A lahar is a volcanic mudflow made from water mixed with ash, rock and other debris.
Lahars are dense slurries of water and volcanic material that flow downhill, often following river valleys on a volcano’s slopes. Their ingredients can include loose ash, pumice, boulders and older deposits left by earlier eruptions. Heavy rain, melting snow or ice, crater-lake water, earthquakes and eruptive heat can all help trigger one.
Unlike lava, a lahar is not molten rock. Unlike a pyroclastic flow, it is primarily a water-driven mixture of sediment and debris. Lahars can occur during an eruption, but they can also happen long afterward when rainfall remobilizes unstable volcanic deposits. That makes them a persistent hazard around dormant as well as active volcanoes.
Lahars can move extremely fast and become extraordinarily destructive. The 1985 Nevado del Ruiz disaster in Colombia produced flows that buried Armero and killed an estimated 23,000 people. At Mount Pinatubo in the Philippines, rain repeatedly transformed loose ash from the 1991 eruption into damaging lahars for years afterward.
The term comes from Javanese and entered geological usage in the early twentieth century.