Lahar - Volcanic Mudflows - PDF

*A **lahar** ( [/ˈlɑːhɑːr/](https://en.wikipedia.org/wiki/Help:IPA/English), from [Javanese](https://en.wikipedia.org/wiki/Javanese_language): ꦮ꧀ꦭꦲꦂ) is a violent type of [mudflow](https://en.wikipedia.org/wiki/Mudflow) or [debris flow](https://en.wikipedia.org/wiki/Debris_flow) composed of a [slurr...

*A **lahar** ( [/ˈlɑːhɑːr/](https://en.wikipedia.org/wiki/Help:IPA/English), from [Javanese](https://en.wikipedia.org/wiki/Javanese_language): ꦮ꧀ꦭꦲꦂ) is a violent type of [mudflow](https://en.wikipedia.org/wiki/Mudflow) or [debris flow](https://en.wikipedia.org/wiki/Debris_flow) composed of a [slurry](https://en.wikipedia.org/wiki/Slurry) of [pyroclastic](https://en.wikipedia.org/wiki/Pyroclastic_rock) material, rocky debris and water. The material flows down from a [volcano](https://en.wikipedia.org/wiki/Volcano), typically along a [river valley](https://en.wikipedia.org/wiki/Valley).[^\[1\]^](https://en.wikipedia.org/wiki/Lahar#cite_note-usgsgloss-1)* *Lahars are often extremely destructive and deadly; they can flow tens of metres per second, they have been known to be up to 140 metres (460 ft) deep, and large flows tend to destroy any structures in their path. Notable lahars include those at [Mount Pinatubo](https://en.wikipedia.org/wiki/Mount_Pinatubo) in the Philippines and [Nevado del Ruiz](https://en.wikipedia.org/wiki/Nevado_del_Ruiz) in Colombia, the latter of which killed more than 20,000 people in the [Armero tragedy](https://en.wikipedia.org/wiki/Armero_tragedy).* The word *lahar* is a general term for a flowing mixture of water and pyroclastic debris. It does not refer to a particular [rheology](https://en.wikipedia.org/wiki/Rheology) or [sediment](https://en.wikipedia.org/wiki/Sediment) concentration.[^\[4\]^](https://en.wikipedia.org/wiki/Lahar#cite_note-Vallance_649%E2%80%93664-4) 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.[^\[4\]^](https://en.wikipedia.org/wiki/Lahar#cite_note-Vallance_649%E2%80%93664-4) 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.^[\[5\]](https://en.wikipedia.org/wiki/Lahar#cite_note-Piersonetal2014-5)[\[6\]](https://en.wikipedia.org/wiki/Lahar#cite_note-Kataokaetal2018-6)^ In addition to their variable rheology, lahars vary considerably in magnitude. The [Osceola Lahar](https://en.wikipedia.org/wiki/Osceola_Lahar) produced by [Mount Rainier](https://en.wikipedia.org/wiki/Mount_Rainier) in modern-day [Washington](https://en.wikipedia.org/wiki/Washington_(state)) some 5600 years ago resulted in a wall of mud 140 metres (460 ft) deep in the [White River](https://en.wikipedia.org/wiki/White_River_(Washington)) canyon and covered an area of over 330 square kilometres (130 sq mi), for a total volume of 2.3 cubic kilometres (^1^⁄~2~ cu mi).[^\[7\]^](https://en.wikipedia.org/wiki/Lahar#cite_note-Crandall-7) 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.[^\[5\]^](https://en.wikipedia.org/wiki/Lahar#cite_note-Piersonetal2014-5) A hyperconcentrated-flow lahar can leave even frail huts standing, while at the same time burying them in mud,[^\[8\]^](https://en.wikipedia.org/wiki/Lahar#cite_note-Jandaetal1996-8) 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](https://en.wikipedia.org/wiki/Quicksand)-like mixture that can remain fluidized for weeks and complicate search and rescue.[^\[5\]^](https://en.wikipedia.org/wiki/Lahar#cite_note-Piersonetal2014-5) 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.[^\[9\]^](https://en.wikipedia.org/wiki/Lahar#cite_note-usgswhat-9) On steep slopes, lahar speeds can exceed 200 kilometres per hour (120 mph).[^\[9\]^](https://en.wikipedia.org/wiki/Lahar#cite_note-usgswhat-9) A lahar can cause catastrophic destruction along a potential path of more than 300 kilometres (190 mi).