Lahars

A lahar (, from jv, ꦮ꧀ꦭꦲꦂ) 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.

Lahars are extremely destructive: they can flow tens of metres per second, they have been known to be up to deep, and large flows tend to destroy any structures in their path. Notable lahars include those at Mount Pinatubo and Nevado del Ruiz, the latter of which killed thousands of people in the town of Armero.

Etymology

The word ''lahar'' is of Javanese origin. Berend George Escher introduced it as a geological term in 1922.

Description

The word ''lahar'' is a general term for a flowing mixture of water and pyroclastic debris. It does not refer to a particular rheology or sediment concentration. Lahars can occur as normal stream flows (sediment concentration of less than 30%), hyper-concentrated stream flows (sediment concentration between 30 and 60%), or debris flows (sediment concentration exceeding 60%). Indeed, the rheology and subsequent behaviour of a lahar may vary in place and time within a single event, owing to changes in sediment supply and water supply. Lahars are described as 'primary' or 'syn-eruptive' if they occur simultaneously with or are triggered by primary volcanic activity. 'Secondary' or 'post-eruptive' lahars occur in the absence of primary volcanic activity, e.g. as a result of rainfall during pauses in activity or during dormancy.

In addition to their variable rheology, lahars vary considerably in magnitude. The Osceola Lahar produced by Mount Rainier in modern-day Washington some 5600 years ago resulted in a wall of mud deep in the White River canyon and covered an area of over , for a total volume of . A debris-flow lahar can erase virtually any structure in its path, while a hyperconcentrated-flow lahar is capable of carving its own pathway, destroying buildings by undermining their foundations. A hyperconcentrated-flow lahar can leave even frail huts standing, while at the same time burying them in mud, which can harden to near-concrete hardness. A lahar's viscosity decreases the longer it flows and can be further thinned by rain, producing a quicksand-like mixture that can remain fluidized for weeks and complicate search and rescue.

Lahars vary in 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. On steep slopes, lahar speeds can exceed . A lahar can cause catastrophic destruction along a potential path of more than .

Lahars from the 1985 Nevado del Ruiz eruption in Colombia caused the Armero tragedy, burying the city of Armero under of mud and debris and killing an estimated 23,000 people. A lahar caused New Zealand's Tangiwai disaster, where 151 people died after a Christmas Eve express train fell into the Whangaehu River in 1953. Lahars have caused 17% of volcano-related deaths between 1783 and 1997.

Trigger mechanisms

Lahars have several possible causes:
* Snow and glaciers can be melted by lava or pyroclastic surges during an eruption.
* Lava can erupt from open vents and mix with wet soil, mud 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 can combine with volcanic material in an eruption.
* Heavy rainfall can mobilize unconsolidated pyroclastic deposits.
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.
* Snow and glaciers can melt during periods of mild to hot weather.
* Earthquakes underneath or close to the volcano can shake material loose and cause it to collapse, triggering a lahar avalanche.
* Rainfall can cause the still-hanging slabs of solidified mud to come rushing down the slopes at a speed of more than , causing devastating results.

Places at risk

Several mountains in the world – including Mount Rainier in the United States, Mount Ruapehu in New Zealand, and Merapi and Galunggung in Indonesia – are considered particularly dangerous due to the risk of lahars. Several towns in the Puyallup River valley in Wa