Figure 13. The blue line delineates the boundary of a surge-type glacier (Chudley and Willis 2019). Two rock glaciers (wkl019 and wkl020) in our inventory are situated in the surroundings.
6 Conclusions
We mapped rock glaciers at a regional scale and quantified their surface kinematics by combining InSAR and image semantic segmentation powered by deep learning. The combined method was applied to map rock glaciers across the West Kunlun Mountains, where the extremely dry climate represents one characteristic environmental setting on the Tibetan Plateau. We draw the main conclusions as follows:
(1) The DeepLabv3+ network trained by manually labelled data based on InSAR and Google Earth images can successfully identify and delineate rock glaciers from Sentinel-2 images, attaining an IoU value of 0.801 for both training and validation datasets. The well-trained model newly mapped 123 rock glaciers to supplement the non-exhaustive InSAR-based sub-inventory of 290 active rock glaciers.
(2) There are 413 rock glaciers mapped over the study area, including 202 glacier-connected rock glaciers (G-RGs), 143 debris-mantled slope-connected rock glaciers (DMS-RGs), 41 glacier forefield-connected rock glaciers (GF-RGs), and 27 talus-connected rock glaciers (T-RGs). The mapped rock glaciers occupy a total area of ~ 108 km2 and are located at altitudes between 3389 m and 5541 m. The average slope angle is 17° and the dominating landform aspect is towards the east.
(3) Among the mapped rock glaciers, the G-RGs and GF-RGs are larger (average areas: 0.40 km2 and 0.38 km2) and occur on gentler slopes (14° and 15°) predominantly facing northeast, whereas the DMS-RGs are the smallest (0.05 km2) and occupy steep (23°) southeastern-facing slopes at the highest altitudes (4889 m). The T-RGs display a medium size (0.20 km2) and slope angle (18°) and mostly occur on southeastern-facing slopes at lower altitudes (4332 m). The GF-RGs have the lowest average altitude (4265 m).
(4) Considering the geomorphologic context, we postulated that the glacier – debris-covered glacier – rock glacier transition is currently inactive due to the abnormal mass gain of glaciers in the West Kunlun: the mapped G-RGs and GF-RGs evolved from glacier to rock glacier during the past Holocene glacial periods, e.g., the Little Ice Age and the Neoglaciation. Surge events of glaciers may provide material supply and promote the glacier-to-rock glacier transition in the future.
(5) Based on the geomorphic characteristics of mapped rock glaciers, we suggest that the genesis of T-RGs and DMS-RGs are controlled by periglacial processes. The DMS-RGs, as a distinct type of rock glaciers in our study area, represent embryonic rock glaciers derived from prevalent mechanical erosion of the slopes and interstitial ice formation during the Little Ice Age. Note that the hypothesis on landform genesis formulated here needs further validation based on measured evidence.
(6) We adopted the spatial average velocity of all pixels within the boundary of each rock glacier to represent the landform surface kinematics. In total, 256 rock glaciers have valid kinematic quantifications. Nearly 90% of the rock glaciers move slower than 50 cm yr-1. The mean downslope velocity is 24 cm yr-1, and the standard deviation is 22 cm yr-1. The median and maximum velocities are 17 cm yr-1 and 127 cm yr-1, respectively.
(7) Among the active rock glaciers, the G-RGs and GF-RGs move faster at mean velocities of 31 cm yr-1 and 35 cm yr-1, respectively. The DMS-RGs and T-RGs creep at a slower average velocity of 17 cm yr-1.
In summary, combining InSAR and high-resolution optical imagery to manually map active rock glaciers proves to be an effective way to quantify rock glacier kinematics consistently in remote areas. With the utilization of deep learning techniques, it is promising to compile rock glacier inventories efficiently over a significant extent of permafrost areas, e.g., the Tibetan Plateau, which provides a baseline dataset and allows the monitoring of rock glaciers as indicators of permafrost degradation and potential water sources in a changing climate.