227-4 How does supraglacial debris grain size influence surface lowering on Emmons Glacier, Mount Rainier, WA?

Session: Recent Advances in Glacial Geology, Geomorphology, and Chronology (Posters)

Presenting Author:

Authors:

Abstract:

Mount Rainier is a 14,410’ active volcano in Washington State, home to 28 glaciers. The largest, Emmons Glacier, covers 4.23 mi² and advanced in the late 20th century, a change attributed to a 1963 rockfall from Little Tahoma Peak that blanketed its lower surface with debris. Supraglacial debris can significantly alter melt rates and mass balance by either insulating the ice or enhancing melting depending on debris thickness and the grain size of the sediment covering the ice. This project investigates how the grain size of supraglacial debris influences surface lowering on Emmons Glacier. Previous work on debris covered glaciers shows that grain size affects debris thermal properties, with finer material providing stronger insulation and coarser material allowing greater heat penetration. Although studies on Emmons Glacier have documented substantial surface elevation lowering in recent decades, the relationship between supraglacial debris grain size and melt remains underexplored. A total of 75 sediment samples were collected over three summers to characterize the grain size of debris covering the glacier. At 48 sites, pits were dug to the ice surface or until the pit walls became unstable and collapsed, preventing further digging. Sediment samples were collected at the surface, base, and at intermediate depths where sediment changes were visible. In the laboratory, samples were dried, weighed, and sieved to quantify grain size distribution. Weight percentages for each fraction were calculated, and grain size distribution curves were generated. These measurements were compared with high-resolution surface elevation profiles to examine how debris characteristics relate to patterns of surface lowering and glacier surface morphology. We expect that finer, more weathered material is located along the glacier margins where ice flow rates are slower and debris has been exposed for longer; and coarser, less weathered material towards the glacier centerline, where debris is recently delivered to the surface. This variation in grain size likely influences melt rates, as finer material at the margins may provide stronger insulation, while coarser material in central areas allows greater heat penetration and enhanced melt.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-7782

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