Frost Blisters of the Bear Rock Spring Area near Fort Norman, N.W.T.

Abstract

Frost mounds of the frost blister type form every winter at the site of a group of cold mineralized springs on the east side of Bear Rock near Fort Norman, Northwest Territories, Canada. During each of four years of observation (1975-1978) three to five frost blisters formed, with measured heights ranging from 1.4 to 4.9 m, and with horizontal dimensions between 20 and 65 m. Locations of the blisters varied somewhat, presumably in response to differences in temperature regime and snow cover. Mature frost blisters consisted of a layer of frozen ground ... and a layer of ice ... covering a cavity which in some cases was over 4.0 m high. The cavities contained water during formation of the frost blisters; they were empty by spring. Time-lapse photography revealed that frost blisters can grow as fast as 0.55 m/d, and that some of them fracture, drain and partially subside one or more times before reaching their full height. During the summer, degradation occurs as a result of thawing and slumping of the soil cover and by melting and collapse of the ice layer; portions of the ice layer, or an uncollapsed section of a frost blister, can survive until the second summer after their formation. Water chemistry and isotope studies revealed that the frost blisters are formed by pressure build-up in subsurface water below seasonal frost and that the ice layers accumulate by gradual downward freezing in a closed (or intermittently opened) system filled with water derived from the Bear Rock spring system. Similar frost blisters are found in other areas of groundwater discharge in a variety of locations.

Key words: frost blisters, hydraulic uplift, springs, icings, permafrost, environmental isotopes, time-lapse photography