An avalanche is a large amount of snow that reaches a high velocity when flowing down the side of a mountain. These avalanches are a big danger and become of threat to life, environment, and anything in its path. There are three basic types of avalanches that are known.
The first which is commonly known as a loose snow avalanche happens when a certain amount of weight of the snow goes beyond the strength of the snow pack. These usually occur of steeper mountain sides or terrain and grow in size as they move down the slope. The loose snow, giving the avalanche of this type its name as it moves very quickly.
Another type of avalanche that is significantly more dangerous is the slab avalanche. This type of avalanche is what causes the most damage and fatalities known today. It accounts for 90% of the avalanches each year. This type occurs when a strong and very stiff 'slab' of snow is released. When the slab falls, the weak layer is broken off and at a very rapid amount of speed can destroy a very large area within a matter of minutes. Ultimately, this type destroys everything in its path without mercy.
The last type of avalanche which isn't as common as the other two is known as the slush avalanche. These occur when the snow is saturated by water. As these avalanches go down the side of the mountain they will grow in size and often do not gain enough gravity current to go as far as the other types.
Additionally, all three types of avalanches are capable of speeds of unknown velocity. In which they are capable of carrying 10,000,000 tons of snow when they do come down a mountain side. Many avalanches have been known to form powder clouds, or air mixed with the flying snow.
Obviously, the danger comes with the avalanche not only being snow, but other elements it has gone over. For example, snow mixed with trees, ice, rock and any other debris such as buildings and other hard surfaces. Some avalanches have been recorded as reaching the towns below the mountains and literally destroying the towns in the avalanche wrath, and can go for long distances along flat bottom terrain or even up and over small hills.