August 12, 2022

Lahar (a word with origin in lahar, avalanche in Javanese, one of the languages ​​of Indonesia) is the designation given to a mass movement exclusive to volcanic regions, formed by the displacement along valleys or steep slopes, in the form of an avalanche, of mud composed of pyroclastic materials and water. Lahars are very frequent during volcanic eruptions, however, they can occur even in their absence on slopes covered by volcanic materials made unstable by heavy rains.


Lahars are mud avalanches formed by the fluidization of water-saturated volcanic materials. Behaving like a viscous fluid of very high density, lahars follow the path of lowest potential energy, so their course is dictated by the topography, generally following the valleys of watercourses. The mud that forms the lahar has the consistency of fresh concrete, maintaining a high degree of fluidity when in motion, but solidifying and losing water almost instantly when standing still. These rheological characteristics allow the lahars to have a high displacement speed and a great capacity to penetrate empty spaces, which leads to the rapid filling with solid material of all cavities that it finds in its path. The high density of the fluid formed allows the transport of large rock masses that float in the mud and are dragged at high speed as if they were light material. This property of lahars results in the appearance in volcanic landscapes of large isolated rocks, left by the weakening of the lahar's transport capacity, usually by the dispersion and loss of velocity and depth of the mud blade due to the widening of the covered area. A notable example of this effect of lahars is the presence of gigantic trachytic blocks, about 8–10 m high and weighing a few thousand tons, isolated on the plateau above Caparica, Biscoitos, on Terceira Island. Those blocks were left there by a gigantic lahar that formed about 25,000 years ago during the last major eruption of Pico Alto. As a result of the high density of the fluid, and its speed, the lahars have an enormous erosive power, ripping large volumes of geological material from the areas crossed by abrasion, transporting it and integrating it in an avalanche process that allows the mass to grow. in motion, thus feeding the lahar. Lahars can travel at very high speeds, and when the terrain is steep and viscosity is low, they can exceed 100 km/h (30 m/s). Lahars can be gigantic: a lahar that occurred 5,600 years ago in Osceola, along the White River valley, during an eruption of Mount Rainier (Washington State), produced a layer of silt 180 m deep and covered a area of ​​320 km². The great lahar that destroyed Vila Franca do Campo on the night of the 21st to the 22nd of October 1522 (the infamous subversion of Vila Franca), causing thousands of deaths, covered about 3.5 km² of land with mud and stones. In the Azores there are many hundreds of geological formations identified as having been formed by lahars, some covering large areas, as is the case of the lahar that from the Pico Rachado area descended along the Ribeira de São Roque, in Altares, Terceira, covering more of 6 km until its end.

Causes of lahars

If there is a sufficient slope and an abundance of water and loose volcanic material, in particular low-density pyroclasts (such as bagasse or hydrothermal clay soils), lahars can be triggered, among others, by the following causes: High and persistent rain during a volcanic eruption, which is common as the presence of fine volcanic ash in the atmosphere, which acts as condensation nuclei, and the air column rises