Abstract

Malaspina Glacier is a piedmont ice sheet covering approximately 850 square miles on the flat coastal foreland of southern Alaska. The Seward-Malaspina glacier system has experienced marked deficits in 6 out of 9 budget years between 1945 and 1954. In 1 of the remaining 3 years it had a good surplus, and in the other 2 years the economy was about balanced. Variation in precipitation has exerted a major influence on regimen.

Seismic explorations show that the ice, 2000 feet thick, lies in a basin at least 825 and possibly 1000 feet below sea level. Data support speculation that the closure of this basin is due about one-third to glacial erosion and two-thirds to peripheral glacial deposition.

Geometrically and structurally, Malaspina Glacier is simple in broad aspects but complex in detail. An extensive system of radial crevasses, a prevailing foliation, and two sets of tight joints are consistent with the broad lobate form of the sheet and with the stresses causing it to spread out on the coastal foreland. Near the center of the glacier the foliation dips too steeply (75°–85°) to be related to near-surface shearing, so it is attributed to plastic deformation at some depth. An extensive and complex system of folds, spectacularly displayed on the glacier's surface, deforms ice streams and debris bands about near-vertical axes. These structures were formed within the Malaspina and are not inherited from higher in the system. They are attributed to slip folding, or to flowage with minor modification by slippage, and developed in response to the strong shove of ice pouring out of the mountains. The Malaspina Glacier spreads out and at the same time maintains sufficient thickness and surface slope to flow uphill across the coastal foreland by folding up accordian fashion.

Deformation of an aluminum pipe in a 1000-foot borehole near the center of this glacier provides information on internal flow. Differential movement in 1 year between top and bottom of the hole was 5.75 feet, with the top moving the most. Deformation recorded in the uppermost 300 feet over a 3-year period extends to within less than 50 feet of the surface, amounts to 1.5 feet, and follows a similar curve. Malaspina borehole observations do not support the concept of extrusion flow. Plots of these data suggest that the strain rate is not an exponential function of depth, but rather that the stress-strain-rate relationship can be described by the commonly used function γ̇ = krn, with a value of about 3 for n. Yielding appears to have occurred at a shear stress as small as 0.01 bar.

Movement of the Malaspina Glacier seemingly involves basal slip or boundary-layer flow as well as internal flowage. Movement of the ice is controlled primarily by surface slope, and steeper parts of the ice surface overlie ascending as well as descending reaches on the subglacial floor. Simple calculations based upon ablation, surface slope, and horizontal velocity show that surfaceward movement of ice is required near the glacier's margin to maintain elevation and profile. Nye's compressive flow seems the most likely mechanism for producing such surfaceward movement.

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