[^\[10\]^](https://en.wikipedia.org/wiki/Lahar#cite_note-hoblitt-10) Lahars from the 1985 [Nevado del Ruiz](https://en.wikipedia.org/wiki/Nevado_del_Ruiz) eruption in [Colombia](https://en.wikipedia.org/wiki/Colombia) caused the [Armero tragedy](https://en.wikipedia.org/wiki/Armero_tragedy), burying the city of [Armero](https://en.wikipedia.org/wiki/Armero) under 5 metres (16 ft) of mud and debris and killing an estimated 23,000 people.[^\[11\]^](https://en.wikipedia.org/wiki/Lahar#cite_note-11) A lahar caused New Zealand\'s [Tangiwai disaster](https://en.wikipedia.org/wiki/Tangiwai_disaster),[^\[12\]^](https://en.wikipedia.org/wiki/Lahar#cite_note-Tangiwai1-12) where 151 people died after a Christmas Eve express train fell into the [Whangaehu River](https://en.wikipedia.org/wiki/Whangaehu_River) in 1953. Lahars have caused 17% of volcano-related deaths between 1783 and 1997.[^\[13\]^](https://en.wikipedia.org/wiki/Lahar#cite_note-13) **Trigger mechanisms** \[(https://en.wikipedia.org/w/index.php?title=Lahar&action=edit&section=3)\] A fallen tree in the forest Description automatically generated[Mudline](https://en.wikipedia.org/wiki/Mudline) left behind on trees on the banks of the [Muddy River](https://en.wikipedia.org/wiki/Muddy_River_(Washington)) after the [1980 eruption of Mount St. Helens](https://en.wikipedia.org/wiki/1980_eruption_of_Mount_St._Helens) showing the height of the lahar Lahars have several possible causes:[^\[9\]^](https://en.wikipedia.org/wiki/Lahar#cite_note-usgswhat-9) - [Snow](https://en.wikipedia.org/wiki/Snow) and [glaciers](https://en.wikipedia.org/wiki/Glacier) can be melted by [lava](https://en.wikipedia.org/wiki/Lava) or [pyroclastic surges](https://en.wikipedia.org/wiki/Pyroclastic_surge) 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](https://en.wikipedia.org/wiki/Gravitational_energy) the flows will have. - A [flood](https://en.wikipedia.org/wiki/Flood) caused by a glacier, [lake breakout](https://en.wikipedia.org/wiki/Lake_breakout), or heavy rainfalls can generate lahars, also called [glacier run](https://en.wikipedia.org/wiki/Glacier_run) or [jökulhlaup](https://en.wikipedia.org/wiki/J%C3%B6kulhlaup). - Water from a [crater lake](https://en.wikipedia.org/wiki/Volcanic_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](https://en.wikipedia.org/wiki/Volcanic_ash) deposits. - Snow and glaciers can melt during periods of mild to hot weather. - [Earthquakes](https://en.wikipedia.org/wiki/Earthquakes) underneath or close to the volcano can shake material loose and cause it to collapse, triggering a lahar avalanche. - [Rainfall](https://en.wikipedia.org/wiki/Rainfall) can cause the still-hanging slabs of solidified mud to come rushing down the slopes at a speed of more than 18.64 mph (30.0 km/h), causing devastating results. **Places at risk** \[(https://en.wikipedia.org/w/index.php?title=Lahar&action=edit&section=4)\] The aftermath of a lahar from the 1982 eruption of [Galunggung](https://en.wikipedia.org/wiki/Galunggung), Indonesia Several mountains in the world -- including [Mount Rainier](https://en.wikipedia.org/wiki/Mount_Rainier)[^\[14\]^](https://en.wikipedia.org/wiki/Lahar#cite_note-14) in the United States, [Mount Ruapehu](https://en.wikipedia.org/wiki/Mount_Ruapehu) in New Zealand, and [Merapi](https://en.wikipedia.org/wiki/Mount_Merapi)^[\[15\]](https://en.wikipedia.org/wiki/Lahar#cite_note-15)[\[16\]](https://en.wikipedia.org/wiki/Lahar#cite_note-16)^ and [Galunggung](https://en.wikipedia.org/wiki/Galunggung) in Indonesia[^\[17\]^](https://en.wikipedia.org/wiki/Lahar#cite_note-17) -- are considered particularly dangerous due to the risk