Effects of rock avalanches on glacier behaviour and moraine formation

Although large rock avalanches are infrequent, sediment production in active orogens is dominated by such events, which may strongly influence geomorphic processes. Where rock avalanches fall onto glaciers, they may affect glacier behaviour and moraine formation. We outline the processes of rock avalanche initiation and motion, and show that supraglacial deposits of rock avalanche debris have distinct sedimentological and thermal properties. Laboratory experiments on the effects of such debris on ice ablation are supplemented by field data from two rock avalanches in the Southern Alps, New Zealand. Their effects are compared with those of the thinner supraglacial debris that results from small rockfalls and melt-out of englacial debris. Implications of rock–avalanche debris cover for glacier behaviour are explored using a mass-balance model of the Franz Josef Glacier in New Zealand, demonstrating a likely supraglacial rock avalanche origin for the Waiho Loop moraine, and considering the potential hazard of a large rock avalanche onto the present-day glacier.