A lahar travels down a river valley in Guatemala near the Santa Maria volcano, 1989.

A lahar ( /ˈlɑːhɑːr/, from Javanese: ꦮ꧀ꦭꦲꦂ, romanized: wlahar) is a violent type of mudflow or debris flow composed of a slurry of pyroclastic material, rocky debris and water. The material flows down from a volcano, typically along a river valley.[1]

Lahars are extremely destructive: they can flow tens of metres per second (22 mph or more), they have been known to be up to 140 metres (460 ft) deep, and large flows tend to destroy any structures in their path. They have even been known to decimate entire settlements. Notable lahars include those at Mount Pinatubo and Nevado del Ruiz, the latter of which killed thousands of people and caused extensive damage to infrastructure.


Excavated 9th century Sambisari Hindu temple near Yogyakarta in Java, Indonesia, was buried 6.5 metres under the lahar volcanic debris accumulated from centuries of Mount Merapi eruptions

The word lahar is of Javanese origin.[2] The geological term was introduced by Berend George Escher in 1922.[3]


A lahar is a volcanic mudflow or debris flow.[4] Lahars have the consistency, viscosity and approximate density of wet concrete: fluid when moving, solid at rest.[5] Lahars can be huge. The Osceola Lahar produced by Mount Rainier (Washington) some 5600 years ago resulted in a wall of mud 140 metres (460 ft) deep in the White River canyon, which covered an area of over 330 square kilometres (130 sq mi), for a total volume of 2.3 cubic kilometres (0.55 cu mi).[6]

A lahar of sufficient size and intensity can erase virtually any structure in its path, and is capable of carving its own pathway, making the prediction of its course difficult. Conversely, a lahar quickly loses force when it leaves the channel of its flow: even frail huts may remain standing, while at the same time being buried to the roof line in mud. A lahar's viscosity decreases with time, and can be further thinned by rain, but it nevertheless solidifies quickly when coming to a stop.

Lahars vary in size and speed. Small lahars less than a few metres wide and several centimetres deep may flow a few metres per second. Large lahars hundreds of metres wide and tens of metres deep can flow several tens of metres per second (22 mph or more): much too fast for people to outrun.[5] With the potential to flow at speeds up to 100 kilometres per hour (60 mph), and flow distances of more than 300 kilometres (190 mi), a lahar can cause catastrophic destruction in its path.[7]

Lahars from the 1985 Nevado del Ruiz eruption in Colombia caused the Armero tragedy, which killed an estimated 23,000 people, when the city of Armero was buried under 5 metres (16 ft) of mud and debris.[8] A lahar caused New Zealand's Tangiwai disaster,[9] where 151 people died after a Christmas Eve express train fell into the Whangaehu River in 1953. Lahars have been responsible for 17% of volcano-related deaths between 1783 and 1997.[10] A lahar can cause fatalities years after its precipitating eruption. For example, the Cabalantian tragedy occurred four years subsequent to the 1991 eruption of Mount Pinatubo.


Mudline left behind on trees on the banks of the Muddy River after the 1980 eruption of Mount St. Helens showing how high the lahar floods reached here

Lahars have several possible causes:[5]

  • Snow and glaciers can be melted by lava and/or pyroclastic surges during an eruption.
  • Lava gushes out of open vents and can mix with wet soil, mud and/or snow on the slope of the volcano making a very viscous, high energy lahar. (The higher up the slope of the volcano, the more gravitational potential energy the flows will have.)
  • A flood caused by a glacier, lake breakout, or heavy rainfalls can generate lahars, also called glacier run or jökulhlaup
  • Water from a crater lake, combined with volcanic elements in an eruption.
  • Heavy rainfall on unconsolidated pyroclastic deposits.
  • Volcanic landslides with water.

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.