Description
"Lahar" is an Indonesian word that describes volcanic mudflows or debris flows. Lahars have the consistency, viscosity and approximate density of concrete: fluid when moving, solid at rest. Lahars can be huge. The Osceola lahar produced by Mount Rainier (Washington) some 5,600 years ago resulted in a wall of mud 140 metres (460 ft) deep in the White River canyon, which covered an area of over 330 square kilometres (130 sq mi), for a total volume of 2.3 cubic kilometres (0.55 cu mi). A lahar of sufficient size and intensity can raze virtually any structure in its path, and is capable of carving its own pathway, making the prediction of its course difficult. Conversely, a Lahar quickly loses force when it leaves the channel of its flow: even frail huts may remain standing, while at the same time being buried to the roof line in mud. A lahar's viscosity decreases with time, and can be further thinned by rain, but it nevertheless solidifies quickly when coming to a stop.
Lahars vary in size and speed. Small lahars less than a few meters wide and several centimeters deep may flow a few meters per second. Large lahars hundreds of meters wide and tens of meters deep can flow several tens of meters per second: much too fast for people to outrun. With the potential to flow at speeds up to 100 kilometres per hour (60 mph), and distances of more than 300 kilometres (190 mi), a lahar can cause catastrophic destruction in its path. Lahars from the 1985 Nevado del Ruiz eruption in Colombia caused the Armero tragedy, which killed an estimated 23,000 people, when the city of Armero was buried under 5 metres (16 ft) of mud and debris. A lahar caused New Zealand's Tangiwai disaster, where 151 people died after a Christmas Eve express train fell into the Whangaehu River. Lahars have been responsible for 17% of volcano-related deaths between 1783 and 1997. A lahar can cause fatalities years after its precipitating eruption. For example, the Cabalantian tragedy occurred four years subsequent to the 1991 eruption of Mount Pinatubo.
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