The onset of a large gravitational slope deformation on Mt. Svínafellsfjall, SE Iceland.

Since the end of the Little Ice Age in the late 19^th century Iceland’s glaciers have experienced dramatic mass loss. Thinning outlet glaciers are exposing oversteepened rock slopes, which sometimes adjust in the form of slow slope deformations that can occasionally result in catastrophic paraglacial rock slope failures. Especially around the country’s deglaciating central volcanoes large landslides have occurred in the past decades. We describe a cluster of gravitational mass movements around the margin of the Svínafellsjökull outlet glacier in Southeast Iceland. The glacier margin is a popular tourist attraction with hundreds of visitors daily. Housing, a hotel, a gas station and the national ring-road are located within one kilometer downstream from the glacier. The largest deformation in the valley is located on the northern slope of Mt. Svínafellsfjall and is evidenced by a 2-km-long fracture system that separates an area of about 1km² and a rock volume in the range of 50-150x10⁶m³ from the mountain slope. The unstable slope is characterized by about elongated 200 sinkholes where soil cover has collapsed into underlying bedrock fractures. In several places across the slope, deep vertical bedrock fractures with no visible vertical displacement were observed. Based on morphological and structural mapping we suggest that the deformation occurs as a composite slide. Remote sensing data, eyewitnesses and field observations indicate that the onset of the deformation occurred between 2003 and 2007. This is parallel with the fastest glacier thinning rate within the 131-year record of existing data. Since 2011 the glacier surface hasn’t lowered significantly, in part due to the deposits of a large debris avalanche from 2013 on the glacial tongue which protect the glacier against ablation. The slope hasn’t shown new signs of deformation since 2018. It is however likely that the slope deformation will start again when glacier thinning continues. Even though deformation rates have been small it is crucial to continue monitoring the slope since several large rock slope failures in Iceland have shown only a short pre-failure deformation period. In a worst-case scenario a catastrophic landslide could travel across the glacier and enter two pro-glacial lakes which may lead to an outburst flood. This study shows how climate change driven glacier thinning has and likely will have further destabilizing effects on paraglacial slopes in Iceland and similar environments elsewhere.