A lahar from the 1982 eruption of Galunggung.

A lahar /ˈlɑːhɑr/ is a 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] The term is a shortened version of "berlahar" which originated in the Javanese language of Indonesia.^{[citation needed]}

Description[link]

"Lahar" is an Indonesian word that describes volcanic mudflows or debris flows.^[2] Lahars have the consistency, viscosity and approximate density of concrete: fluid when moving, solid at rest.^[3] Lahars can be huge. The Osceola lahar produced by Mount Rainier (Washington) some 5,600 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)^[4]. A lahar of sufficient size and intensity can raze 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.

Lahar flows can be deadly because of their energy and speed. With the potential to flow at speeds up to 100 kilometres per hour (60 mph), and distances of more than 300 kilometres (190 mi), a lahar can cause catastrophic destruction in its path.^[5] 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.^[6] A lahar caused New Zealand's Tangiwai disaster, where 151 people died after a Christmas Eve express train fell into the Whangaehu River. Lahars have been responsible for 17% of volcano-related deaths between 1783 and 1997.^[7] 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.

Causes[link]

This mudline left behind on the trees on the banks of the Muddy River after the 1980 eruption of Mount St. Helens shows how high the lahars reached here.

Lahars have several possible causes:^[3]

• Snow and glaciers can be melted by lava or pyroclastic flows during an eruption
• A flood caused by a glacier, lake breakout, or heavy rainfall can release a lahar, also called glacier run or jökulhlaup
• Water from a crater lake, combined with volcanic material in an eruption

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 weather
• Earthquakes underneath or close to the volcano can shake material loose and cause it to collapse triggering a lahar avalanche.
• Rainfall or typhoons can cause the still-hanging slabs of solidified mud to come rushing down the slopes at a speed of more than 30 Kilometers per hour, causing devastating results.

Places at risk[link]

An evacuation route sign in case of volcanic eruption or lahar.

Several mountains in the world, including Mount Rainier in the US, Mount Ruapehu in New Zealand, and Galunggung in Indonesia, are considered particularly dangerous due to the risk of lahars. Several towns in the Puyallup River valley in Washington state, including Orting, are built on top of lahar deposits that are only about 500 years old. Lahars are predicted to flow through the valley every 500-1,000 years, so Orting, Sumner, Puyallup, Fife, and the Port of Tacoma face considerable risk. The USGS has set up lahar warning sirens in Pierce County, Washington, so that people can flee an approaching debris flow in the event of a Mount Rainier eruption.

A lahar warning system has been set up at Mount Ruapehu by the New Zealand Department of Conservation and hailed as a success after it successfully alerted officials to an impending lahar on 18 March 2007.

Since mid-June 1991, when violent eruptions triggered 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 manned 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.^[8] Physical preventative measures by the Philippine government were not adequate to stop over 20 feet (6.1 m) of mud from flooding many villages around Mount Pinatubo from 1992 through 1998.

Scientists and governments try to identify areas with a high risk of lahars based on historical events and computer models. An example of such a model is TITAN2D. 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.

Examples[link]

Nevado del Ruiz[link]

The lahar from the 1985 eruption of Nevado del Ruiz that wiped out the town of Armero in Colombia.

In 1985, the volcano Nevado del Ruiz erupted. As pyroclastic flows erupted from the volcano's crater, they melted the mountain's glaciers, sending four enormous lahars down its slopes at 60 kilometers per hour (40 miles per hour). The lahars picked up speed in gullies and coursed into the six major rivers at the base of the volcano; they engulfed the town of Armero, killing more than 20,000 of its almost 29,000 inhabitants.^[9] Casualties in other towns, particularly Chinchiná, brought the overall death toll to 23,000. Footage and photographs of Omayra Sánchez, a young victim of the tragedy, were published around the world. Other photographs of the lahars and the impact of the disaster captured attention worldwide and led to controversy over the degree to which the Colombian government was responsible for the disaster. A banner at a mass funeral in Ibague read, "The volcano didn't kill 22,000 people. The government killed them."

Mount Pinatubo[link]

Before and after photographs of a river valley filled in by lahars from Mount Pinatubo

The 1991 eruption of Mount Pinatubo caused lahar flows: the first eruption itself killed only six people, but the lahar flows killed more than 1500. The eye of Typhoon Yunya passed over the volcano during its eruption on June 15, 1991. The rain from the typhoon triggered the flow of volcanic ash, boulders, and water down the rivers surrounding the volcano. Angeles City was damaged by the volcano's sticky mud when Sapang Balen Creek and the Abacan River became the channel for the lahar and carried it to the heart of the city. Over 20 feet (6 m) of mud destroyed or damaged Castillejos, San Marcelino, Botolan, Porac, Mabalacat, Tarlac City, Capas, Concepcion, and Bamban. The mud flow down the Sacobia Bamban River scoured all structures in its path, including the bridges and dikes by the Parua river in Concepcion. The Tarlac River in Tarlac City was inundated by over 20 feet (6 m) of lahar, causing the river to lose the ability to hold water.

On the morning of October 1, 1995, pyroclastic material which clung to the slopes of Pinatubo and surrounding mountains rushed down because of heavy rain, and turned into a 25-foot (8 m) lahar. The mudflow killed hundreds of people in Barangay Cabalantian in Bacolor. The Philippine government under then-president Fidel V. Ramos ordered the construction of the FVR Mega Dike in an attempt to protect people from further mudflows.

