WHAT DO VARIATIONS IN DEBRIS THICKNESS ON EMMONS GLACIER, MOUNT RAINIER, WA TELL US ABOUT ITS FUTURE?

Mount Rainier is a 14,410’ active volcano in Washington State, and home to 28 glaciers. The largest, Emmons Glacier, covers an area of 4.23 mi2 and showed signs of growth from 1966 to 1991/2. This advance has been attributed to a 1963 rock fall from Little Tahoma Peak that covered the lower glacier with rock debris. Rock debris on a glacier's surface can significantly impact its melt rate and overall mass balance. Thicker debris layers can insulate the ice beneath, reducing the amount of melting that occurs. Conversely, thinner debris layers absorb solar radiation and accelerate ice melt, leading to increased mass loss. Debris cover thickness was measured in pits dug on Emmons Glacier during summer 2018 - 2023. 27 out of 58 pits were unsuccessful, with pit wall failure or a large obstruction preventing access to the debris-ice interface. These failed pits varied in depth, ranging from 12 to 121 cm, and all but two were located near the glacier margins. Closer to the centerline of the glacier, 31 pits reached ice, documenting debris thickness ranging from 2 to 62 cm. Our preliminary results suggest that Emmons Glacier debris cover is thicker near the glacier margins, where landslides from valley walls may deliver debris; this finding is consistent with published results from other glaciers. We do not find a consistent trend of debris cover thickening down glacier, contradicting the expectation of debris accumulation at the glacier terminus. This finding may be attributed to the glacier's rapid recession in recent years. We will collect additional debris cover thickness measurements in summer 2024, including revisiting past pit locations to understand how debris cover thickness may change over time. These new measurements will clarify our preliminary findings and identify areas of the glacier that may experience higher surface melt rates due to thin debris cover.