Rapid sequestration of rock avalanche deposits within glaciers

Abstract

Topographic development in mountainous landscapes is a complex interplay between tectonics, climate and denudation. Glaciers erode valleys to generate headwall relief, and hillslope processes control the height and retreat of the peaks. The magnitude-frequency of these landslides and their long-term ability to lower mountains above glaciers is poorly understood; however, small, frequent rockfalls are currently thought to dominate. The preservation of rarer, larger, landslide deposits is exceptionally short-lived, as they are rapidly reworked. The 2013 Mount Haast rock avalanche that failed from the slopes of Aoraki/Mount Cook, New Zealand, onto the glacier accumulation zone below was invisible to conventional remote sensing after just 3 months. Here we use sub-surface data to reveal the now-buried landslide deposit, and suggest that large landslides are the primary hillslope erosion mechanism here. These data show how past large landslides can be identified in accumulation zones, providing an untapped archive of erosive events in mountainous landscapes.

Figure 1. Recent rock avalanches from the ridges of Aoraki/Mt Cook.

(a) Hillshaded elevation model of the Mt Cook area showing recent rock avalanche deposit outlines as emplaced. (b) NASA EO-I ALI Satellite image of the landslide deposit captured on 02/13/13, with GPR profile marked. (c) NASA Landsat 7 image captured on 5 November 2013, in which no evidence of the landslide is visible, in situ deposit outline for context. Scale bars in (a) is 2 km, and in (b) and (c) 1 km.

Figure 2. The 2013 Mount Haast rock avalanche deposit.

The image was taken 25 days after failure and shows features discussed in the text. X–Y is the approximate position of the 50-MHz radar line (Fig. 4). Source: Charlie Hobbs, Southern Alps Guiding, New Zealand.

Figure 3. Reworked rock avalanche debris emergence.

(a) View from the helicopter above Tasman Glacier looking north west at rock avalanche debris in the Hochstetter Icefall, with none visible on the Grand Plateau behind; Plateau Hut to the right for scale. (b) View from Grant Plateau looking north east (up flow) at rock avalanche debris in a crevasse on the inferred Eastern deposit margin buried beneath the 2013/14 snow and firn. The debris thins to a dust layer to the right.

Figure 4. GPR data obtained from the Grand Plateau.

(a) The 100-MHz data with snow layers overlying the 2013 rock avalanche deposit showing raised lateral rim, variable thickness and a poorly resolved base due to snow/entrainment. (b) Zoomed portion of (a) highlighting the dielectric contrasts between snow layering in the upper third and the clear rock avalanche surface reflector at 2,193 m. (c) The 50-MHz data showing the longer profile as marked in Fig. 1 with penetration to bedrock at the south-eastern end of the survey. Hyperbolas at depth relate to non-rock avalanche debris point returns and are interpreted as minor, older rockfalls.