See also[link]

Wikimedia Commons has media related to: Lahars

• Mudflow
• Pyroclastic flow
• Pyroclastic surge

References[link]

External links[link]

• Schools page about lahars and pyroclastic flows
• USGS web page about lahars
• Mount Rainier, Washington
• USGS fact sheet - "Mount Rainier -- Living Safely With a Volcano in Your Back Yard"

Landsat image of Lake Toba, Indonesia, the largest volcanic crater lake in the world.

Lake Pinatubo, Philippines, formed after the 1991 eruption of Mount Pinatubo.

Space shuttle imagery of Manicouagan Reservoir / Manicouagan impact crater, Canada, the largest impact crater lake in the world.

Heaven Lake, North Korea / China.

Mount Aso crater lake, Japan.

Taal volcano in Lake Taal, Philippines.

Irazú crater lake, Costa Rica.

Maderas crater lake (Ometepe Island), Nicaragua.

Crater Lake in Oregon, USA.

Cuicocha, Ecuador.

Niuafo'ou crater lake, Tonga.

Katmai crater lake, Alaska, USA.

Kelimutu crater lake, Indonesia.

Kerið crater lake, Iceland.

A crater lake is a lake that forms in a volcanic crater or caldera, such as a maar; less commonly and with lower association to the term a lake may form in an impact crater caused by a meteorite, or in an artificial explosion caused by humans. Sometimes lakes which form inside calderas are called caldera lakes, but often this distinction is not made. Crater lakes covering active (fumarolic) volcanic vents are sometimes known as volcanic lakes, and the water within them is often acidic, saturated with volcanic gases, and cloudy with a strong greenish color. Lakes located in dormant or extinct volcanoes tend to have fresh water, and the water clarity in such lakes can be exceptional due to the lack of inflowing streams and sediment.

Lava lakes, a rare phenomenon where a volcanic crater can sustainably contain a significant volume of molten lava, are discussed in a separate article.

Formation[link]

Volcanic crater lakes[link]

Crater lakes form as precipitation within the rim fills the created depression. The water level rises until an equilibrium is reached between the rate of incoming and outgoing water. Sources of water loss singly or together, may include evaporation, subsurface seepage, and in places, surface leakage or overflow when the lake level reaches the lowest point on its rim. At such a saddle location, the upper portion of the lake is contained only by its adjacent natural volcanic dam; continued leakage through or surface outflow across the dam can erode its included material, thus lowering lake level until a new equilibrium of water flow, erosion and rock resistance is established. If the volcanic dam portion erodes rapidly or fails catastrophically, the occurrence produces a breakout or outburst flood. With changes in environmental conditions over time, the occurrence of such floods is common to all natural dam types.

A well-known crater lake, which bears the same name as the geological feature, is Crater Lake in Oregon, USA. It is located in the caldera of Mount Mazama. It is the deepest lake in the United States with a depth of 594 m (1,949 ft). Crater Lake is fed solely by falling rain and snow, with no inflow or outflow at the surface, and hence is one of the clearest lakes in the world.^[1]

The highest volcano in the world, 6,893 metres (22,615 ft) Ojos del Salado in Chile, has a permanent crater lake about 100 metres (300 ft) in diameter at an elevation of 6,390 m (20,960 ft) on its eastern side.^[2] This is most likely the highest lake of any kind in the world.

Due to their unstable environment, some crater lakes exist only intermittently. Caldera lakes in contrast can be quite large and long-lasting; for instance, Lake Toba (Indonesia) formed after its eruption around 70,000 years ago and has an area of over 1,000 square kilometres.

While many crater lakes are picturesque, they can also be deadly. Gas discharges from Lake Nyos (Cameroon) suffocated 1,800 people in 1986, and crater lakes such as Mount Ruapehu's (New Zealand) often contribute to destructive lahars.

Certain bodies of water, although their formation is directly related to volcanic activity, are not usually referred to as crater lakes, including:

• Lakes created by volcanic dams due to lava flowing outside of the volcanic edifice/caldera (such as Garibaldi Lake in Canada, Fuji Five Lakes in Japan),
• Closed atoll lagoons (such as Clipperton lagoon), whose formation process also implies subsequent biogeomorphologic processes,
• Ponds encountered at the bottom of waterfalls occuring in volcanic canyons in a volcanic context but not within a volcanic edifice/caldera (such as Trou de Fer on Réunion Island).

Other kinds of crater lakes[link]

Lakes can also fill impact craters, but these are not usually referred to as crater lakes except in a few isolated cases. Example of such impact crater lakes include Manicouagan in Canada, Lake Bosumtwi in Ghana and Siljan in Sweden.

More rarely they can also fill craters caused by artificial explosions, such as the radioactive Lake Chagan in Kazakhstan.

Other geological features that can be mistaken for a crater lake[link]

Some geomorphological features, when filled with water, can sometimes be confused with crater lakes:

• Pingos (soil-covered ice mounts) whose summital part has collapsed,^[3]
• Sinkholes (karstic holes, also called cenotes), such as Otjikoto Lake in Namibia.

Some circular open-pit mines can also present a similar appearance, such as Big Hole in Kimberley, South Africa, a diamond mine (in a kimberlite pipe) where water has accumulated in the artificially created depression.

List of volcanic crater lakes[link]

This list is incomplete; you can help by expanding it.

List of meteor crater lakes[link]

List of artificial crater lakes[link]