A giant Early Holocene tsunamigenic rock-ice avalanche in South Greenland preconditioned by glacial debuttressing

Deposits of a giant (more than 1 km³) rock-ice avalanche with a runout of 15.8 km and an associated failure scar have been discovered in Tupaasat Valley, South Greenland (Kalaallit Nunaat). To study its formation, a geomorphological map of the area has been produced using mainly remote sensing, while the age of the landforms has been constrained based on ¹⁰Be cosmogenic nuclide exposure dating. The rock-ice avalanche landforms include a 1100 m wide mound of displaced material located approximately 6.4 km from the scar, at a change in slope from around 4° to 1.5°. A boulder field containing boulders up to 45 m across lies 0.5 km farther down the valley. Beyond the boulder field are kettle holes up to 45 m in diameter and debris cones interpreted as molards reaching heights up to 36 m and diameters up to 140 m. The source area of the rock-ice avalanche is on a mountain crest above a present-day glacier. The rock-ice avalanche landforms yielded ¹⁰Be ages ranging from c. 12.3 to 9.5 ka with a weighted mean of 10.9 ± 0.5 ka, which coincides with the generally known deglaciation age of the valley. Based on our results, we suggest that the rock-ice avalanche was preconditioned by glacial debuttressing during the deglaciation of the valley and created a tsunami when it impacted a nearby fjord. Such events are expected to be more frequent due to climate change and future ice loss and pose a hazard for populations located near glaciated valleys. A better understanding of past events can help mitigate future large rock-slope failures.