Emplacement dynamics of supraglacial rock avalanches: details from the 2016 Lamplugh event, Alaska

In Glacier Bay Park and Preserve, Alaska, at least 25 rock avalanches occurred since the mid-1980s. The 2016 Lamplugh rock avalanche, with roughly 70 Mm³ deposit volume, is one of the larger events within the park. It originated from a north-facing bedrock ridge without any obvious trigger, and spread 10 km down Lamplugh Glacier. Based on field surveys, high-resolution digital elevation models, and continuous seismic data, we show that the emplacement dynamics of this supraglacial rock avalanche can be described by two distinct stages. Clear long-period seismic signals during Stage-1 record strong interactions of the rock avalanche debris with the ground, suggesting dynamic processes such as grain collisions and fragmentation ('active or dynamic emplacement' of a granular flow). During this first stage, the debris traveled about 5 km from the base of the slope; its deposit is thin and stretched with a dominant dry and flat area in the center, and has narrow raised margins. Stage-2 was essentially aseismic at long periods and dominated by low-friction sliding at slow deceleration rates ('passive sliding'). This sliding produced the distal roughly third of the total runout length where the deposit has a higher density of flowbands and more prominent, raised margins from entrainment and bulldozing of snow. The higher apparent mobility of supraglacial landslides (relative to their counterparts in other runout environments) may be explained by this two-stage model.