A preliminary study of the formation of landfast ice at Barrow, Alaska, winter 1973-74

Abstract

During late December 1973, the landfast ice sheet at Barrow broke loose and drifted offshore, following which an intense storm drove the pack ice into the nearshore zone. As a result, a new landfast ice mass was formed, which included a complex array of pressure ridges, shear ridges and hummock fields. The entire process was monitored by radar, and study of the imagery provides information on the velocity vector of the pack ice during this time, and on the sequence of events which lead to the formation of the landfast ice. This data, in conjunction with field observations, gives some insight into the mechanisms by which some of the structural features of the landfast ice mass were formed. An important feature of the landfast ice was a linear hummock field about 4 km long, 135 m wide, and with an average elevation of about 3 m. This feature, termed here as “ice pile”, was approximately aligned along a shoal 4-5 m deep which is oriented at an angle of about 10 degrees to the drift vector of the pack ice. On its offshore side, the ice pile was bounded by a shear ridge which built shortly after the pile under the same conditions of ice drift. The formation of both these features took less than 1.5 hours, and the transition between them is hypothesized to have resulted from the change in water depth along the outer boundary of the shoal. Measurements of the size and shape of blocks in the pressure ridges and hummock fields suggest that the ice failed in bending during the formation of these features. This may serve to put an upper limit on the forces involved in these processes, but the possibility remains that stresses approaching the crushing strength of the ice are attained during the growth of shear ridges.