These lethal mixtures of water and tephra have the consistency of wet concrete, yet they can flow down the slopes of volcanoes or down river valleys at rapid speeds, similar to fast-moving streams of water. These mud slurries carry debris ranging in size from ash to lapilli , to boulders more than 10 meters in diameter.
Lahars can vary from hot to cold, depending on their mode of genesis.
The maximum temperature of a lahar is 100 degrees Centigrade, the boiling temperature of water. The majority are produced by intense rainfall during or after an eruption. A tragic example of such an event was the 1991 eruption of Mt. Pinatubo in the Philippines, which was contemporaneous with the arrival of a major hurricane. An estimated 700 people died from buiral by the ensuing lahars, together with the collapse of structures benearth the wet ash. As demonstrated by the graph below, lahars can also be generated directly from a volcanic eruption as massive amounts of water are generated either by the rapid melting of ice and snow, or by the disruption of crater lakes.