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Roger Weller, geology instructor

Lindsey Patterson
Physical Geology
Fall 2011


Snow Avalanches


Millions of people each year vacation to winter destinations and take part in snow recreations such as snowboarding, skiing, and snowmobiling. But what they don’t realize is that they may be putting themselves in life threatening danger. What’s even worse is that they very well may cause their own death accidentally. Avalanches kill more than 150 people worldwide every year, and 90% of avalanche incidents are caused by the victim themselves. One in every 10 people caught in an avalanche will die.



Avalanches are sudden slides or drops of snow. The event is sudden but there are often warning signs. If you are heading into an area where an avalanche is possible, you should first check the local avalanche report before leaving home. If the avalanche danger is high, this should be your first warning sign. Other warning signs include:


·         Evidence of an avalanche, or witnessing an avalanche

·         You see cracks at the top of the slope or around your feet or skis

·         The ground feels hollow under your feet or skis

·         You see patterns on the snow caused by strong winds

·         You hear a “whumping” sound as you walk.



Causes of an avalanche can include storms that produce 12 inches or more of snow can cause an avalanche. The quick pileup overloads the packed snow underneath and causes a weak layer beneath the slab to fracture. Strong winds can cause snow that has fallen on dangerous drifts to slide. The snow in these avalanches will most likely have patterns across the top caused by the wind. Slope angling is in indicator for the most likely spots for an avalanche to occur. Slopes between 30 and 45 degrees are most common avalanche sites, which incidentally is the perfect angle for skiing. Earthquakes are also another cause of avalanches. When the plate tectonics make a sudden shift below the Earth’s surface, it causes an earthquake above ground. When this occurs, the shaking can cause snow it slide and fall. Once an avalanche starts, it can reach speeds of more than 200 miles per hour!



There are three basic types of snow avalanches:


·         Powder or Sluff avalanches

·         Slab avalanches

·         Wet avalanches


Many avalanches are “sluffs” or powdery, formless masses. Powder avalanches can gain momentum as they go, allowing them to collect more snow and grow larger. They have been known to injure skiers by pushing them off cliffs and rock bands.



            Slab avalanches are the most fatal type of avalanches. The majority of avalanche related deaths are by slab avalanches. These occur most often when a more cohesive or harder layer of snow collects on a weaker or less cohesive layer of snow. The under layer of weak snow becomes very fragile and can collapse with even the slightest bit of excess pressure. (i.e. skiers or snowboarders). Typically, slab avalanches are large volumes of fast moving snow.


Anatomy of a Slab Avalanche



Wet avalanches occur mostly near the end of winter when the temperature is beginning to warm. When temperatures are above 32 degrees for an extended period of time, the snow begins to melt and is more susceptible to sliding and falling off cliffs. Once it begins to slide, the snow can flow like water.



        When you hear the word “avalanche” most people think of snow avalanches. But what most people don’t know is that there are other types of avalanches. Rock and mud slides are considered avalanches. Sometimes, much like a Slab avalanche, a whole side or chunk of a mountain can fall. Excessive amounts of rain can cause mud slides. Rock instability and earthquakes can cause rock slides. Volcanic eruptions can cause an avalanche of rocks and mud. Massive clouds of volcanic ash, or pyroclastic flows, are also considered avalanches. These avalanches are extremely dangerous because if you are caught in one of these, there is no way out. They have a blast wave in front of them that can stretch long distances. Even if you aren’t close to the avalanche itself, you could still be caught in the range of the blast wave. The temperatures of the blast wave are so hot that it will kill you instantly.



        The best defense is a good offense when it comes to avoiding avalanches. You should ALWAYS be prepared when you are in an area where avalanches are possible. You should never travel alone; use the buddy system. Be sure to check your local avalanche reports so you can avoid areas with high avalanche warnings. Keep an eye out for warning signs that may indicate an avalanche is possible in the area and know what kinds of environments are more prone to avalanches.

            Avalanche preparation and safety classes are available in most areas, especially areas that receive snow regularly. Your local police and fire department should know when and where classes are available as well as online.




Photos by Ilan Adler (,r:9,s:0&tx=35&ty=112