Logo link to homepage

Types and Processes Gallery - Lahars (mud flows)

Lahars (mud flows)
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.

Lahars are water-saturated mixtures of volcanic debris that sweep down volcanoes and onto 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. Lahars can remain a hazard long after an eruption ends, when heavy rainfall redistributes deposits produced by the eruption.

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

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

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

Lahars from Pinatubo fill the broad Santo Tomás River valley SW of the volcano. Erosion along the south bank of the river has cut into the town of San Rafael. This photo was taken a month after the end of the 1991 eruption. Lahars produced by the redistribution of thick deposits of ashfall and pyroclastic flows caused extensive long-term economic devastation which continued for years after the eruption ended.

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

Floodwaters back up over the villages of Aglao and Dalanawan on the SW flank of Pinatubo volcano in the Philippines. Lake Mapanuepe was formed when lahars from the 1991 eruption traveled down the Marella River and dammed its tributary; several villages were submerged by the rising water. The lake was stabilized in late 1992 at about this level after excavation of a trench through bedrock, seen in the background. This prevented catastrophic rapid draining of the lake.

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

These relatively fine-grained layers are lahar deposits produced by successive overflows of dikes along the Bambam River, about 35 km NE of Pinatubo volcano in the Philippines. The photo was taken on 13 October 1991, a little more than a month after the end of the devastating 1991 eruption. Note the pen at the upper left for scale. By the end of 1991, rainfall-induced lahars had traveled 50 km down the Bambam River.

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

Owners of a service station in the city of Bacolor, 38 km SE of Pinatubo in the Philippines, adapted to ongoing lahars over many years; note the garage door opening at right and the height of the canopy in each photo. On 12 September 1991 (upper left), 10 days after the end of the 1991 eruption, they dug out gas pumps buried by 1-m-thick lahar deposits. By 30 November 1991 (upper right) they had raised the pumps to the new ground level. Three years later, in September 1994 (bottom left), the pumps had again been raised, to a ground surface now half the height of the original garage opening. A year later (bottom right) a 5-m-high lahar deposit filled the garage, and the station had been abandoned.

Photos by Chris Newhall, 1991-1995 (U.S. Geological Survey).

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

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

Lahars produced devastating social and economic disruption for many years after the 1991 eruption of Pinatubo in the Philippines. The towns of Barangay Manibaug Pasig in the foreground of this 26 February 1994 view, and Mancatian in the distance, were progressively buried over several years. Construction of lahar levees provided only temporary protection from the massive downstream redistribution of ash and pumice from the 1991 eruption.

Photo by Ray Punongbayan, 1994 (Philippine Institute of Volcanology and Seismology).

A house on the SW flank of Unzen volcano is buried to its eaves by lahar deposits. Redistribution of material that collapsed from the lava dome (background) produced lahars that devastated populated areas near the volcano. Dome growth, which had begun in May 1991, ceased at about the time of this 3 February 1995 photo.

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

This massive boulder (top photo) was carried down the Biwasawa valley on the east side of Bandai volcano in a lahar during an eruption in 1888. The deposit covers the broad floor of the Nagase valley. In addition to this lahar, the 1888 eruption included a pyroclastic flow on the east side and a 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.

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

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

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

St. Helens
The twisted girders of a highway bridge within lahar deposits from Mount St. Helens. This 18 May 1980 mudflow was produced by dewatering (meltwater coming out) of the debris avalanche deposit in the North Fork Toutle River that traveled as far as the Columbia River, decreasing the depth of the navigational channel from 11 to 4 m.

Photo by Dan Miller, 1980 (U.S. Geological Survey).

A thick lahar deposit resulted from a 100,000-year-old debris avalanche produced by a collapse of the N side of Mount Hood. It swept down the Hood River valley and traveled across the Columbia River, temporarily damming it to a depth of 30 m. This thick outcrop that contains rounded boulders in a clay-rich matrix, is located N of Underwood, Washington, on the other side of the Columbia River.

Photo by Willie Scott, 1994 (U.S. Geological Survey).

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

Photo by Lee Siebert, 1988 (Smithsonian Institution).

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 13 November 1985. The boulders were carried within the lahar and deposited against the tree that 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, 1985 (U.S. Geological Survey).

This massive boulder on the SW side of Cotopaxi was carried in a lahar, possibly during a major eruption in 1877. Scale is provided by volcanologists Minard Hall and Patty Mothes. More than 130,000 people live in areas subject to lahar risk from Cotopaxi. The 1877 eruption produced lahars that covered this valley, swept into eastern river drainages, and reached the Pacific Ocean along valleys to the NW.

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