It appears that researchers have located a volcano under a thick mantle of Antarctic ice. They found the volcano's approximate position by mapping a
layer of ash and glass shards within the glacial ice. The volcano erupted in or around 325
B.C., say Hugh Corr and David Vaughan, based on their study. (Both men work for the British Antarctic Survey.)
They initially detected the layer of volcanic debris through airborne radar-reflectance measurements. (At first they thought the reflective layer was the bedrock at the bottom of the ice, since it provided such a strong reflection.) Then they looked at the thickness of snow overlying this layer and correlated the ash deposit with eruption-linked acid-rich snow strata in ice cores that were taken in adjacent areas. The image here shows the radar-wave reflectance profile.
(According to my rough calculations, the vertical exaggeration of the cross-section is about 6x. )
This has been billed as the first time we've seen clear evidence of a volcano pushing its way up through the ice sheet in Antarctica, though similar eruptions have been observed in historical times in Iceland (like Grimsvotn in 2004). However, just this past weekend I watched an episode of the PBS series
NOVA, which showed scientists working on a big ice coring project near what they interpreted to be a sub-ice volcano. There was a big depression, and ice was flowing into the depression (downhill) from all directions. Ergo that ice had to be going somewhere.
NOVA's scientists posited it was being melted, and that
meltwater was greasing the skids of the bottom of multiple ice streams which were cruising out of that area of the ice sheet. (These ice streams are just faster-flowing areas of the ice sheet, like currents zooming through ocean water, sometimes 50x as fast as the "background" rate of flow.)
The show got me thinking about another study, coincidentally also published in
Nature Geoscience, although this one was in the inaugural January issue. It's a study of the Kennicott Glacier, in Alaska's Wrangell-St. Elias National Park:
The study was conducted by three researchers, all associated with the Institute of Arctic and Alpine Research: Timothy Bartholomaus, Robert Anderson & Suzanne Anderson. They measured a bunch of variables on the Kennicott Glacier, seeing which of them correlated with a rise in the glacier's speed. They found that an annual flood event from Hidden Creek Lake (HCL in part d of the diagram,
orange line) occurred at the same time as the glacier's maximum speeds during the measured interval, the maximum discharge of the (downstream) Kennicott River, and a maximum electrical conductivity of the water in the Kennicott River (the bedrock beneath the glacier is halite-bearing). As this whopper of a graphic shows, Not only does the glacier speed up its horizontal motion during the flood (part b), but the whole thing actually rises up vertically too! (part c) Also, Donoho Falls Lake (DHL in part d of the diagram,
blue line) downstream experiences a huge surge in water as the flood passes over it. Conductivity spikes during this same interval. Bartholomaus and the two Andersons propose that when the ice dam of the lake gives way and all that water surges into the glacier's channel, it overwhelms the capacity of the sub-glacial network of channels & raises the pore pressure of water within the ice. This extra pressure "inflates" the space between glacial ice & underlying bedrock, and the whole thing slides like an air hockey puck. At least, as long as the super-high pressure lasts. Once the flood ebbs, pore pressure in the glacier drops back down to levels that are easily counteracted by friction. The glacier slows once more to a "normal" pace.
This is very reminiscent to me of studies done on how an increase in pore pressure along a fault plane can trigger movement along that fault. When I took structural geology in college, the professor described an example from Colorado (I think) where the Army (I think) was injecting nerve gas down into the ground to get rid of it. The nasty nerve gas was dissolved in water, and the periodic injections of this solution correlated with a series of earthquakes (movement) along a previously-unknown subterranean fault. The injections increased fluid pressure in the pore space of the rock, and that "inflated" the space between the fault blocks, and the relatively minor shear acting on them was then enough to get the two to slide. I won't get into the whole Mohr Circle here, but I do recommend you
check out the famous Beer Can Experiment to get an idea of how an increase in fluid pressure can cause an otherwise "stuck" fault to slide. Anyhow, I guess
the base of a glacier is essentially a big fault, with one kind of rock below and another (ice) above. Same phenomenon, in other words, but different geologic context.
The Bartholomaus + 2 Andersons study also has some big global warming implications. The recent surge noted in Greenland's glaciers (e.g. Zwally,
et al., 2002) may be explained by higher rates of surface melting (due to elevated Arctic air temperatures) which then produces lots of meltwater, which flows down through the glaciers to the bottom via meltwater channels which plunge through the ice. Via the mechanism explained above, the great ice sheet atop Greenland is reduced more rapidly than without the surface melting. One of these meltwater channels was featured prominently on the cover of the June 2007 issue of
National Geographic.
So, with that, I think I'll end this blog post -- my thoughts went from volcanoes to ice streams & subglacial meltwater to fault slippage to global warming. I reckon that's just about enough... just about... but I also noticed something else...
A tangent about publication: The Corr & Vaughan findings will be published in the second issue of the new spinoff journal
Nature Geoscience, but they were posted online over the weekend in advance of the actual print publication of that issue. An article in the
New York Times alerted me to the study. I'm not surprised that
Nature, like the
Proceedings of the Royal Society, has taken to hatching specialty sub-journals to convey more articles each month. (An "
about the journal" page appears on their website, if you're curious.) The image shown here with this post is from the
Times, not the actual
Nature Geoscience article.
References:
Hugh F. J. Corr & David G. Vaughan. (2008) "
A recent volcanic eruption beneath the West Antarctic ice sheet." Nature Geoscience. Published online: 20 Jan. 2008. doi:10.1038/ngeo106
Timothy C. Bartholomaus, Robert S. Anderson & Suzanne P. Anderson. (2008) "
Response of glacier basal motion to transient water storage."
Nature Geoscience 1, 33-37. Published online: 20 December 2007 doi:10.1038/ngeo.2007.52
H. Jay Zwally, Waleed Abdalati, Tom Herring, Kristine Larson, Jack Saba, & Konrad Steffen. (2002) "
Surface melt-induced acceleration of Greenland Ice-Sheet flow."
Science 297, 218-222. doi: 10.1126/science.1072708
Also see:
Kenneth Chang. "
Scientists find active volcano in Antarctica."
The New York Times. Published online: 21 Jan. 2008.
Labels: antarctica, global warming, tv, volcano
