Don’t judge a glacier by its cover. That’s the message that has emerged (both figuratively and literally) from research carried out at Grey Glacier, a body of ice in the Southern Patagonian Icefield.
Using a side-scan sonar device, scientists have established that the eastern portion of Grey Glacier, which butts into Chile’s Grey Lake, forms an icy ‘tongue’ beneath the water. This subaqueous protrusion extends about one hundred metres beyond what’s visible above the water and appears to be tens of metres deep beneath the lake’s surface.
The team, led by Shin Sugiyama of Hokkaido University, first collected sonar data of the structure in 2016. In October of the same year, a tourist guide reported that a chunk of ice had broken off the glacier and part of the submerged terrace was suddenly lifted above the water’s surface. This event, along with subsequent sonar measurements, confirmed the existence of the glacial tongue. It all came as a complete surprise to Sugiyama, who says he had expected to find nothing at all.
There are currently very few observations of subaqueous ice available from other freshwater glaciers, and so it isn’t yet possible to say if glacial tongues are a common feature in lakes. Nevertheless, Sugiyama thinks that they may well exist. ‘I can speculate that similar structures would exist in other freshwater lakes, largely because the temperature structure in lakes should be similar in front of glaciers,’ he says. The team’s theory is that these tongues occur because slightly warmer water near a lake’s surface melts subsurface ice, forming a terrace-like protrusion. It’s something that doesn’t appear to happen in the waters beneath ocean-dwelling glaciers. In contrast, previous measurements of glaciers in Greenland have revealed underwater ice fronts that under-cut the main glacial body.
Underwater calving at Viedma Glacier, another glacier in Patagonia. Reseacrhers aren’t not sure if the underwater ice structure was similar to Grey Glacier, but ice remaining in the lakewater is consistent at the two glaciers.
Grey Glacier’s underwater structure is particularly significant because the team’s observations strongly suggest that the buoyancy of the submerged ice affects the stability of the above-water portion. Most of Patagonia’s freshwater glaciers have been receding for several decades, with large chunks of ice frequently ‘calved’ from the main bulk. Research such as this into underwater ice geometry is one of the keys to understanding this process.
The findings also have implications for human safety, as the sudden emergence of the tongue from the depths of Lake Grey revealed in 2016. Sugiyama now advises caution for anyone visiting the lakes and admits that his own research techniques will probably need to change. ‘What I found from this study was that these glacier fronts are not very safe, so I am hesitant to take the next measurements,’ he says. ‘But there is another instrument with a longer lens than the side-scan sonar, so using that we could measure glacier fronts more safely.’
This was published in the June 2019 edition of Geographical magazine
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