Cold-climate shattering (1974 to 1993) of 200 glacial erratics on the exposed bottom of a recently drained Arctic lake, western Arctic Coast, Canada

The shattering of 200 glacial erratics on the exposed bottom of an Arctic l ake that drained rapidly, probably in 1955, was studied from 1974 to 1993. Most of the erratics were igneous rocks derived from the Canadian Shield. T he erratics, which were unshattered before 1974, had already survived, in v arying degrees, at least three prior stages of shattering: first, when many of the rocks were in the thin active layer of the glacial till that covere d the area; second, when all of the rocks, after submergence by lake enlarg ement, underwent annual freeze-thaw cycles under saturated conditions; and third when, after rapid lake drainage, the rocks were exposed to cold sub-a erial climate conditions before being marked for study in 1974. The 200 roc ks were checked in 1977, 1978, 1979, 1987, 1988 and 1993. In 1993, the last year of observation, 180 of the original 200 rocks were relocated. The res ults showed that at least 10 of the 200 rocks had shattered, these being: a t least 2 out of about 136 granites; 1 out of about 6 gneisses; 1 out of 2 sandstones; and 6 out of about 22 dolomites. The impervious granites probab ly hydrofractured from the freezing of water in closed to semi-closed syste ms or from thermal shocks. Rocks which facilitated the entry of water, such as those with a foliation, schistosity or porosity, broke the most frequen tly, many probably from ice segregation. Some of the dolomites probably sha ttered explosively. In support of the ice segregation theory of shattering for some types of rocks, an example is given of present-day ice segregation in a Cretaceous shale at the mouth of nearby Horton River, NWT. Copyright (C) 1999 John Wiley & Sons, Ltd.