Deep seated gravitational slope deformation north of the Tungnakvíslarjökull outlet glacier, in western Mýrdalsjökull ice cap, S-Iceland

A large deep seated gravitational slope deformation has been detected in a mountain slope north of the Tungnakvíslarjökull outlet glacier, in the western part of the Mýrdalsjökull ice cap in South Iceland. Mýrdalsjökull also hosts the Katla central volcano, which erupted spectacularly last in 1918. Based on comparison of Digital Elevation Models (DEMs) obtained from aerial photographs, lidar and Pléiades stereoimages, the slope has been showing slow gravitational slope deformation since 1945 to present. The total vertical displacement in 1945-2020 is around 200 m. The deformation rate has not been constant over this time period and the maximum deformation occurred between 1999 and 2004 of total of 94 m or about 19 m/year.

The mountain slope north of the Tungnakvíslarjökull outlet glacier reaches up to around 1100 m height. The head scarp of the slide, which is almost vertical, is around 2 km wide rising from about 400-500 m in the western part up to the Mýrdalsjökull glacier at 1100 m in the east. The area of deformation, from the head scarp down to the present-day ice margin is around 1 km². The total volume of the moving mass is not known as the depth of the sliding plane is not known, but the minimum mobile rock volume is between 100 to 200 million m³. The entire slope shows signs of displacement and is heavily fractured. Continuous GNSS stations which were installed in the uppermost part of the slope in August 2019 and in the lower part of the slope in 2020 provide real-time displacements. The GNSS time series show evidence of seasonal motion of the landslide, with highest deformation rates occurring in late summer or fall. Historically, seismicity in the area has been at maximum in the fall, although little seismicity has been observed since the GNSS stations were installed.

There are two main ideas of the causes for this deformation. One is the consequences of slope steepening by glacial erosion, followed by unloading and de-buttressing due to glacial retreat. Another proposed cause for the deformation is related to its location on the western flank of the Katla volcano. Persistent seismic activity in this area for decades may be explained by a slowly rising cryptodome into the base of the slope, which may also explain the slope failure.