J. Ross Mackay

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 lake that drained rapidly, probably in 1955, was studied from 1974 to 1993. Most of the erratics were igneous rocks derived from the Canadian Shield. The erratics, which were unshattered before 1974, had already survived, in varying 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 covered the area; second, when all of the rocks, after submergence by lake enlargement, underwent annual freeze–thaw cycles under saturated conditions; and third when, after rapid lake drainage, the rocks were exposed to cold sub-aerial climate conditions before being marked for study in 1974. The 200 rocks 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 results showed that at least 10 of the 200 rocks had shattered, these being: at 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 probably hydrofractured from the freezing of water in closed to semi-closed systems or from thermal shocks. Rocks which facilitated the entry of water, such as those with a foliation, schistosity or porosity, broke the most frequently, many probably from ice segregation. Some of the dolomites probably shattered 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

Growth of Ibyuk Pingo, Western Arctic Coast, Canada, and some implications for environmental reconstructions

Abstract Ibyuk Pingo, which is 49 m high and 300 m in basal diameter, is one of the largest pingos in the world. Precise surveys carried out for the 1973 to 1983 period indicate that the lower half of the pingo, below a height of 25 m, shows no perceptible growth. From 25 m to the top, the growth steadily increases to a maximum of 2.3 cm/yr at the summit. The source of growth is believed to lie at a depth of about 65 m below the highest peak. Evidence based upon the depth of permafrost near the pingo, radiocarbon dating of wood in the pingo overburden, heat conduction theory, and the measured growth rate suggests an age of about 1300 ± 200 yr. Ibyuk Pingo is in an unstable state and collapse could be initiated by further slumping, a thermal disturbance to the crater, or erosion along a radial dilation crack. A study of the potential collapse pattern of Ibyuk Pingo suggests a number of criteria that might be employed in the environmental reconstruction of large circular features that might be identified as collapsed pingo remnants.