Mudflow on a volcano is called lahar and is typically caused by heavy rains during or after volcanic eruptions. Lahars can also occur when nearby ice or snow melts, carrying ash and rock debris down a volcano's slopes.
As a lahar moves down a volcano it increases in size as it picks up rock and other debris. The lahar can range in size from just a few meters to hundreds of meters wide. It can travel very quickly or very slowly, depending on the amount of water contained in the mixture. Once a lahar travels down the slope of the volcano it tends to move through valleys, causing serious economic and environmental damage.