The connection between glacier debris and melt

Many ice cliffs on Kennicott Glacier seen with an undercutting stream.

Eric Petersen

Many of Alaska's largest valley glaciers are covered, in their lower reaches, by a thin but prominent layer of rocky debris. This debris originates in lateral moraines along the sides of upstream tributaries. As tributaries converge, these lateral moraines are pinched between ice lobes and become conspicuously striped medial moraines. As the glacier carries this debris even further downslope, the thin (1-24" deep) layer of debris insulates the glacier surface, causing areas with thicker debris to melt more slowly and become superelevated in comparison to adjacent white ice. The debris thus gradually spreads out, eventually blanketing the glacier terminus. Though this process is well understood, glaciologists have long been puzzled by an apparent contradiction: debris cover slows the pace of surface melt, and yet long-term melt rates averaged across debris-covered glacier termini generally keep pace with those measured on other debris-free glaciers. How can that be?

Part of the answer is ice cliffs. Debris-covered glaciers are typically quite hummocky, responding as they do to subtle differences in debris thickness and consequent melt rate. Scattered among the hummocks are ice cliffs too steep for debris to rest upon. These cliffs interrupt gentler glacier surfaces mantled with debris, and as they rapidly melt backwards, the cliffs are subject to a nearly constant avalanche of debris sliding from the head to the base of the cliff. Rapid melt on these ever-evolving cliffs helps debris-covered glacier surfaces to waste away at a pace comparable to their cleaner-ice counterparts. But what creates the cliffs in the first place? And why don't the cliffs rapidly bury themselves in the cascade of undermined rocks?

In this recent study, researchers from Alaska, Norway, and the National Park Service conducted field measurements on the debris-covered (and highly visited) terminus of Kennicott Glacier, in Wrangell-St. Elias National Park and Preserve, to investigate the role of supraglacial streams on ice cliff formation and survival. Supraglacial streams, which are simply meltwater streams flowing on the surface of the glacier (rather than below or adjacent to it), turn out to have a highly influential role in the ice cliff lifecycle. Through careful field measurements, remote sensing, and a simple conceptual model, the researchers demonstrate that over 90% of ice cliffs are stream-influenced. They attribute that influence to two main processes: (1) streams thermally erode ice at the glacier surface, creating and then maintaining the relief of ice cliffs; and (2) streams mechanically transport rocks away from the bases of ice cliffs after they cascade down from the head of the cliffs, preventing cliff burial. This research provides a clearer understanding of how debris-covered glaciers work, a process of growing importance as the amount of debris-covered ice in Alaska's national parks grows due to increasing summer melt rates.

Stream hydrology controls on ice cliff evolution and survival on debris-covered glaciers

Abstract

Ice cliffs are melt hot spots that contribute disproportionately to melt on debris-covered glaciers. In this study, we investigate the impact of supraglacial stream hydrology on ice cliffs using in situ and remote sensing observations, streamflow measurements, and a conceptual geomorphic model of ice cliff backwasting applied to ice cliffs on Kennicott Glacier, Alaska. We found that 33 % of ice cliffs (accounting for 69 % of the ice cliff area) are actively influenced by streams, while half are nearer than 10 m from the nearest stream. Supraglacial streams contribute to ice cliff formation and maintenance by horizontal meandering, vertical incision, and debris transport. These processes produce an undercut lip at the ice cliff base and transport clasts up to tens of centimeters in diameter, preventing reburial of ice cliffs by debris. Stream meander morphology reminiscent of sedimentary river channel meanders and oxbow lakes produces sinuous and crescent ice cliff shapes. Stream avulsions result in rapid ice cliff collapse and local channel abandonment. Ice cliffs abandoned by streams are observed to be reburied by supraglacial debris, indicating a strong role played by streams in ice cliff persistence. We also report on a localized surge-like event at the glacier's western margin which drove the formation of ice cliffs from crevassing; these cliffs occur in sets with parallel linear morphologies contrasting with the crescent planform shape of stream-driven cliffs. The development of landscape evolution models may assist in quantifying the total net effect of these processes on steady-state ice cliff coverage and mass balance, contextualizing them with other drivers including supraglacial ponds, differential melt, ice dynamics, and collapse of englacial voids.

Petersen, E., R. Hock, and M. G. Loso. 2024. Stream hydrology controls on ice cliff evolution and survival on debris-covered glaciers. Earth Surface Dynamics (12)3: 727-745.