A massive avalanche of unknown cause was reported in the Aru Range of Tibet in July 2016. The avalanche sent glacial ice and rock tumbling down the valley, and nine people got killed on the occasion. A second such avalanche happened again in September, only a few kilometers from the area of the first incident.
The scientists who studied the occurrence weren't able to determine the exact cause behind the event, as temperatures and rainfall amount were in a normal range several months before. The most surprising fact was that the part of the glacier that collapsed was situated on a fairly flat part of the terrain.
Several months later, a similar avalanche was reported only several kilometers to the south of the first. According to Andreas Kääb, a glaciologist at the University of Oslo, both of these gigantic avalanches were very unusual, and two of them within close geographical and temporal vicinity is an unprecedented event.
On acquired satellite images, the older of the two avalanches seems significantly darker, meaning that it likely had a smoother and wetter surface, probably because the ice on its surface was longer exposed and had time to melt only partially. However, it's impossible to distinguish between wetness and surface roughness based on a satellite image.
The area before the first avalanche occurred, Tibet, June 24, 2016. Image credit: Landsat-8
The first avalanche, Tibet, July 21, 2016. Image credit: Sentinel-2
Second avalanche, Tibet, September 24, 2016. Image credit: Sentinel-1A
The scientists from the International Association of Cryospheric Sciences and the International Permafrost Association have been working to discover what had caused the avalanches. In the optical and radar satellite data, acquired months before the collapse, they found a distinctive pattern of crevasses on glacier surface, in addition to changes in the height. The pattern indicated the glacier was in the surging process, meaning the upper part of it experienced the unusually rapid ice flow in comparison to the lower part.
Ice in a surging process can flow about 10 to even 100 times faster than usual. However, no documented prior cases caused such violent and swift collapses. According to the data analysis, the first glacier started to surge by September 2015, afterward experiencing a stalling period as the flow reached a narrow valley.
A computer simulation, conducted by a team of Swiss researchers showed the blockage would have caused the extra water to build up beneath and inside the glacier, and the extra lubrication made the glacier more prone to collapse. The pooling of melt water, indicated in the light blue color on the image, could provide another evidence the excess lubrication may have been a part of the avalanche collapse.
An image from the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) on NASA’s Terra satellite shows both avalanches in false color, October 4, 2016. Image credit: NASA/Terra MODIS
According to the scientists, a second avalanche was waiting to happen, as indicated by the same telltale crevasses and changes in height which preceded the first one.On September 21, the researchers warned the Chinese scientists and government that a second avalanche was imminent to the south. However, only a few hours before the warning arrived, the second avalanche already occurred.
Still, the precise cause remains unknown, as there is no proof of a direct physical connection between the glaciers or their collapse. The resemblance between the two events suggest the shared factors such as short-term weather conditions, longer-term climate change, and the geological and topographical environment could have played an important role.
Featured image: An image from the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) on NASA’s Terra satellite shows both avalanches in false color, October 4, 2016. Image credit: NASA/Terra MODIS