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. The term “lahar” originated in the Javanese language of Indonesia. In Hindi “lahar” means “wave”.
Lahars can be best described 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. 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).
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. 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. 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 usually travel down valleys. They have a wide range of velocities varying from 1 m/s to 40 m/s. The velocity of a lahar depends on the channel width, channel slope, volume of the flow, and grain size composition (Scott, 1989). Lahars can travel long distances. Some lahars have traveled hundreds of kilometers from their source (Scott, 1989). The deposits of a lahar that traveled 60 km from its source at Mount Rainier can be found near the large city of Seattle, Washington (Pierson et al., 1992). The lahar’s origin at Mount Rainier helped make that volcano a decade volcano.
Lahars have been known to transport very large boulders. At Mount Pinatubo, boulders measuring 1.5 m long were not uncommon in lahar deposits (Pierson et al., 1992). The lahars from Nevado del Ruiz transported a boulder with a volume of 208 cubic meters, 300 m downstream (Mileti, 1991).
When a lahar travels down valley, the high point of the lahar is usually marked by the mudline it leaves on trees, valley walls, and buildings. This mudline marks the upper limit of how high a lahar will go. This upper limit is important because it defines how high people must go to be out of danger from the lahar. The small eruption of Nevado del Ruiz in 1987, produced large lahars that destroyed the city of Armero. Unfortunately, the 30,000 people who lost their lives might have been saved had they established an appropriate line of communication and evacuated to higher ground (Francis, 1993).
In 1991, Mount Pinatubo erupted. Some of the pyroclastic flows initiating from this eruption were transformed into lahars as they moved downslope through river valleys. Secondary lahars were formed when rain mixing with ash from the eruption became unstable. The formation of these lahars often occured within 30 minutes of as little as 10-15 mm of precipitation falling on the loose ash near the summit of Pinatubo (Primer, 1992). Secondary lahars are still forming today from the unconsolidated ash.
Lahars are extremely dangerous especially to those living in valley areas near a volcano. Lahars can undercut banks and cause houses on those banks to be destroyed. Lahars can bury and destroy manmade structures including roads and bridges. At Nevado del Ruiz, lahars destroyed an entire city; filling the first floor of a hospital with mud, breaking windows, floating cars, and leaving debris in the tops of trees (Mileti, 1991).
Causes of Lahar
Lahars have several possible causes:
- 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 jokulhlaup
- 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.