of lahars. Several towns in the [Puyallup River](https://en.wikipedia.org/wiki/Puyallup_River) valley in Washington state, including [Orting](https://en.wikipedia.org/wiki/Orting,_Washington), are built on top of lahar deposits that are only about 500 years old. Lahars are predicted to flow through the valley every 500 to 1,000 years, so Orting, [Sumner](https://en.wikipedia.org/wiki/Sumner,_Washington), [Puyallup](https://en.wikipedia.org/wiki/Puyallup,_Washington), [Fife](https://en.wikipedia.org/wiki/Fife,_Washington), and the Port of [Tacoma](https://en.wikipedia.org/wiki/Tacoma,_Washington) face considerable risk.[^\[18\]^](https://en.wikipedia.org/wiki/Lahar#cite_note-18) The [USGS](https://en.wikipedia.org/wiki/United_States_Geological_Survey) has set up [lahar warning sirens](https://en.wikipedia.org/wiki/Mount_Rainier_Volcano_Lahar_Warning_System) in [Pierce County, Washington](https://en.wikipedia.org/wiki/Pierce_County,_Washington), so that people can flee an approaching debris flow in the event of a Mount Rainier eruption.[^\[19\]^](https://en.wikipedia.org/wiki/Lahar#cite_note-USGS-19) A lahar [warning system](https://en.wikipedia.org/wiki/Warning_system) has been set up at [Mount Ruapehu](https://en.wikipedia.org/wiki/Mount_Ruapehu) by the [New Zealand Department of Conservation](https://en.wikipedia.org/wiki/New_Zealand_Department_of_Conservation) and hailed as a success after it successfully alerted officials to an impending lahar on 18 March 2007.[^\[20\]^](https://en.wikipedia.org/wiki/Lahar#cite_note-20) Since mid-June 1991, when violent eruptions triggered [Mount Pinatubo](https://en.wikipedia.org/wiki/Mount_Pinatubo)\'s first lahars in 500 years, a system to monitor and warn of lahars has been in operation. Radio-telemetered rain gauges provide data on rainfall in lahar source regions, acoustic flow monitors on stream banks detect ground vibration as lahars pass, and staffed watchpoints further confirm that lahars are rushing down Pinatubo\'s slopes. This system has enabled warnings to be sounded for most but not all major lahars at Pinatubo, saving hundreds of lives.[^\[21\]^](https://en.wikipedia.org/wiki/Lahar#cite_note-pinatuboLahar-21) Physical preventative measures by the [Philippine government](https://en.wikipedia.org/wiki/Philippine_government) were not adequate to stop over 6 m (20 ft) of mud from flooding many villages around Mount Pinatubo from 1992 through 1998.[^\[22\]^](https://en.wikipedia.org/wiki/Lahar#cite_note-22) Scientists and governments try to identify areas with a high risk of lahars based on historical events and [computer models](https://en.wikipedia.org/wiki/Computer_model). Volcano scientists play a critical role in effective hazard education by informing officials and the public about realistic hazard probabilities and scenarios (including potential magnitude, timing, and impacts); by helping evaluate the effectiveness of proposed risk-reduction strategies; by helping promote acceptance of (and confidence in) hazards information through participatory engagement with officials and vulnerable communities as partners in risk reduction efforts; and by communicating with emergency managers during extreme events.[^\[23\]^](https://en.wikipedia.org/wiki/Lahar#cite_note-FOOTNOTEPiersonWoodDriedger2014-23) An example of such a model is [TITAN2D](https://en.wikipedia.org/wiki/TITAN2D).[^\[24\]^](https://en.wikipedia.org/wiki/Lahar#cite_note-24) These models are directed towards future planning: identifying low-risk regions to place community buildings, discovering how to mitigate lahars with dams, and constructing evacuation plans.[^\[25\]^](https://en.wikipedia.org/wiki/Lahar#cite_note-25) **Examples**