Sherman Glacier Rock Avalanche, Alaska, U.S.A.

Many rock avalanches fell during a major earthquake in Alaska in 1964; most fell on glaciers where they attracted considerable attention. Two distinct forms can be recognized among them: a lobate form, where debris moved largely as one coherent spreading lobe; and a digitate form, where debris separated into many, long, narrow, curving streams. The most studied lobate form, the Sherman Glacier rock avalanche, fell within minutes of initiation of shaking in the earthquake. The estimated 10.1 × 106 m3 mass of already highly fractured rock may have behaved thixotropically in the shaking, but, in any case, was soon fluidized and flowed and slid from Shattered Peak to spread over 8.25 km2 of Sherman Glacier to an average depth of 1.65 m. The fluidized debris behaved as a complex, perhaps dilatant Bingham plastic, with an estimated shear strength of 2 kN/m2 and two apparently stress-and strain-rate-dependent Bingham viscosities (about 106–107 Ns/m2). Although the energy used for deformation and dispersal was very high (2.67 × 1014 J), the debris was always so very viscous that it appeared to slide rather than flow, as a thin flexible sheet, as it spread under its own weight; this energy was dissipated largely as heat through friction against the snow-covered glacier. The mass slid at speeds averaging 26 m/s, but reaching as high as 67 m/s. The low coefficient of basal friction (0.11) apparently resulted from rolling and sliding of clasts against soft wet snow. Rebounds from elastic collisions in the deforming debris caused a statistical loss of contact between particles, so reducing internal friction (μ = 0.03) that they behaved like molecules in a fluid. A mechanism of mechanical fluidization, first suggested by Albert Heim more than 90 years ago, and the consequent supercritical laminar flow as a Bingham plastic, can form almost all of the many features seen in lobate rock avalanches without recourse to special lubricants at the base such as a trapped layer of compressed air. The few features not explained by the supercritical plastic flow are features derived from the parent block and its style of disintegration. The digitate avalanche form, such as the Allen II rock avalanche of 1964, may result from a delayed mode of disintegration as compared to the dominant fragmentation before flow of the lobate form. The curving ribbons of debris in digitate avalanches may be left as thin carpets “unrolled” by crushing from the bases of large rolling boulders. If boulders are more conical than cylindrical about their axes of rotation, they roll in curved paths whose direction may change abruptly as the taper, or even the direction of taper, changes during fragmentation.