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A lahar (, from ) is a violent type of mudflow or debris flow composed of a slurry of Pyroclastic rock material, rocky debris and water. The material flows down from a volcano, typically along a valley.
Lahars are often extremely destructive and deadly; they can flow tens of metres per second, they have been known to be up to deep, and large flows tend to destroy any structures in their path. Notable lahars include those at Mount Pinatubo in the Philippines and Nevado del Ruiz in Colombia, the latter of which killed more than 20,000 people in the Armero tragedy.
In addition to their variable rheology, lahars vary considerably in magnitude. The Osceola Lahar produced by Mount Rainier in modern-day Washington some 5600 years ago resulted in a wall of mud deep in the White River canyon and covered an area of over , for a total volume of . A debris-flow lahar can erase virtually any structure in its path, while a hyperconcentrated-flow lahar is capable of carving its own pathway, destroying buildings by undermining their foundations. A hyperconcentrated-flow lahar can leave even frail huts standing, while at the same time burying them in mud, which can harden to near-concrete hardness. A lahar's viscosity decreases the longer it flows and can be further thinned by rain, producing a quicksand-like mixture that can remain fluidized for weeks and complicate search and rescue.
Lahars vary in speed. Small lahars less than a few metres wide and several centimetres deep may flow a few metres per second. Large lahars hundreds of metres wide and tens of metres deep can flow several tens of metres per second (22 mph or more), much too fast for people to outrun. On steep slopes, lahar speeds can exceed . A lahar can cause catastrophic destruction along a potential path of more than .
Lahars from the 1985 Nevado del Ruiz eruption in Colombia caused the Armero tragedy, burying the city of Armero under of mud and debris and killing an estimated 23,000 people. A lahar caused New Zealand's Tangiwai disaster, where 151 people died after a Christmas Eve express train fell into the Whangaehu River in 1953. Lahars have caused 17% of volcano-related deaths between 1783 and 1997.
A lahar warning system has been set up at Mount Ruapehu by the New Zealand Department of Conservation and hailed as a success after it successfully alerted officials to an impending lahar on 18 March 2007.
Since mid-June 1991, when violent eruptions triggered Mount Pinatubo's first lahars in 500 years, a system to monitor and warn of lahars has been in operation. Radio-telemetered rain gauges provide data on rainfall in lahar source regions, acoustic flow monitors on stream banks detect ground vibration as lahars pass, and staffed watchpoints further confirm that lahars are rushing down Pinatubo's slopes. This system has enabled warnings to be sounded for most but not all major lahars at Pinatubo, saving hundreds of lives. Physical preventative measures by the Philippine government were not adequate to stop over of mud from flooding many villages around Mount Pinatubo from 1992 through 1998.
Scientists and governments try to identify areas with a high risk of lahars based on historical events and . Volcano scientists play a critical role in effective hazard education by informing officials and the public about realistic hazard probabilities and scenarios (including potential magnitude, timing, and impacts); by helping evaluate the effectiveness of proposed risk-reduction strategies; by helping promote acceptance of (and confidence in) hazards information through participatory engagement with officials and vulnerable communities as partners in risk reduction efforts; and by communicating with emergency managers during extreme events. An example of such a model is TITAN2D. These models are directed towards future planning: identifying low-risk regions to place community buildings, discovering how to mitigate lahars with dams, and constructing evacuation plans.
Casualties in other towns, particularly Chinchiná, brought the overall death toll to over 25,000. Footage and photographs of Omayra Sánchez, a young victim of the tragedy, were published around the world. Other photographs of the lahars and the impact of the disaster captured attention worldwide and led to controversy over the degree to which the Colombian government was responsible for the disaster.
Over of mud inundated and damaged the towns of Castillejos, San Marcelino and Botolan in Zambales, Porac and Mabalacat in Pampanga, Tarlac City, Capas, Concepcion and Bamban in Tarlac. The Bamban Bridge on the MacArthur Highway, a major north-south transportation route, was destroyed, and temporary bridges erected in its place were inundated by subsequent lahars.
From 3 September to 1 October 1995, pyroclastic material which clung to the slopes of Pinatubo and surrounding mountains rushed down because of heavy rain, and turned into an lahar. This mudflow killed at least 100 people in Barangay Cabalantian in Bacolor.
Typhoon Reming triggered additional lahars in the Philippines in 2006.
(2026). 9780123859389 ISBN 9780123859389 The Philippine government under President Fidel V. Ramos ordered the construction of the FVR Mega Dike in an attempt to protect people from further mudflows.
Mayon
Lahars that followed the eruption of Mayon Volcano in 1814 buried the town of Cagsawa.
Mount Ruapehu
A lahar that occurred while Mount Ruapehu was not erupting undermined a train bridge and caused the Tangiwai disaster.
See also
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