A lahar is a type of mudflow composed of pyroclastic material and water that flows down from a volcano, typically along a river valley. The term 'lahar' originated in the Javanese language of Indonesia.

Lahars have the consistency of concrete: fluid when moving, then solid when stopped. Lahars can be huge: the Osceola lahar produced 5,600 years ago by Mount Rainier in Washington produced a wall of mud 180 m (600 feet) deep in the White River canyon and extends over an area of over 320 km².

Lahars can be extremely dangerous, 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. The lahars from the Nevado del Ruiz eruption in Colombia in 1985 killed an estimated 21,000 in the city of Armero, buried under 8 m (26 ft) of mud and debris. The 1953 Tangiwai disaster in New Zealand was caused by a lahar.

Lahars have several possible causes:

• Snow and glaciers can be melted by 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.

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 a success after it successfully alerted officials to an impending lahar on 18 March 2007.

The 1991 Mount Pinatubo eruption caused lahar 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 only killed 6 people, 1500 were killed in the resulting lahar, showing the destructive nature of lahars.

• Mudflow

• Schools page about lahars and pyroclastic flows
• USGS web page about lahars
• Mount Rainier Volcano Lahar Warning System
• USGS fact sheet - "Mount Rainier -- Learning to Live with Volcanic Risk"