Why do volcanic eruptions sometimes trigger mudflows called lahars?
Quick Answer
Ash and loose volcanic debris mix with water from rain, snow, ice, or crater lakes and flow downhill like wet concrete. Lahars can happen during or long after an eruption.
The Full Story
Lahars are fast-moving mixtures of water, volcanic ash, and rock fragments. They can be triggered by heavy rain on fresh ash, the sudden melting of snow and ice, or the collapse of a crater lake or volcanic dam. Because they follow river valleys, they can travel far from the volcano and bury communities with little warning. Mount Pinatubo in the Philippines generated major lahars after its 1991 eruption, especially during subsequent typhoons. Lahars are dangerous because they can move quickly and remain hazardous long after the eruption ends.
Key Facts
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