Late Holocene Activity of Sherman and Sheridan Glaciers, Prince William Sound, Alaska

Two adjacent glaciers in the Chugach Mountains of south-central Alaska have markedly different histories on decadal to perhaps centennial timescales. Sheridan Glacier has advanced and retreated hundreds of metres during the latest Holocene. Its recent fluctuations have markedly altered local base level of Sherman River, which drains Sherman Glacier and flows into Sheridan Lake. Sheridan Glacier advanced to its greatest extent during the Little Ice Age, raising base level of Sherman River and inducing aggradation there of up to 17 m of sediment. Retreat of Sheridan Glacier formed a series of lakes that have coalesced. As lower lake outlets have become available, base level of Sherman River has dropped, resulting in the evacuation of substantial volumes of sediment from Sherman River valley. In about 2000, the terminus of Sheridan Glacier began to disintegrate; retreat accelerated dramatically in 2010. By 2016, the glacier had retreated an average of 600 m from its 2010 terminus, although some areas retreated up to 1.9 km and others did not retreat at all. Meanwhile, Sherman Glacier continued a slow advance initiated by a rock avalanche that blanketed much of its ablation area in the 1964 Alaska earthquake. © 2018 Elsevier Lt

Dynamic rock fragmentation causes low rock-on-rock friction

The low frictional resistance to rock-on-rock sliding reported in large blockslides, in coseismic fault rupture and in laboratory-scale rock friction tests has been attributed to a variety of causes. Herein we propose a mechanical explanation for the reduced friction, which seems likely to be universally relevant to complement other mechanisms. Rock-on-rock sliding of intact brittle rocks always generates a layer of comminuted debris. Rock must fail in order to form and further comminute debris; at local strain rates >> 100 s-1, recycling of elastic strain energy stored in accomplishing fragmentation generates instantaneous, local, GPa-range isotropic pressures similar to the rock’s Hugoniot elastic limit (Q). Under rapid strain, simultaneously fragmenting grains deliver large normal forces to the boundaries of the comminuting layer, reducing the confining stress on the debris (and hence its resistance to shear), thus lowering the frictional resistance to slip. This behaviour corresponds quantitatively to published laboratory data on granite friction; to the dynamics of low-angle blocksliding and faulting; and to our data on rapid shearing of fragmenting dry coal