Evaluation of factors that led to the 1967 paraglacial slope failure at Grewingk Lake and Glacier, Alaska

The interaction between glacier retreat and rock slopes has gained considerable attention in the past years due to climate change. Glaciers shape mountain slopes and can daylight zones of weakness as they recede. Glaciers can act as a preparatory factor or trigger for slope failure. A retreating glacier at the slope's toe is often cited as the cause of failure. However, the relationship between glacier retreat and rock-slope stability is much more complicated, particularly for landslides that lack an explicit trigger. We studied a paraglacial slope failure at Grewingk Lake and Glacier in southcentral Alaska, United States. The collapse occurred on October 14, 1967, with no specific trigger, such as heavy rain or seismic activity on the day of the event. Grewingk Glacier is a lake-terminating glacier that has experienced and continues to experience rapid retreat, as have most glaciers around the world. The rapid retreat and the location of the glacier at the time of the collapse could lead to the conclusion that this was the cause of the collapse. However, a thorough examination of the structural geology of the slope and processes that could contribute to reduce the slope stability showed that the retreat of the glacier is only part of the tale. The structural preconditioning, together with an accumulation of seismic activity and daylighting fracture planes progressively contributed to the slope's destabilization. Our study emphasizes the value of examining the temporal trends of paraglacial rock-slope failures in situations in which there was no evident trigger at the time of the collapse.