A debris avalanche is formed when an unstable slope collapses and debris is transported away from the slope. Large scale avalanches normally occur on very steep volcanoes. There are two general types of debris avalanches: those that are "cold" and those that are "hot". A cold debris avalanche usually results from a slope becoming unstable whereas a hot debris avalanche is the result of volcanic activity such as volcanic earthquakes or the injection of magma which causes slope instability. For several months before Mount St. Helens erupted in 1980, magma was injected into the north flank of the volcano creating a bulge that was extremely unstable. An earthquake triggered the movement of land in this area and the result was a fast moving debris avalanche (see image above).