Glide-snow and snow block avalanches in South Africa: documentary and photo-based observations

Field observations of snow mass movement phenomena, as transient features of snow-covered mountains, are underreported from many global regions and as such, their triggers, flow characteristics and potential geomorphic expressions remain relatively unknown. Here, we report unusual glide-snow and snow block avalanches from the Drakensberg of southern Africa, that took place in September 2024, in which a sudden and relatively heavy spring snowfall resulted in multiple snow mass movement events. Our fortuitous photo-based field observations show that glide-snow and snow block avalanches are generated at the foot of rock cliffs and extend some tens of metres (max = ∼140 m) downslope as thread-like channels. They developed here under specific climatological conditions that led to snowpack instability, triggered by warming temperatures and liquid water infiltration into the snowpack. The glide-snow avalanches terminate at different mid-slope locations when kinetic inertia becomes lower than frictional resistance. This is also influenced by slide geometry and roughness. The short time over which glide-snow and snow block avalanches develop and melt (scale of a few hours), and their lack of residual geomorphic expression, are the reasons why they are under-reported mass movements and snow hazards in mountain environments.