ICE-MOTION CONTROL ON SUPER-GLACIAL DEBRIS THICKNESS AND MATURITY, MATANUSKA GLACIER, ALASKA

Our study was designed to provide new information concerning an observed transition in super-glacial material from unstable glacial debris to stable debris upon which soil is developing and vegetation is growing along the northern lateral moraine of the Matanuska Glacier, Alaska. This transition, generally 15 to 40m wide, is hypothesized to result from either a boundary between moving and stagnant ice, or a threshold of debris thickness at which stable soils become established and can support vegetation. This study indicates the transition is caused by a boundary between moving and stagnant ice. A threshold of debris thickness correlates to the transition, but it is not the cause of the transition; rather, it is a function of ice movement and time elapsed since motion ceased. The nature of the stagnant ice at the northern lateral moraine resembles the stagnant ice-cored-moraine that is detached from actively flowing glacier ice due to a change in ice dynamics.

A model of the subsurface structure and movement of ice was constructed based upon several sources of information collected during summer 2002: 2D and 3D ground penetrating radar (GPR) surveys, 5 hand-dug test pits, global positioning system (GPS) data, topographical surveys, and relative velocity determined from fixed markers. Data indicate that ice covered by unstable debris moved a maximum of 6m relative to ice covered by stable vegetated soils over an 18-day period. Additionally, data indicate a change in debris thickness from 0.17m (unstable glacial debris) to 10m (stable vegetated soils). Debris thickens northward toward the Talkeetna Mountains and away from the glacier, closely following surface topography. A threshold of debris thickness correlates with the transition between moving and stagnant ice. Unstable glacial debris exists until the depth to buried ice reaches 5.5m to 6.5m. At this point, vegetated soils emerge and the depth to buried ice changes little, 6.5m to 7.5m, followed by the development of densely vegetated soils. This change in depth to buried ice manifests itself as the transition from unstable glacial debris to stable vegetated soils, indicating a threshold depth to buried ice of 5.5m to 6.5m, where stable soil can exist and support vegetation.