Types of Avalanches
by Auri Clark
On our expedition to the Stikine Icefield in spring 2018, we witnessed many, MANY avalanches. We saw small, loose, dry avalanches cascading off the steep rock faces around us as new snow accumulated; a massive, wet, loose avalanche triggered by a cornice fall from far above us; and pieces of the entire snowpack glide off a slope down valley from our camp. Each avalanche type forms, behaves, and is triggered in different ways…so it is important to know the different types when traveling through avalanche country!
Dry Slab Avalanche: This type of avalanche is the cause of most avalanche fatalities in North America. Dry slab avalanches form when a strong, cohesive layer of snow forms on top of a weak layer. The cohesive layer may be formed by wind, large deposits of snow during a storm, or settlement over time. The weak layer may be faceted snow (forms within or on top of the snowpack when there are large temperature gradients), surface hoar (also known as frost; forms on the surface of the snowpack during cold, clear nights), low density layers within new snow, or a weak interface (such as an ice crust, sun crust, melt-freeze crust, or wind-hardened snow). All slab avalanches start as one unit of cohesive snow that collapses the weak layer. As the slab starts sliding down the bed surface, it begins to fracture and break apart like a pane of glass. Within three seconds, they can reach a speed of 20 mph. After about six seconds, they can get up to an extraordinary speed of 80 mph.
You will sometimes see dry slab avalanches broken down into subcategories: wind slabs, storm slabs, persistent slabs, and deep slabs. All follow the structure of a strong layer on top of a weak layer, usually on top of a bed surface.
The cover photo of Staying Alive In Avalanche Terrain by Bruce Tremper shows a great example of a slab avalanche in action. The crown, where the initial fracture occurred, can be seen in the upper left corner, with the cohesive layer just beginning to fracture further as it slides downhill on the bed surface.
Loose Snow Avalanche: Loose snow avalanches usually start from a point and move downhill in a fan shaped trail. They form from loose, unconsolidated snow on the surface of the snowpack. Loose snow avalanches may also be called point-releases or sluffs. While they tend to be small and start beneath the trigger (instead of slabs which can fracture above the human trigger and easily sweep you off your feet), they can still cause serious damage. Most loose snow avalanche fatalities have been climbers caught in naturally triggered slides from above them. They can also carry skiers and climbers through various terrain traps, such as cliffs, crevasses, or gullies.
The massive wet-loose avalanche the rope team witnessed during our third attempt through the icefall. We believe the avalanche was triggered by a cornice breaking from the peak far above. You can see the slow, cement-flow motion of this wet avalanche.
Wet Avalanches: Wet avalanches can occur in either as slabs or loose snow avalanches, but they move differently and are caused by a different mechanism than dry slab or dry loose avalanches. Loose wet avalanches can occur when the surface layer of snow warms to 32 degrees Farenheit. This can happen via rain, warming, or solar radiation. Wet slab avalanches occur when the weak layer beneath a cohesive layer is penetrated by liquid water, via these same natural methods. Wet avalanches are common in the spring or during rain events. These avalanches tend to be slower moving, which makes them similar to flowing cement and nearly impossible to escape.
Icefall Avalanche: Icefall avalanches, for the most part, occur randomly and are not triggered by people nearby. An icefall is created when a glacier flows over a steep section of terrain, causing the glacier to break apart as it spreads across a convex surface. Icefall avalanches occur when a block of ice breaks off, creating an avalanche of ice beneath it. These avalanches rarely kill people, unless they happen to be in the wrong place at the wrong time. The only pattern with icefall avalanches is they happen more frequently in the heat of the day or during warming events. Avoid traveling beneath icefalls or seracs as much as possible, especially in the afternoon on a sunny day.
Photo taken by Mary Gianotti during our Stikine Icefield expedition in May 2018. This was the first sunny, warm day after several days of mixed snow and rain. With a close look you can see the debris from several wet loose avalanches (or sluffs) on the far right. On the far left and middle, you can see where ice broke off of the icefall above and caused small icefall avalanches. If you look REALLY close, you can see a cornice on the ridge to the upper left. With the rapid warming and sun, we were seeing an avalanche almost every ten minutes.
Cornice Fall Avalanche: Cornices are formed by wind drifting snow blowing across the top of a ridge. The wind tumbles and breaks snow particles into small grains that compact and bond to form an overhanging piece of snow. Cornices can either break off and start an underlying avalanche on the slope below them or they may be large enough to form their own avalanche. Be aware of cornices above you on warm, sunny days, or windy, stormy days. Rapid warming or deposition of more snow can cause cornices to become unstable. Cornices also have the tendency of breaking much farther back than one expects. Observe ridgelines from a distance to look for cornices before approaching, and stay far from the edge!
Glide Avalanche: Glide avalanches occur when the entire snowpack slides, usually due to melt water getting between the snowpack and the ground underneath. It is a slow process (unlike a slab avalanche that fractures and takes off in seconds), and may take several days between the first signs of glide cracks and the release of snow. Much like icefall avalanches, it is nearly impossible to predict when they will occur.
Debris from avalanches occurring throughout the first sunny day. The arrow points to the crown of the (glide or slab…I’m not sure) avalanches that were occurring as sections of this snowpack broke off. The snow was sliding across the smooth rock face at the bottom of the snowpack.
If you want to learn more about types of avalanches or avalanches in general, here are some great resources:
-Staying Alive in Avalanche Terrain by Bruce Tremper
-Allen & Mikes Avalanche Book: A Guide to Staying Safe in Terrain
But the most important resource is an avalanche safety course! All of our expedition members have taken at least a level 1 course, and it is the best way to learn and start practicing safe travel through avalanche terrain.