Evidence for Headwall Weathering Zones, Boundary Glacier, Canadian Rocky Mountains

Abstract

Temperature records from several randklu/t sites at the margins and headwall of Boundary Glacier (Iat. 52°12'N., long. 117°12'W.) in the Canadian Rocky Mountains are presented. These records indicate that during the ablation season a diurnal freeze-thaw air and rocksurface temperature regime occurs in a 2 m wide zone centred on the randklu/t lip. Deeper in the randklu/t , stable sub-zero conditions prevail whereas above the randklu/t an above-freezing temperature regime prevails. The freeze-thaw temperature regime, observed freezing of melt water and rain water on randklu/t rock surfaces, and copious in-situ loose weathering products on the head wall and marginal free faces suggest an active frost-shattering environment. Downslope migration of the randklu/t lip during the ablation season results in a migration of the freeze-thaw zone and thus a seasonal extension of the rock surface area exposed to the favorable weathering environment. Longer-term glacier fluctuations, with attendant thinning and thickening of the ice body, could result in large areas of marginal and headwall rock surfaces being exposed to the randklu/t weathering environment over long periods of time. The data and observations from Boundary Glacier support an idea, suggested by Battle (1960), that open and shallow randklu/ts, rather than closed and deep bergschrunds, are a focus of weathering at glacier margins and thus a potential factor in cirque development.

Weathering by segregation ice growth in microcracks at sustained subzero temperatures: Verification from an experimental study using acoustic emissions

“…However, Battle (1 960) cast doubt on these ideas through measurements in deep bergschrunds, where he found stable temperatures at or below freezing, instead of frequent temperature oscillations through 0 "C. He suggested that open randklufts rather than deep bergschrunds are focal zones for frost shattering. On the basis of records of diurnal freeze-thaw air and rock-surface temperature regime in a 2 m wide zone centred on the randkluft lip, as well as observations of water freezing on nearby rock surfaces and of copious in situ loose weathering products on the headwall, Gardner (1987) supported the notion that the randkluft lip is an active frost shattering environment. Although diurnal freeze-thaw activity is localized within a couple of metres of the randkluft lip at any one time, he proposed that seasonal fluctuations of its elevation and much longer term glacier level fluctuations could result in large areas of headwalls being exposed to the narrow randkluft weathering environment over long time periods.…”

“…A strong possibility is that segregation ice growth is responsible for the active weathering, because randklufts and bergschrunds constitute ideal sites for the process. The data of Battle (1960) and Gardner (1987) suggest that ice and rock surface temperatures at depths exceeding 1 or 2 m below the ice or snow surface are maintained for extended periods slightly below 0 "C, near the temperature range favourable for segregation ice growth in rock microfractures. Any trickle of water from solar warming of the exposed headwall surface or rain would tend to bring not only moisture to the rock surface but also heat due to latent heat release upon freezing.…”

“…Moreover, the concept offers a new perspective on the widely recognized, but imperfectly understood, evidence for intense cryogenic weathering in the vicinity of snow patches (e.g. Gardner, 1987).…”

Fracture dynamics in an unstable, deglaciating headwall, Kitzsteinhorn, Austria

“…Previous findings commonly highlight the importance of moisture availability as well as rock temperature for timing and magnitude of frost shattering (Matsuoka et al, 1997) and support the efficiency of frost wedging in glacier headwalls (Matsuoka, 2001, Gardner, 1987, Sanders et al, 2012.…”

“…Protected from solar radiation and daily/seasonal temperature extremes a similar setting may exist in the randkluft, which represents the lowermost part of the investigated headwall system. Surface run-off can enter the randkluft system during the whole summer period and refreeze due to sustained negative rock temperatures inside the randkluft (Gardner, 1987, Sanders et al, 2012. Conditions in randkluft systems may thus favour intense frost action (Matsuoka 2001) and ice segregation (Sanders et al, 2012), and thus serve as important preparatory factors for subsequent slope destabilisation.…”