Interpretation of Volume and Flux Changes of the Laurichard Rock Glacier Between 1952 and 2019, French Alps

International audienceRock glaciers are creeping bodies of ice and rock that account for an important part of the mountain cryosphere. In this study, we investigated long-term changes of the Laurichard rock glacier (French Alps), to understand how this rock glacier is responding to climate change. Using feature-tracking and photogrammetric measurements between 1952 and 2019, we quantified changes in thickness, flow velocities and from which we derived the ice/rock flux of the rock glacier at a decadal time scale. This is the first time that emergence velocity and surface mass balance changes have been reconstructed for a rock glacier. Our results reveal a very small surface mass balance ranging from -0.1 m a(-1) to +0.05 m a(-1), reflecting the role of debris in damping the melt rate of the underlying ice. Surprisingly, we found a more negative surface mass balance in the upper part than in the lower part of the rock glacier during the 1952-1971 cold period, likely due to a reduction in rock and snow mass accumulation. Our study shows that thickness changes are mainly driven by changes in surface mass balance except during the most recent period in the lower part of the rock glacier, which was also influenced by a compressive flow related to a protrusion that prevented the rock glacier from advancing. We conclude that the period 1994-2019 witnessed a marked acceleration in rock glacier flow, in agreement with the observations of other rock glaciers in the European Alps. This strong increase in surface speed is likely a consequence of changes in the basal conditions