the rate of drainage “exceeded the average flow rate over Niagara Falls.”
There was only one place all that water could have gone, down into the ice sheet, where researchers feared it could lubricate its base and hasten its slide into the ocean. Little understood phenomena like this, they wrote, could add to the dynamism and rapidity with which Greenland, which contains enough ice to raise sea levels 20 feet – responds to global warming by melting and pouring water into the ocean.
“The difficult thing about Greenland is, it’s so important for sea level rise because [compared to other countries with massive ice sheets] it’s quite far south,” said study co-author Timothy James, a professor of geography at Swansea University in the United Kingdom. Watching a glacial earthquake unfold back in 2010 for a prior mission to study glacial earthquakes “was a really lucky experience,” he said. “Every once in a while, you’d hear a crack and a bang,” he added, “but by the time the sound actually got to you, you turned and didn’t really see anything.”
“We found that we were actually having to sit there very carefully, looking at it and going, ‘Do you see anything moving? I think the front’s getting higher.’ It was just all kind of quite slow to look at, but the noise was absolutely chaotic. I think that was the most surprising thing,” James said.
The research was conducted on Helheim Glacier in Greenland for 55 days in 2013. The researchers employed cameras, GPS sensors and the global seismographic network for recording theses glacial upheavals. The researchers were able to record ten such large-scale glacier retreat events and saw the glacier retreat by about 1 mile following the shaking events.
Nettles informed that ‘Calving’ is an important component of the mass loss in both Greenland and Antarctica and glacial earthquakes induced by calving occur seven times more frequently than they did in the early 1990s.
Using radar data from NASA’s Operation IceBridge, scientists have built the first-ever comprehensive map of the layers deep inside Greenland’s ice sheet. (NASA)