Debris flows have the viscous consistency of wet concrete, and there is a complete transition to watery floods. Lahars are composed of volcanic particles and originate directly or indirectly from volcanic action. Lahars have destroyed many villages and lives living on Indonesian volcanoes because most people live in valleys where lahars flow. The 21,000 lives lost at Armero, Colombia, was from a lahar that formed during the eruption of Nevado del Ruiz in 1985. It was generated by meltwater from the interaction of pyroclastic surges with snow and ice, from a very small eruption.
Lahars can transform into regular floods as they become increasingly diluted with water downstream.
When the landslide occurred, it decreased the pressure on the pressurized interior of the volcano which expanded explosively to form a lateral blast that devastated the countryside north of the volcano. Most of the debris flow avalanche was diverted down the North Fork Toutle River, but some moved directly northward over a 300 meter ridge and down into the next valley. Since the 1980 Mount St. Helens eruption, dozens of volcanoes that have given rise to avalanches have been discovered. The hilly topography north of Mount Shasta in northern California is now known to be the result of a have debris-flow avalanche. Some are known to extend up to 85 km from their sources and to cover tens to more than 1000 km2 in area.