Mechanisms of debris supply onto rock glaciers

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Recent global warming has caused melting of rock glaciers in mid-latitude high mountains, possibly triggering rock avalanches and debris flows. This research aims at modeling rock glacier dynamics in terms of debris production processes. Field observations in the Swiss Alps involve : (1) automated monitoring of rock joint propagation and rock temperature on rockwalls ; (2) measurements of the size and volume of rockfall debris ; (3) longitudinal variation in the composition of rock debris ; and (4) DC resistivity tomography on rock glacier structure.The research provided the following outcomes. (1) Seasonal freezing enlarges rock joints and subsequent thawing triggers boulder falls, which means that the volume of rockfall debris depends mainly on the maximum freeze-thaw depth. (2) Fallen large boulders can deposit beyond the talus slope, which eventually develop the upper open-work blocky layer overlying the lower frozen debris layer with permafrost creep. Such fall sorting results in the two-layer structure. (3) Recent warming has accelerated the movement of many rock glaciers located near the lower limit of the mountain permafrost belt. However, this in turn decreases the thickness of the debris layer and surface gradient, eventually inactivating the rock glaciers. The rock glacier movement also shows significant inter-year variability in response to snow conditions.Finally, a model of the contemporary debris budget in rock glaciers was proposed on the basis of the field data on the rate of debris production from rockwalls and the tree-dimensional movement of rock glaciers.