File:Unzen pyroclastic and lahar deposits.jpg

"Lahar" is an Indonesian word that describes volcanic mudflows or debris flows.[2] Lahars have the consistency of concrete: fluid when moving, then solid when stopped.[3] Lahars can be huge: the Osceola lahar produced 5,600 years ago by Mount Rainier in Washington produced a wall of mud 140 metres (460 ft) deep in the White River canyon and covered an area of over 330 square kilometres (130 sq mi) for a total volume of 2.3 cubic kilometres (0.55 cu mi).

Lahars can be deadly because of their energy and speed. Large lahars can flow 100 kilometres per hour (60 mph) and can flow for more than 300 kilometres (190 mi), causing catastrophic destruction in their path.[5] The lahars from the Nevado del Ruiz eruption in Colombia in 1985 caused the Armero tragedy, which killed an estimated 23,000 when the city of Armero was buried under 5 metres (16 ft) of mud and debris.[6] New Zealand's Tangiwai disaster in 1953, where 151 people died after a Christmas Eve express train fell into the Whangaehu River, was caused by a lahar. Lahars have been responsible for 17% of volcano-related deaths between 1783 and 1997.

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• 1792
• Mount Unzen