Avalanches:
Sudden downhill movement of snow.
Most avalanches occur spontaneously during storms under increased load due to snowfall. The second largest cause of natural avalanches is metamorphic changes in the snowpack such as melting due to solar radiation. Other natural causes include rain, earthquakes, rock fall and icefall. Artificial triggers of avalanches include skiers, snowmobiles, and controlled explosive work.
Slab avalanches
Form frequently in snow that has been deposited, or redeposited by wind.
Look like a block (slab) of snow cut out from its surroundings by fractures.
Features include: crown fracture at the top of the start zone, flank fractures on the sides of the start zones, and a fracture at the bottom called the staunchwall. The crown and flank fractures are vertical walls in the snow delineating the snow that was entrained in the avalanche from the snow that remained on the slope.
Vary in thickness from a few centimetres to three metres.
90% of fatalities in backcountry users are slab avalanches
Powder snow/dry snow avalanches
Form turbulent suspension currents known as powder snow avalanches or mixed avalanches.
Consist of a powder cloud, which overlies a dense avalanche.
Usually occur with fresh dry powder.
Speeds of 300+ km/h, and masses of 10,000,000 tonnes; their flows can travel long distances along flat valley bottoms and even uphill for short distances.
Wet snow avalanches
Low velocity suspension of snow and water
The low speed of travel is due to the friction between the sliding surface of the track and the water saturated flow.
Wet snow avalanches are capable of generating powerful destructive forces, due to the large mass, and density.
They can plough through soft snow, and can scour boulders, earth, trees, and other vegetation; leaving exposed, and often scored, ground in the avalanche track.
Initiated from either loose snow releases, or slab releases, and only occur in snow packs that are water saturated and isothermally equilibrated to the melting point of water.

How do avalanches begin?
The snow packed down on the surface cannot support itself with all the weight. When another factor is introduced, such as a person’s step, this helps to loosen the snow and an avalanche occurs. Major temperature changes, rapid wind speed and man-made influences are the main causers of why avalanches occur.
Most avalanches begin within weak layers of snow, which evolve within the snowpack or form on top of the snow and become buried. Eventually these weak layers can no longer hold up the weight on the overlying snow, and will give way causing the snow above them to break free and slide downhill. Common Causes:
1. Snowstorm and Wind Direction: Heavy snowstorms are more likely to cause Avalanches. The 24 hours after a storm are considered to be the most critical. Wind normally blows from one side of the slope of mountain to another side. While blowing up, it will scour snow off the surface which can overhang a mountain.
2. Heavy snowfall: Heavy snowfall is the first, since it deposits snow in unstable areas and puts pressure on the snow-pack. Precipitation during the summer months is the leading cause of wet snow avalanches.
3. Human Activity: Humans have contributed to the start of many avalanches in recent years. Winter sports that require steep slopes often put pressure on the snow-pack which it cannot deal. Combined with the heavy deforestation and soil erosion in mountain regions, it gives the snow little stability in the winter months. Further natural causes include earthquakes and tremors, since they can often create cracks in the snow-pack.
4. Vibration or Movement: The use of All Terrain Vehicles and Snowmobiles creates vibrations within the snow that it cannot withstand. Coupled with the gravitational pull, it is one of the quickest ways to cause an avalanche. The other is construction work done with explosives, which tend to weaken the entire surrounding area.
5. Layers of Snow: There are conditions where snow is already on the mountains and has turned into ice. Then, fresh snow falls on top which can easily slide down.
6. Steep Slopes: Layers of snow build up and slide down the mountain at a faster rate as steep slopes can increase the speed of snow. A rock or piece of huge ice can shake the snow and cause it to come down.
7. Warm Temperature: Warm temperatures that can last several hours a day can weaken some of the upper layers of snow and cause it to slide down.
Examples:
Mount Everest 2015:
Triggered by Nepal Earthquake (7.8 magnitude)
Avalanche from Pumori (mountain on the Nepal-Tibet border) into the basecamp on Mount Everest.
22 people were killed
Mount Everest was approximately 220 kilometres (140 mi) east of the epicentre, and the avalanche built up spped as it headed for base camp, where climbing expeditions have been preparing to make their summit attempts
Yungay, Peru, 31 May 1970:
An earthquake off the coast of Peru caused a substantial section of the north slope of Mt. Huascaran to collapse. [Situated in the Cordillera Blanca, the world’s highest tropical mountain range, Mount Huascarán rises to 6768 m above sea-level.]
The avalanche moved dowsnhill at a speed of 100 MPH with a mass of roughly 80 million cubic feet of ice, mud and rock.
It ran nearly 11 miles, burying the towns of Yungay and Ranrahirca in up to 300 feet of rock and debris.
Estimates suggest that the earthquake killed over 20,000 people.