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.