Photo of crevasse on Worthington Glacier, AK.
Crevasses
Crevasses are one of the most commonly known formations on glaciers. The conditions for crevasses to occur are simple, when a glacier undergoes a tension force and the surface tension of the ice is less than the force of tension on the glacier in that area, the surface splits open to relieve the stress of the tension force. “The firn and surface ice are rigid, so when the tension due to its flow becomes too great the ice cracks forming crevasses” (Fountain). This typically happens when a glacier must move over an underlying rise in topography, such as a small hill. The surface moves over the hill but is pulled apart. This is a bit similar to the skin on our knuckles when we bend our fingers. The ice of a glacier is not as elastic however, and splits apart as it moves. For these reasons, glaciers that move over a depression or lower point in the ground are less likely to have crevasses because they are undergoing compression. (“Seminar…”)Photo of crevasses on the surface of Worthington Glacier near Valdez, Alaska.
"Cryoconite Holes on Greenland Ice Sheet, Greenland, 10 July, 2008. Photo credit: James Balog" (Patowary)
Cryoconite Holes
One surface formation on glaciers that I find fascinating are cryoconite holes. These holes occur with the presence of cryoconite dust, and most noticeable in the ablation section of a glacier. It is stated that, “Cryoconite dust is made of a combination of small rock particles, soot and microbes which is deposited and builds up on snow, glaciers, or icecaps. Cryoconite may contain dust from far away continental deserts or farmland, particles from volcanic eruptions or power plant emissions, and soot” (Patowary). Due to its origins listed, the dust is usually a dark color. Once it settles on the glacier, it absorbs radiant energy from the sun, which transforms to thermal energy and causes the dust to heat up, melting the ice around it. Through this process the dust forms long holes in the ice or snow as the cryoconite dust sinks through the surface.Moulins
Moulins are holes in a glacier that penetrate through the entire thickness, caused by melting water eroding the surface of a glacier, occurring at some deformation such as a crack in the ice surface. According to Matt Hoffman of the Los Alamos National Lab, “Sometimes they penetrate a whole kilometer or a mile thick body of ice” (Hoffman). He goes on to explain that once the water reaches the bottom of the glacier it can speed up the overall movement of the ice. Hoffman was part of a team that set out to find the largest factor in the formation of moulins. They tested how stresses of the seasons affected the potential for moulin formation but ultimately found these did not make cracks where moulins could form. He reports, “However during a brief lake drainage event that lasted only a few hours the stresses were so great that most of the surface ice in this region was being pulled apart, and that would allow new moulins to form anywhere” (Hoffman). A drainage event such as described happens when a dam of water caused by ice or moraine (debris at bottom of glacier) suddenly overflows. This floods the glacial streams, and adds extra force of water across the glacier’s surface.
Small moulin on Worthington Glacier.
Ice Caves
Similar to moulins, ice caves are attractive to many adventurers. These form in the same way that moulins do, as glacial runoff erodes the ice to exit the glacier, forming a hole. Temperatures in the summer rise and melt the hole even more. 