Lahar ( Mudflows )

Mudflows are somewhat of a misnomer, because these volcanic flows include not only mud, but debris ranging up to boulder size. The term, however, has been commonly applied to water-saturated volcanic flows and is well entrenched in the literature. The Indonesian word lahar refers to mudflows in volcanic terrain. Lahars can occur both during an eruption and as secondary flows long after an eruption is over, as rainfall remobilizes volcanic ash deposits. Lahars have lower velocities than debris avalanches formed by volcanic landslides, but can travel long distances beyond a volcano, inundating large areas in low-lying terrain. The most catastrophic lahar in historical time took place on 13 November 1985 at Nevado del Ruiz volcano in Colombia, when a lahar struck the city of Armero in the dark of night, catching people unawares and causing more than 21,000 fatalities. Extremely destructive along the axis of the flows, at the margins lahars can passively enter houses through openings without destroying the building. After emplacement of a mudflow, dewatering can cause the deposit to harden almost like concrete.

Photo by Robin Holcomb, 1983 (U.S. Geological Survey).

Galunggung

Volcanic mudflows, also known by their Indonesian name, "lahars," are water-saturated mixtures of volcanic debris that sweep down volcanoes and valley floors far beyond. These lahars from the 1982 eruption of Galunggung volcano on the Indonesian island of Java caused extensive damage to houses and croplands. Unlike pyroclastic flows, mudflows are generally low temperature, and these houses were not ignited. Mudflows can remain a hazard long after an eruption ends as heavy rainfall redistributes tephra produced by the eruption.

Photo by Ernesto Corpuz, 1984 (Philippines Institute of Volcanology and Seismology).

Mayon

A hot lahar sweeps down a channel on the SW flank of Mayon volcano in the Philippines on September 14, 1984, five days after the onset of an eruption. The water temperature of this lahar was about 80 degrees Centigrade. Note the large block in the center of the channel that is being transported by the lahar.

Photo by Chris Newhall, 1991 (U.S. Geological Survey).

Pinatubo
A lahar, or volcanic mudflow, fills the banks of the Pasig-Potrero River on the east side of Pinatubo volcano in the Philippines on October 13, 1991. The lahar moved at a velocity of 3-5 m/sec, and carried a few meter-sized boulders. This lahar was not directly produced by an eruption, but was triggered by minor rainfall, which remobilized thick deposits of ash and pumice that blanketed the landscape. Devastating mudflows occurred at Pinatubo for years after the catastrophic 1991 eruption.

Photo by Tom Pierson, 1995 (U.S. Geological Survey).

Unzen

A house on the SW flank of Unzen volcano is buried to its eaves by deposits from lahars (volcanic mudflows). Redistribution of material shed off of Fugen-dake lava dome (background) produced lahars that devastated populated areas near the volcano. The lahars had low temperatures, unlike pyroclastic flows, and did not ignite the houses. Dome growth, which had begun in May 1991, ceased at about the time of this February 3, 1995 photo.

Top Photo by Fukushima Minposha Newspaper, 1888; bottom photo by Lee Siebert, 1988 (Smithsonian Institution).

Bandai

This massive boulder (top photo) was carried down the Biwasawa valley on the east side of Bandai volcano in a mudflow during an eruption in 1888. The mudflow deposit covers the broad floor of the Nagase valley. In addition to this mudflow, the 1888 eruption included a pyroclastic flow on the east side and catastrophic debris avalanche that swept over villages to the north of the volcano. The bottom photo is taken from the same location a century later. The identical boulder now forms part of the landscaping of a house in the town of Inawashiro.

Photo by Tom Miller, 1990 (Alaska Volcano Observatory, U.S. Geological Survey).

Redoubt

Lahars that formed during the 1989-90 eruptions of Redoubt Volcano in Alaska accumulated in the Drift River Valley NE of the volcano. The largest lahars, such as this one from the February 15, 1990 eruption, covered the valley floor nearly wall-to-wall and extended more than 35 km to the Cook Inlet.

Photo by Lee Siebert, 1988 (Smithsonian Institution).

Santa Maria

Lahar deposits produced by redistribution of material shed off the Santiaguito lava dome, visible below the steam plume to the left of Guatemala's Santa María volcano, have had dramatic effects on downstream drainages. This December 1988 photo shows the Río Tambor, SW of Santa María, filled bank-to-bank with debris. Bridges such as the one in the foreground have been frequently destroyed during rainy-season lahars, which have traveled 35 km or more from the volcano.

Photo by Tom Pierson, 1985 (U.S. Geological Survey).

Nevado Del Ruiz

A cluster of rounded boulders was deposited on a river terrace by a lahar in the Río Chinchina valley, 59 km WNW of the summit of Colombia's Nevado del Ruiz volcano on November 13, 1985. The boulders were carried as bedload and deposited against the tree, which served as an obstruction to flow. Note the mudline on the tree that marks the upper flow surface of the lahar.

Photo by Tom Pierson, 1992 (U.S. Geological Survey).

Cotopaxi

This massive rounded boulder on the SW side of Cotopaxi volcano was carried in a lahar, or volcanic mudflow, possibly during a major eruption in 1877. Scale is provided by volcanologists Minard Hall and Patty Mothes of Ecuador's national university, who have spent many years assessing hazards from Ecuador's volcanoes. More than 130,000 persons live in areas subject to lahar risk from Cotopaxi. The 1877 eruption produced lahars that covered this valley, swept into the Amazon basin, and reached the Pacific Ocean along valleys to the NW.