The direction of ice-wedge cracking in permafrost: downward or upward?

Field studies have been carried out along the western arctic coast of Canada in an attempt to determine whether all ice-wedge cracks originate at the ground surface and therefore propagate downward or whether some cracks originate near the top of permafrost and then propagate both upward and downward. The field studies have been concentrated upon (1) low- and high-centred tundra polygons a few thousand years old; and (2) ice wedges, growing for the first time, on the bottom of a lake experimentally drained in 1978. The field instrumentation has included electronic crack direction indicators, electronic elapsed timers, and continuous temperature measurements. The field studies reveal that many of the ice-wedge cracks originated near the top of permafrost and then propagated upward to the ground surface as well as downward into ice-wedge ice. For the 1974–1982 period, the field observations showed that about 57% of the ice wedges cracked from the ground surface downward and 43% cracked both upward and downward. Furthermore, the vertical direction of ice-wedge cracking was not consistent for any given wedge, presumably because of year-to-year variations in the physical and thermal conditions of the polygons and their troughs.