Pressure Release and Glacial Erosion

Abstract

ABSTRACT.Two tunnels were dug through a small cirque glacier to the rock wall behind (a) the neve, and (b) the lower glacier. The upper tunnel revealed shattering of the rock wall due to the freezing of melt water. The bedrock at the end of the lower tunnel was both smoothed and fractured. The problem is to account for the fracturing of the resistant gneiss where the temperature remains at 0°C. throughout the year. The solution offered is that the release of pressure beneath a thick glacier immediately downstream from a roche moutonnee, or from a small steep step, enables the stresses locked within the gneiss to crack the rock. These stresses result from the gradual removal of the overburden, since the gneiss consolidated under the pressure of thousands of metres of rock strata. The glacier can later remove the jointed and broken rock.

Paraglacial rock-slope stability

“…Stress-release joints weaken the rock mass and also form failure surfaces for mass movements. We do not support an ice-unloading origin for stressrelease joints for reasons given in the preceding section, but instead favour an alternative view that they form from erosion (Balk, 1939;Jahns, 1943;Lewis, 1954;Bradley, 1963;Wyrwoll, 1977;Lo, 1978). The fact that stress-release joints are also found in fluvial valleys (Matthes, 1936) adds significant support to this hypothesis.…”

“…The relative contributions of ice unloading and erosional unloading have been long debated (Matthes, 1936;Jahns, 1943;Lewis, 1954;Harland, 1957;Twidale, 1973) but it seems that the loading and unloading of ice itself may seldom be sufficient to produce sheeting joints, as indicated by field evidence of sheeting joints and rock slope failure distributions (Section. 2.1.2.1).…”

Slope instability in relation to glacial debuttressing in alpine areas (Upper Durance catchment, southeastern France): Evidence from field data and 10Be cosmic ray exposure ages

“…At the smaller scale, Lewis (1954) noted that glacial retreat may be associated with shattering or disruption of glacially-polished bedrock surfaces. Lewis interpreted displacement along joints as due to glacial unloading because (i) joint geometry was not controlled by the local structural pattern and (ii) many joints formed parallel to the former glacier bed, consistent with stress release following deglacial unloading.…”

On the origin of A-tents (pop-ups), sheet structures, and associated forms

“…He cited evidence of recent spalling in cirque heads (Lewis, 1954;Harland, 1957;Gage, 1966) that had been attributed to relief of load consequent on glacier melting. But neither erosional nor glacial unloading is now considered a viable explanation.…”