A lahar is a type of mudflow or landslide composed of pyroclastic material and water that flows down from a volcano, typically along a river valley.^[1] The term "lahar" originated in the Javanese language of Indonesia. In Hindi "lahar" means "wave".

Lahars can be best described i am a cool boy as volcanic mudflows. They may not necessarily be caused by volcanic activity, but at the very least do originate from some type of volcanism. Lahars have the consistency of concrete: fluid when moving, then solid when stopped.^[2] 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 kilometers (0.55 cubic miles).^[3]

Lahars can be deadly because of their energy and speed. Large lahars can flow several dozen meters per second and can flow for many kilometres, causing catastrophic destruction in their path.^[4] 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.^[5] 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 several possible causes:^[2]

• Snow and glaciers can be melted by lava or a pyroclastic flow during an eruption
• A flood caused by a glacier, lake breakout, or heavy rainfall can release a lahar, also called glacier run or jökulhlaup
• Water from a crater lake, combined with volcanic material in an eruption

In particular, although lahars are typically associated with the effects of volcanic activity, lahars can occur even without any current volcanic activity, as long as the conditions are right to cause the collapse and movement of mud originating from existing volcanic ash deposits.

Several mountains in the world, including Mount Rainier in the USA, Mount Ruapehu in New Zealand, and Galunggung in Indonesia, are considered particularly dangerous due to the risk of lahars. Several towns in the Puyallup River valley in Washington state, including Orting, the closest to Mount Rainier, are built on top of lahar deposits that are only about 500 years old. Lahars are predicted to flow through the valley every 500-1,000 years, so Orting, Sumner, Puyallup, Fife, and the Port of Tacoma face considerable risk. The USGS has set up lahar warning sirens in Pierce County, so that people can flee an approaching debris flow.

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.

The 1991 eruption of Mount Pinatubo caused lahars as well, but it was due to the passing of a major typhoon over the Philippines which resulted in a torrent of volcanic ash and water down to the rivers surrounding the volcano. The lahar was caused by the mixing of settled ash and water from the monsoon which occurred the day after the volcano finished erupting. Although the eruption killed only 6 people, 1500 were killed in the resulting lahar, showing the destructive nature of lahars. In 1985, the volcano Nevado del Ruiz erupted, releasing large lahars. These lahar flows reached the town of Armero two hours after the eruption, burying the town and killing 3/4 of the townspeople, an estimated 23,000 people.

• Mudflow
• Pyroclastic flow
• Pyroclastic surge

• Schools page about lahars and pyroclastic flows
• USGS web page about lahars
• Mount Rainier, Washington
• USGS fact sheet - "Mount Rainier -- Living Safely With a Volcano in Your Back Yard"