Abstract
AbstractThick supraglacial debris layers often have an undulating, hummocky topography that influences the lateral transport of debris and meltwater and provides basins for supraglacial ponds. The role of ablation and other processes associated with supraglacial debris in giving rise to this hummocky topography is poorly understood. Characterizing hummocky topography is a first step towards understanding the feedbacks driving the evolution of debris‐covered glacier surfaces and their potential impacts on mass balance, hydrology and glacier dynamics. Here we undertake a geomorphological assessment of the hummocky topography on five debris‐covered glaciers in the Everest region of the central Himalaya. We characterize supraglacial hummocks through statistical analyses of their vertical relief and horizontal geometry. Our results establish supraglacial hummocks as a distinct landform. We find that a typical hummock has an elongation ratio of 1.1:1 in the direction of ice flow, length of 214 ± 109 m and width of 192 ± 88 m. Hummocky topography has a greater amplitude across‐glacier (15.4 ± 10.9 m) compared to along the glacier flow line (12.6 ± 8.3 m). Consequently, hummock slopes are steeper in the across‐glacier direction (8.7 ± 4.3°) than in the direction of ice flow (5.6 ± 4.0°). Longer, wider and higher‐amplitude hummocks are found on larger glaciers. We postulate that directional anisotropy in the hummock topography arises because, while the pattern of differential ablation driving topography evolution is moderated by processes including the gravitational redistribution of debris across the glacier surface, it also inherits an orientation preference from the distribution of englacial debris in the underlying ice. Our morphometric data inform future efforts to model these interactions, which should account for additional factors such as the genesis of supraglacial ponds and ice cliffs and their impact on differential ablation.
Highlights
Debris-covered glaciers are widespread throughout tectonically active mountain ranges (Herreid & Pellicciotti, 2020) where high rates of rock uplift provide sediment to glacier surfaces by landsliding (Schomacker, 2008)
In High Mountain Asia, 30% of the ice mass in glacier ablation areas is covered with rock debris (Kraaijenbrink et al, 2017)
Observations from glaciers with less mature debris layers than those in the Everest region indicate that hummocky topography initially forms from pronounced medial moraines (Mölg et al, 2020), and these longitudinal forms appear to persist as the debris layer thickens to cover the entire glacier width
Summary
Debris-covered glaciers are widespread throughout tectonically active mountain ranges (Herreid & Pellicciotti, 2020) where high rates of rock uplift provide sediment to glacier surfaces by landsliding (Schomacker, 2008). Focusing on a sample of five debriscovered glaciers in the Everest region, we undertook extensive statistical characterization of the topography of their debris-covered sections and developed a first-order classification of hummocks as a supraglacial landform We used these results to discuss the physical factors and processes that determine the range of observed hummock forms, the glacier-scale controls on their morphometry and the conditions for their formation and evolution. To complement the statistical characterization, which treats individual hummocks as countable, a surface roughness method was employed to analyse the continuous topography of each sampled area with no a priori assumptions of what constitutes a recognizable landform (Gadelmawla et al, 2002) This approach quantifies the landscape so that information on processes can be inferred over a length-scale continuum. Unequal variance t-tests were conducted on the H2 values using a nominal difference of 10% (Shepard et al, 2001) to determine if their mean values differed significantly in the two directions
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