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SEA ICE CRACKS


Sea ice cracks are either blatantly obvious or spotted as roughly straight raised features on the sea ice. A sea ice crack can be jammed closed or still open with snow drifted over obscuring its presence visually. This sea ice crack was at Couloir Cliffs at Granite Harbor and its open stretch was about a half meter across. The sea ice surrounding a crack becomes reduced in thickness approaching the center of the crack and field teams have to be vigilant when approaching a sea ice crack. Ice drills are used to determine the thickness of the sea ice surrounding a crack if a crack is questionable for supporting the passage of tracked vehicles.

Sea ice cracks form due to forces working on the sea ice sheet. Ice sheets move in response to current and wind and they rise and fall with tidal rhythms. Deposition of snow by wind builds up pressure on the sea ice surface. Waves move into shallow water, increase in height and cause stress in the overlying ice sheet. Where the ice sheet comes in contact with an island or shoreline, sea ice cracks and pressure ridges will be evident. When a fast moving ice sheet comes into contact with a slow moving or stationary ice sheet, a pileup of ice blocks of angular shapes may form.

Sea ice sheets fold, fault and crumple; they compress and stretch. Sea ice cracks are an inevitable result. Here is a tidal pressure ridge at the Cape Evans shoreline with the Royal Society Range in the background across McMurdo Sound. Tidal sea ice cracks are observed near sea ice bound to a shoreline; tidal action lifts the sea ice sheet above or below the level at which it is bound to the shoreline. Pressure ridges or blocks may jut up from these shoreline cracks. Sea ice cracks tend to form parallel to shore at a distance dependent on the sea ice thickness; they open, refreeze and reopen and are of variable width.


The Weddell seal depends on sea ice cracks in stable contiguous sea ice for breathing and for entry/exit. The Weddell seal keeps a hole open year round with its teeth; its strong teeth project forward and are used to ream ice. Here a Weddell sea mother and pup sun themselves next to a sea ice crack; the sea ice blocks in the background were thrown up by forces on the sea ice sheet.



Weddell seal mothers and pups float in shallow water under a sea ice crack near where the mother gave birth to the pup. The seals' entry/exit/breathing holes in the sea ice crack are visible as bright lights above them. When young, pups do not swim very far away from the hole in which they enter and exit the water.



Sea ice cracks break up the deep blue visual continuity of the sea ice ceiling for the diver. On sunny days, sunbeams may stream down through open sea ice cracks and illuminate the seafloor with a band of light. Sea ice cracks that are jammed closed are seen by the diver as a light fine line running along the sea ice ceiling.

1: Ice Shelves of Antarctica. NI Barkov. Washington DC: Office of Polar Programs, National Science Foundation, 1985; 2: Ice and Snow: Properties, Processes, and Applications. Proceedings of a Conference held at the Massachusetts Institute of Technology, February 12-16, 1962. WD Kingery, ed. Cambridge, Massachusetts : MIT Press, 1963. pp.322-334

Text ©Peter Brueggeman. Photographs ©Peter Brueggeman & Norbert Wu. Photographs may not be used in any form without the express written permission of Peter Brueggeman & Norbert Wu. Norbert Wu no longer grants permission for uncompensated use of his photos under any circumstances whatsoever; want more info?