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How not to become a “crevasse bait”
Our lead dog Lasse is pushing the crevasse radar ahead with its long snout. I am sitting in the cab 6 meters behind the antenna, reading the screen in real-time to spot reasons for alarm. Mostly it shows nothing but horizontal layers in the snow, and the show gets tedious in a matter of minutes. But today I was in for more excitement.
Location: 85° 41’ S, 18°24’ E
Weather: Scattered clouds and diamond dust, -20 C, wind 5 kts
The crevasse detector is a 400 MHz ground penetrating radar, sending its energy waves down into the snowpack and receiving reflections from any icy boundary. This could be – and mostly is – the horizontal layering in the snow, or it could be the vertical walls of a crevasse. Snow layers appear as, well, layering in the screen image, while a crevasse will reveal itself a few meters in advance as curved reflections growing up from the bottom of the image. Finally, when the antenna is directly on top of it, the gap will appear as a vertical, black void on the screen. Not until the antenna has crossed the crevasse will we know its full extent.
Seeing none of this is of course good news. We are hoping to avoid crevassed ground completely, and we have selected our route with this in mind. So far the going has indeed been smooth – hence the tedium for the screen reader – but we know there are crevassed regions around the Recovery Lakes and try to remain alert. So, after days of seeing nothing but the usual horizontal layering, imagine my delight when a series of vertical, black voids finally presented themselves on my screen today!
Crevasses? Hardly. The curved reflections (hyperbolas) were totally absent, and all I could see were the black, vertical stripes. But they did indeed signal voids in the snowpack. In glazed areas a scattered network of fine cracks appear on the snow surface, a bit like a white but imperfect cobweb. If putting a spade or a crowbar into it, we would discover a crack that may extend for several meters down and widen to a few centimetres. These things form in windswept areas with little or no snow accumulation. The same surface layer is exposed to the annual temperature changes for years, and cracks up due to thermal tension. Eventually a little space will form under the surface, allowing air to circulate and the snow to recrystallize. The gap widens – but never to a dangerous extent.
The forces of the snow we can take delight in. The forces of the ice play on a more sinister note and will give us reason for caution and concern when we meet them. But until then I will be enjoying whatever scenery that dances across my little radar screen.
Having concluded this day of driving, it’s time for a little celebration. Happy New Year to all of you!
The cracks as they appear on the radar screen, and in real life at the surface and below. Photo: Stein Tronstad/NPI
