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Abstract Regional assessments of whether a rock glacier is active are mainly based on expert interpretation of surface characteristics. This study presents a quantitative approach to assess rock glacier activity based on magnitudes (average and standard deviation) of rock glacier thickness changes and displacement rates calculated from repeated airborne laser scanning data. In contrast to the established three‐part activity classification, the activity index ρ R indicates the degree of rock glacier activity, with values ranging between 0 (not active, because no surface changes are observed) and 1 (very active, because the highest magnitudes of surface changes are observed). The activity index values determined correspond well with the activity type determined in the Tyrolean rock glacier inventory: rock glaciers with ρ R values > 0.65 are mainly classified as active (77% active, 22% inactive, 1% relict), whereas 88 per cent of all relict rock glaciers correspond to ρ R values < 0.4. For some rock glaciers, the ρ R does not support the classification given in the Tyrolean rock glacier inventory and the activity index provides valuable information to improve and complement existing inventories. The workflow developed can be adapted to other rock glacier data‐sets, allowing quantitatively comparable results to be obtained between different regions. Copyright © 2015 John Wiley & Sons, Ltd.
Bollmann et al. (Fri,) studied this question.