During eruptive activity or precursors to eruptions, local government officials may ask form prompt evacuation of areas likely to be affected. They can travel over 50 miles downstream, commonly reaching speeds between 20 and 40 miles per hour. Close to the volcano they have the strength to rip huge boulders, trees, and houses from the ground and carry them downvalley. Further downstream they simply entomb everything in mud. Historically, lahars have been one of the most deadly volcanic hazards.
Lahars can form in a variety of ways, either during an eruption or when a volcano is quiet.
Rock debris in lahars ranges in size from clay to blocks several tens of meters in maximum dimension. When moving, lahars resemble masses of wet concrete and tend to be channeled into stream valleys. Outburst floods at Mount Rainier form from sudden release of water stored at the base of glaciers or within the glacier ice. Outburst floods become debris flows by incorporating large quantities of sediment from valley floors and walls, often by triggering landslides that mix with the flood waters. Outburst floods at Mount Rainier form from sudden release of water stored at the base of glaciers or within the glacier ice.