Surface motion of active rock glaciers in the Sierra Nevada, California, USA: inventory and a case study using InSAR

Despite the abundance of rock glaciers in the Sierra Nevada of California, USA, few efforts have been made to measure their surface flow. Here we use the interferometric synthetic aperture radar (InSAR) technique to compile a~benchmark inventory describing the kinematic state of 59 active rock glaciers in this region. Statistically, these rock glaciers moved at speeds range from 15 cm yr 1 to 88 cm yr 1 with a mean value of 55 cm yr 1 in the late summer of 2007. We also find a spatial gradient: rock glaciers in the southern Sierra Nevada moved faster than the ones in the central Sierra Nevada. In addition to the inventory mapping, we also conduct a case study to measure the surface flow of the Mount Gibbs rock glacier in fine spatial and temporal detail. The InSAR measurements over this target reveal (1) that the spatial pattern of surface flow is influenced by surface geomorphological features and (2) a significant seasonal variation of flow speed whose peak value was 48 cm yr 1 in the fall, more than twice the minimum value observed in the spring. The seasonal variation lagged air temperatures by three months and likely results from temporal changes in mechanical strength of mixing debris and ice, internal melting of ice, and surface snow cover. Our finding on the seasonal variation of surface speed reinforces the importance of a long time series with high temporal sampling rates to detect possible long-term changes of rock glaciers in a warming climate.