Slab avalanches are a significant natural hazard in mountainous regions, involving rapid movements of snow down a slope. They are particularly common and hazardous, responsible for over 90% of avalanche-related fatalities. Understanding these powerful snow slides is important for safety in snow-covered mountains.
Understanding Slab Avalanches
A slab avalanche involves a cohesive block of snow sliding down a slope, distinct from loose snow avalanches that start at a single point and fan out. This type of avalanche is identified by a broad fracture line that forms as the cohesive layer breaks away. Slab avalanches can propagate widely.
The formation of a slab avalanche relies on specific snowpack layers. The “slab” is a bonded layer of snow, often composed of multiple fused layers ranging from centimeters to meters deep. Beneath this slab is a “weak layer,” made of less stable snow, which behaves like a layer of marbles with little resistance. This weak layer has less strength compared to surrounding snow.
The “bed surface” is the stable layer or ground beneath, providing the surface over which the slab slides when the weak layer collapses. The combination of a cohesive slab over a weak layer is a fundamental requirement for a slab avalanche. The properties of both the weak layer and the slab determine how far the fracture propagates.
How Slab Avalanches Form
A cohesive slab forms as snow crystals settle and bond over time. New snowfall contributes, and wind significantly impacts this by breaking crystals into smaller particles, packing them densely on leeward slopes, creating a harder, denser slab. Warmer temperatures also encourage snow grains to round and bond, consolidating the snow through a process called snow metamorphism.
Weak layers involve specific snow crystal types that resist bonding well. Persistent weak layers are particularly concerning, sometimes lasting for weeks or months. Faceted crystals develop from significant temperature differences within the snowpack. These sharp, sugar-like grains have minimal cohesion, preventing strong bonds.
Depth hoar, another weak layer, forms near the snowpack’s base under cold, clear conditions and a strong temperature gradient. Surface hoar, feathery ice crystals forming on the snow surface, can become a buried weak layer if new snow covers it. These crystal types maintain their weak structure, forming the unstable foundation required for a slab avalanche.
Conditions and Triggers
Several environmental conditions contribute to slab avalanche instability. Slope steepness is a primary factor, with most slab avalanches occurring on slopes between 30 and 45 degrees. Slopes flatter than 25 degrees or steeper than 60 degrees generally pose less risk for large slabs. Slope aspect influences sun exposure and wind loading, impacting snowpack stability. North, northeast, and east-facing slopes often retain colder, weaker snow.
Recent snowfall, particularly heavy or rapid accumulation, adds significant weight, stressing weak layers. Temperature changes, especially rapid warming or rain, can weaken snowpack bonds and increase instability. Wind loading also creates dense, potentially unstable, wind slabs on leeward slopes.
Slab avalanches are set off by various triggers. Natural triggers include heavy snowfall, rapid temperature increases, and cornice falls. Cornices, overhanging snow on ridges, can break off and initiate slides. Human triggers are common, with skiers, snowboarders, snowmobilers, or hikers adding the load needed to collapse a weak layer. Often, the individual caught in the avalanche is the one who initiated it.
Identifying Avalanche Hazards
Recognizing signs of unstable snow is important for safety. Observable clues include recent avalanche activity, such as visible fracture lines or debris piles. Hearing a “whumpfing” sound indicates a weak layer collapsing within the snowpack, a clear sign of instability that can precede an avalanche on steep slopes.
Shooting cracks, visible fractures radiating from beneath a person on the snow surface, also signal an unstable snowpack. Identifying areas with recent heavy snowfall or significant wind loading suggests potential instability. Assessing snowpack conditions, potentially through snow pits, helps reveal weak layers. Consulting local avalanche forecasts offers current danger levels and specific problems, providing valuable insight before entering mountainous areas.