Lava flows destroy everything in their path, but most move slowly enough that people can move out of the way. The speed at which lava moves across the ground depends on several factors, including the type of lava erupted, the steepness of the ground, and the rate of lava production at the vent. Historically, lahars have been one of the deadliest volcano hazards. Lahars are mudflows or debris flows composed mostly of volcanic materials on the flanks of a volcano. These flows of mud, rock, and water can rush down valley and stream channels at speeds of 20 to 40 miles per hour and can travel more than 50 miles.
Some lahars contain so much rock debris that they look like fast-moving rivers of wet concrete.
They can occur both during an eruption and when a volcano is quiet. The water that creates lahars can come from melting snow and ice, intense rainfall, or the breakout of a summit crater lake. Large lahars are a potential hazard to many communities downstream from glacier-clad volcanoes, such as Mount Rainier in Washington. Volcanic ash is actually fine, glassy rock fragments and can affect people and equipment hundreds of miles away from the cone of the volcano. Volcanic ash can contaminate water supplies, cause electrical storms, disrupt the operation of all machinery, and collapse roofs.