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Lahar on Mount St. Helens (March 1982)

A lahar is a stream of mud and debris that emanates from a volcano . The word comes from Javanese . In the process, eruptive material from blocks, some of which are meter-sized, mixes with loose sediments and water. Depending on the slope of the terrain, lahars can reach speeds of up to 100 km / h due to gravity, flow over 100 km and flood large areas. They can be triggered by a volcanic eruption, but can also arise completely independently of it. Depending on how they are formed, lahars can be up to 100 ° C.


The definition is not entirely uniform in the literature. The Encyclopedia of Volcanoes limits the term to mud flows in the broader sense, which have a sediment content between 20% and 90% and originate from a volcano. In sedimentological terms, this corresponds to a debris flow (sediment proportion approx. 50–60% to approx. 90%) and a hyperconcentrated flow (sediment proportion between 20% and 50–60%). Mud streams with lower sediment concentrations transport sediment like rivers, either in suspension or as ground transport. The Encyclopedia of Volcanoes also recommends limiting the term lahar to the process and not to the deposit, although the term in the literature has meanwhile been extended to include the deposits ("fossil lahars").

Origin and cause

Mud masses after the eruption of the Indonesian volcano Gunung Galunggung in 1982

Lahars can form during or shortly after a volcanic eruption. But they can also be triggered completely independently of a volcanic eruption.

The prerequisites for the formation of lahars are:

  • a large reservoir of water in the form of snow, ice or other bodies of water
  • large amounts of unconsolidated material, e.g. B. pyroclastic stream or fall deposits, glacial deposits , soils or colluvium
  • steep slopes and a certain relief of the delivery area
  • a trigger that produces the right mix (see above) for a lahar

Typical trigger mechanisms:

  • Volcanic activity above the snow line. Snow and ice are quickly melted (see also glacier run ) and mix with loose sediments. This process can trigger very large lahars.
  • Heavy rains that meet volcanic loose sediment. Heavy rains are frequent companions of volcanism as a result of eruption thunderstorms , when a lot of water vapor is released from the glowing melt , rises in the already water-containing earth atmosphere , cools down and condenses around the finest ash particles . Usually only smaller, but numerous, lahars are triggered by this.
  • As a result of volcanic activity, lakes that were previously dammed up by volcanic deposits can suddenly flow into lower-lying areas when these natural dams break. This mechanism can also produce very large lahars.
  • Collapse Induced Lahars. Due to the instability of volcanic loose masses, slopes or smaller parts of a volcanic building can collapse. If the sediment contains enough pore water, this can liquefy the sediment. Shallow intrusions of magma into a volcanic building are the most common causes. Usually only smaller lahars are triggered by this.


Lahars can have a strong erosive effect on their way down the valley, especially in loose volcanic masses. In the process, embankments are washed away and slopes are undermined, and secondary smaller landslides can also be triggered. In general, the more water the sludge flows, the higher the erosion. The decisive factor for erosion, however, is that very large amounts of water or a mixture of mud and water drain off in a short period of time. A tidal wave erodes more than a continuous runoff.

Change in the lahars when draining

Traces of the catastrophic lahar of Armero , Colombia, 1985

Flood currents, i.e. water currents with less than 20% sediment flow that originate from volcanoes, can also change to lahars through the absorption of sediment. This slows down the flow rate, but it happens rarely. Lahars can also be “diluted” by further absorption of water and thus become “normal” flood currents. Most of the time, however, lahars absorb sediment and water and do not change significantly in their flow behavior until they are deposited. When draining, lahars literally “collect” everything that lies on their way.

Natural disasters caused by lahars

While the immediate vicinity of volcanoes, which is most threatened by eruptions, usually remains unpopulated, the wider surroundings of the volcanoes are often densely populated because of the fertile soils. The threat from the volcanoes there is rated as low. Lahars therefore have a considerable potential for destruction, primarily due to their large flow range.

  • On December 24, 1953, New Zealand's worst railway accident occurred when a lahar washed away a railway bridge at Tangiwai. Shortly after the collapse, the Wellington-Auckland night train reached the spot and fell into the mud stream. 151 people were killed, many of the victims were never found.
  • In the southern Andes of Chile , a lahar from the Villarrica volcano ( 2847  m ) dug a river channel 14 km away on December 29, 1971, which was 128 m wide and 8 m deep at its upper edge.
  • A lahar during the eruption of Mount St. Helens in southern Washington state in the USA on May 18, 1980 contributed to the extent of the devastating disaster.
  • On November 13, 1985, a lahar from the Nevado del Ruiz volcano in Colombia caused the second largest death toll from a volcanic eruption in the 20th century. The up to 5 m high mud flow reached the town of Armero, 47 km away, about two and a half hours after the eruption and cost two thirds of the 28,700 inhabitants their lives.
  • In March 2007, like in 1953, a lahar fell from the Ruapehu volcano in New Zealand . There were no human lives to be mourned this time, as the authorities along the Whangaehu River had taken timely measures.

Lahar deposits

The sediment structures of a lahar do not differ from those of a debris flow and a hyperconcentrated current. Depending on the initial composition , debrites , diamictites , paraconglomerates and breccias are formed . Basically, they only differ from “normal” rubble and mud flows in the volcanogenic material of which they are predominantly made. There are transitions to "normal" mud flows.

Due to the sudden covering with mud masses, many living things can also be fossilized . A fossil deposit can arise.

Lahar fossil deposits

View of Mont-Dore

About 5600 years ago, a huge lahar was formed at Mount Rainier in Washington state . The volume was calculated to be around 3.8 km³. It filled valleys with up to 200 m of sediment, covered a distance of up to 120 km and continued to flow 20 km under water at the bottom of Puget Sound .

At the Monts Dore in the French Auvergne , a lahar covered more than 30 km in the Tertiary .


  • Hans Füchtbauer : Sediments and sedimentary rocks , part 2. In: Sediment-Petrology. 4th, completely revised edition, Schweizerbart, Stuttgart 1988, ISBN 3-510-65138-3 .
  • Christopher G. Newhall, Raymundo S. Punongbayan (Eds.): Fire and Mud. Eruptions and Lahars of Mount Pinatubo, Philippines. University of Washington Press, Seattle 1997, ISBN 0-295-97585-7
  • Haraldur Sigurðsson , Bruce F. Houghton et al. (Ed.): Encyclopedia of Volcanoes. Academic Press, San Diego 2000, ISBN 0-12-643140-X

Web links

Wiktionary: Lahar  - explanations of meanings, word origins, synonyms, translations
Commons : Lahars  - collection of images, videos and audio files

Scientific contributions

Eyewitness accounts