Other forms: lahars
A lahar is a dangerous combination of mud, rock, and water that flows down the side of a volcano. Some lahars are cause by volcanic eruptions.
In geology, the Javanese word lahar is used to describe a specific type of mudflow. Moving down the slopes of a volcano and often flowing into riverbeds, lahars are particularly destructive, described as "rivers of concrete." Like a muddy, rocky avalanche, they grow in size and speed as they move, picking up rocks and debris in their path. An eruption can trigger a lahar, but they're sometimes caused by extremely heavy rain.