Lahar

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.

The Osceola Lahar produced by Mount Rainier in modern-day Washington some 5600 years ago resulted in 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 kilometres (1⁄2 cu mi).

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.

[11] A lahar caused New Zealand's Tangiwai disaster,[12] where 151 people died after a Christmas Eve express train fell into the Whangaehu River in 1953.

Several towns in the Puyallup River valley in Washington state, including Orting, are built on top of lahar deposits that are only about 500 years old.

Lahars are predicted to flow through the valley every 500 to 1,000 years, so Orting, Sumner, Puyallup, Fife, and the Port of Tacoma face considerable risk.

[18] The USGS has set up lahar warning sirens in Pierce County, Washington, so that people can flee an approaching debris flow in the event of a Mount Rainier eruption.

[21] Physical preventative measures by the Philippine government were not adequate to stop over 6 m (20 ft) of mud from flooding many villages around Mount Pinatubo from 1992 through 1998.

The lahars picked up speed in gullies and coursed into the six major rivers at the base of the volcano; they engulfed the town of Armero, killing more than 20,000 of its almost 29,000 inhabitants.

[31] On the morning of 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 8-metre (25 ft) lahar.

[32] 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.