What is a lahar?
Quick Answer
A lahar is a fast-flowing slurry of volcanic debris and water that rushes down the slopes of a volcano like wet concrete.
The Full Story
A lahar forms when water mixes with loose volcanic ash and rock, often from melted snow and ice, heavy rain or a breached crater lake. The resulting mudflow can have the consistency of wet concrete and travel at speeds of tens of kilometres per hour down river valleys. The word comes from Javanese, reflecting Indonesia's long experience with these flows. In 1985 lahars from Colombia's Nevado del Ruiz buried the town of Armero, killing more than 23,000 people. Lahars can occur years after an eruption, whenever rain remobilises the ash left on a volcano's slopes.
Key Facts
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Suggested video title for this topic:
"The volcanic hazard that can strike far from the eruption itself"