A destructive mudflow or debris flow composed of volcanic material mixed with water, generated when volcanic ash, rock and debris on a volcano's slopes are mobilized by melting snow and ice, heavy rainfall, or the sudden release of a crater lake. A lahar can travel tens of miles from its source at speeds rivaling a fast-moving river, burying valleys in dense, cement-like mud long after an eruption itself has ended, as in the 1985 Nevado del Ruiz disaster in Colombia, where a comparatively small eruption's lahars killed more than twenty thousand people.
Facts
PeriodicityLahars are not tied to a fixed schedule; they can be triggered by the rapid melting of snow and ice by hot volcanic debris during an eruption, by earthquakes that mobilize saturated volcanic debris into landslides, or by the transition of saturated landslide masses into debris flows. 1 ScaleLahars can travel more than 100 km from their source volcano; the 1877 lahar from Ecuador's Cotopaxi volcano traveled more than 300 km (186 mi), while 1980 Mount St. Helens lahars advanced at an average velocity of about 67 kph (41.6 mph). Volume varies widely, from a few cubic centimeters of debris to massive flows. 1 First DescribedThe word lahar comes from Indonesian, reflecting how frequently volcanic debris flows occur in that region; the term was adopted into English language volcanology from this Indonesian usage. 1 Identification
Distinguishing AppearanceA lahar moves as a fluid mass with the general consistency of wet concrete. 1 Connections
Associated With
Loose pyroclastic-flow deposits on a volcano's flanks are readily remobilized by rainfall into lahars long after the flow itself has stopped, a documented secondary hazard of pyroclastic deposition.
Lahars are volcanic mudflows commonly triggered directly by an eruption, when hot pyroclastic material rapidly melts snow and ice or mixes with a volcano's crater lake and heavy rainfall on its flanks.
Sources
1. Encyclopedia.com: Lahar
Causes and triggers paragraph
can also be associated with the rapid melting of snow and ice by hot volcanic (pyroclastic) debris during an eruption
Introduction
For example, a lahar that occurred during an eruption of Cotopaxi Volcano ( Ecuador ) in 1877 traveled more than 186 mi (300 km) at an average velocity of 16.7 mph (27 kph).
Etymology paragraph
The word lahar is of Indonesian origin, reflecting the frequency of volcanic debris flows in that region.
Physical description paragraph
lahars move as fluid masses with the general consistency of wet concrete.
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