What causes avalanches

Overview

Avalanches are rapid flows of snow down a slope. They can range from small, harmless events to massive destructive forces capable of destroying forests and villages. Understanding what causes them is crucial for safety in mountainous, snow-covered regions.

The Anatomy of an Avalanche

Avalanches are not random events; they are the result of complex interactions between snowpack characteristics, terrain, and weather. The fundamental cause is the failure of a layer of snow, which then slides down the slope under the influence of gravity. This failure typically occurs when a cohesive slab of snow is pulled away from a weaker, less stable layer beneath it.

Snowpack Structure and Weak Layers

The internal structure of a snowpack is critical. Over time, snow layers accumulate, each with different properties influenced by temperature, wind, and precipitation. These layers can bond together to varying degrees. An avalanche begins when the bond between two layers fails, or when a layer itself becomes too weak to support the weight of the snow above it. Common weak layers include:

• Surface Hoar: Large, delicate ice crystals that form on the snow surface during clear, cold, calm nights. If buried by subsequent snowfall, they can remain a persistent weak layer.
• Depth Hoar: Granular ice crystals that form at the bottom of the snowpack due to temperature gradients. This layer is very weak and poorly bonded.
• Chainsaw Cut: Layers formed during rapid snowfall where individual snowflakes retain some of their original shape, creating poor bonding.
• Old Snow Surface: A crust or surface formed on older snow that becomes buried by new snow.

The Role of Gravity

Gravity is the constant force pulling snow down any slope. When the snowpack's internal strength is less than the gravitational stress acting upon it, an avalanche can occur. Steeper slopes (generally between 30 and 45 degrees) are more prone to avalanches because gravity's pull is more directly aligned with the slope.

Triggers of Avalanches

While the snowpack's instability is the underlying condition, an avalanche needs a trigger to initiate the slide. These triggers can be natural or human-induced.

Natural Triggers

• New Snowfall: Heavy snowfall in a short period adds significant weight to the snowpack, increasing stress on weak layers. A common rule of thumb is that 12 inches of new snow can increase avalanche risk considerably.
• Wind: Wind can redistribute snow, creating drifts and loading certain areas more heavily than others. It can also erode snow from wind-exposed slopes and deposit it on lee slopes, creating unstable slabs.
• Temperature Changes: Rapid warming can weaken the snowpack by melting the bonds between snow crystals, especially at lower elevations or on sun-exposed slopes. Conversely, rapid cooling after a warm spell can sometimes lead to surface crusts that become buried.
• Rain: Rain falling on snow can saturate the snowpack, adding weight and lubricating layers, leading to instability.
• Earthquakes and Tremors: Seismic activity, even minor ones, can shake the snowpack sufficiently to trigger a slide.
• Cornices: Overhanging masses of snow formed by wind, cornices can break off and trigger avalanches on slopes below.

Human-Induced Triggers

Humans are often the direct cause of avalanches, particularly in recreational areas. The additional weight or vibration introduced by a person or machine can be enough to break a critically weak layer.

• Skiers and Snowboarders: Dropping onto a slope from above, or even traversing a sensitive slope, can apply enough force.
• Snowmobiles: The weight and vibration from a snowmobile, especially when it travels across or near a potential avalanche slope, can be a significant trigger.
• Snowshoers and Hikers: While less common than skiers or snowmobilers, they can still trigger avalanches.
• Animals: Large animals like elk or deer can also trigger avalanches, though this is less frequent.
• Explosives: Controlled avalanches are sometimes triggered intentionally using explosives for safety purposes in avalanche-prone areas, like ski resorts or along highways.
• Sound: While often cited, loud noises like shouting or gunshots are rarely sufficient to trigger an avalanche on their own unless the snowpack is already critically unstable.

Terrain Factors

The shape and aspect of the terrain also play a role:

• Slope Angle: As mentioned, slopes between 30 and 45 degrees are most avalanche-prone.
• Elevation: Snowpack conditions can vary significantly with elevation.
• Aspect: The direction a slope faces (north, south, east, west) affects how it is heated by the sun and how wind affects it, influencing snowpack stability. Sun-exposed slopes can develop melt-freeze crusts or become wet and heavy.
• Elevation of Starting Zone: The area where the avalanche begins is crucial.
• Path and Runout Zone: The terrain features that guide the avalanche's path and where it eventually stops.

Conclusion

Avalanches are a natural phenomenon driven by gravity acting on an unstable snowpack. Understanding the factors that contribute to snowpack weakness—such as persistent weak layers, heavy snowfall, and temperature fluctuations—and the various triggers, both natural and human-induced, is essential for anyone venturing into avalanche terrain. Always check local avalanche forecasts and practice safe travel techniques.