Decadal and millennial velocities of rock glaciers, Selwyn Mountains, Canada

Abstract

We compare surface velocities of nine rock glaciers over decadal and millennial time scales using independently derived velocities. We surveyed 15 rock glaciers in the Selwyn Mountains in 1983 and again in 1995, thereby obtaining 12-year average velocities. We also determined millennial scale velocities using lichenometric ages and rock glacier length on nine of the 15 rock glaciers for which lichenometric ages were available. The mean surveyed velocity of the nine rock glaciers was 0.20 ± 0.11 m/yr, and the mean age-length velocity was 0.20 ± 0.13 m/yr. A paired t-test of the two independently derived data sets shows no difference at the 0.01 level of significance. Given that both measures have inherent errors, the decadal and millennial velocities should be considered as being the same order of magnitude. The similarity of results of these two independent methods suggests that (1) decadal scale velocities of rock glaciers are comparable to millennial scale velocities in this area, and (2) either method can resolve long-term velocities. We examined surveyed velocities in relation to rock glacier characteristics in order to identify controls on velocities. Surface velocity is expected to vary with density, the sine of surface slope, thickness and temperature. Rock glaciers facing NE were significantly longer and moved faster than rock glaciers facing other directions. This may be due to greater snow, ice and debris accumulation causing increased mass. Maximum surveyed velocity was significantly correlated with rock glacier thickness (r2 = 0.88) and with the sine of surface slope (r2 = 0.50), when one and two outliers were excluded from analyses, respectively. Future work focusing on the systematic measurement of the dimensions, age, and velocities of rock glaciers should help to improve our understanding of controls on velocities of rock glaciers.