An avalanche is a rapid mass of snow sliding down a slope, typically a hill or mountain. This movement of snow, air, and sometimes ice can achieve high speeds and possess immense destructive power. For an avalanche to occur, snow must rest on a steep slope, and an underlying structural weakness must exist within the snowpack. A separate force is then required to initiate the release.
The Essential Ingredients: Snowpack Structure
The foundation for nearly all avalanches is a layered snowpack where strong layers rest upon weak ones. Snow accumulates in distinct layers throughout the winter, with each layer possessing unique characteristics based on the weather conditions during its deposition. The overall snowpack strength is determined by its weakest internal layer.
A temperature gradient is the primary driver for forming these failure points. When the ground is warm and the surface is cold, water vapor moves rapidly through the snow, causing ice crystals to change shape in a process called metamorphism. This process creates unstable crystal types that do not bond well with the snow above them.
One common weak layer is depth hoar, which forms near the ground in shallow snowpacks exposed to prolonged cold. These crystals grow into large, cup-shaped grains that resemble coarse sugar. Another type is surface hoar, which forms on the snow’s surface during clear, calm, and cold nights, similar to frost. When subsequent snowfall buries this layer, it creates a persistent zone of weakness that can last for weeks or an entire season.
Primary Types of Avalanches
Avalanches are classified based on the physical characteristics of the moving snow. Slab avalanches are the most dangerous type, accounting for most avalanche fatalities. These occur when a cohesive layer of snow, known as the slab, fractures and slides as a single unit over a weak layer beneath it. The resulting fracture line is linear and distinct, often stretching for tens or hundreds of meters across the slope.
In contrast, loose snow avalanches, also known as point releases, begin at a single point and fan out as they travel downhill, creating a characteristic inverted ‘V’ shape. These slides involve non-cohesive, loose snow and are generally smaller, posing less risk of burial than a slab avalanche. They most often occur during or immediately following heavy snowfall when the surface snow has not yet bonded.
Avalanches are also categorized by the moisture content of the snow: dry and wet types. Dry avalanches move quickly and can produce a large powder cloud of suspended snow. Wet avalanches, typically occurring in the spring or after a rain event, contain liquid water that weakens the bonds between snow grains. While wet slides move slower than dry ones, the snow is heavier and can be dense and destructive.
Immediate Triggers: The Release Mechanism
The immediate mechanism that transforms an unstable snowpack into a moving avalanche is the application of stress that exceeds the strength of the buried weak layer. The weight of a person or a machine is a frequent trigger for slab avalanches. A skier, snowboarder, or snowmobiler introduces a concentrated stress into the snowpack, and if the weak layer is within three to five feet of the surface, this added weight can cause it to fracture and collapse. In approximately 90% of avalanche accidents, the slide is triggered by the victim or someone in their immediate group.
Natural factors also frequently initiate avalanches by adding weight or reducing the strength of the snowpack. Heavy snowfall and wind loading are common causes, with wind effectively piling snow onto leeward slopes and rapidly increasing the load on the layers below. Cornice falls, where an overhanging mass of snow breaks off a ridge, can also trigger a slide by sending a sudden impact load onto the slope below.
Rapid warming, whether from increased air temperature or solar radiation, can also act as a trigger. As meltwater percolates down through the snowpack, it weakens the bonds between ice grains and lubricates the contact surface of a weak layer, leading to failure. This failure involves the internal collapse of the weak layer, causing a fracture that propagates rapidly, removing the support for the cohesive slab above it and allowing gravity to pull the mass down the slope.