Lahar

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

A lahar ( /ˈlɑːhɑːr/ , from Javanese : 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. [1]

Contents

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. Notable lahars include those at Mount Pinatubo and Nevado del Ruiz, the latter of which covered entire towns and killed thousands of people.

Etymology

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

Description

Excavated 9th century Sambisari Hindu temple near Yogyakarta in Java, Indonesia. The temple was buried 6.5 metres under the lahar volcanic debris accumulated from centuries of Mount Merapi eruptions Sambisari 01.jpg
Excavated 9th century Sambisari Hindu temple near Yogyakarta in Java, Indonesia. The temple was buried 6.5 metres under the lahar volcanic debris accumulated from centuries of Mount Merapi eruptions

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] On steep slopes, lahar speeds can exceed 200 kilometres per hour (120 mph). [5] With the potential to 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]

Causes

Mudline left behind on trees on the banks of the Muddy River after the 1980 eruption of Mount St. Helens showing the height of the lahar. MSH80 mudline muddy river with USGS scientist 10-23-80.jpg
Mudline left behind on trees on the banks of the Muddy River after the 1980 eruption of Mount St. Helens showing the height of the lahar.

Lahars have several possible causes: [5]

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.

Places at risk

The aftermath of a lahar from the 1982 eruption of Galunggung, Indonesia. Galunggung lahar.jpg
The aftermath of a lahar from the 1982 eruption of Galunggung, Indonesia.

Several mountains in the world, including Mount Rainier in the United States, Mount Ruapehu in New Zealand, Merapi [11] [12] 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 to 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. [13] 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. 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. [14] 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

Nevado del Ruiz

The lahar from the 1985 eruption of Nevado del Ruiz that wiped out the town of Armero in Colombia. Armero aftermath Marso.jpg
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 in central Colombia. 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 (37 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. [15]

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.

Mount Pinatubo

A before-and-after photograph of a river valley filled in by lahars from Mount Pinatubo. River valley filled in by pyroclastic flows, Mt. Pinatubo.jpg
A before-and-after photograph of a river valley filled in by lahars from Mount Pinatubo.

The 1991 eruption of Mount Pinatubo caused lahars: the first eruption itself killed six people, but the lahar 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. In Pampanga, Angeles City and neighbouring cities and towns were damaged by the volcano's lahar when Sapang Balen Creek and the Abacan River became the channels for the mudflows and carried it to the heart of the city and surrounding areas.

Over 6 metres (20 ft) of mud inundated and damaged the towns of Castillejos, San Marcelino and Botolan in Zambales, Porac and Mabalacat City in Pampanga, Tarlac City, Capas, Concepcion and Bamban in Tarlac. The lahar in 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 6 metres (20 ft) 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 an 8-metre (25 ft) lahar. This mudflow killed hundreds of people in Barangay Cabalantian in Bacolor. The Philippine government under President Fidel V. Ramos ordered the construction of the FVR Mega Dike in an attempt to protect people from further mudflows.

Another typhoon-caused lahar hit the Philippines in 2006; see Typhoon Reming.

See also

Related Research Articles

Mount Ruapehu Active stratovolcano at the south of the North Island of New Zealand

Mount Ruapehu is an active stratovolcano at the southern end of the Taupo Volcanic Zone in New Zealand. It is 23 kilometres (14 mi) northeast of Ohakune and 23 km (14 mi) southwest of the southern shore of Lake Taupo, within Tongariro National Park. The North Island's major ski resorts and only glaciers are on its slopes.

Stratovolcano Tall, conical volcano built up by many layers of hardened lava and other ejecta

A stratovolcano, also known as a composite volcano, is a conical volcano built up by many layers (strata) of hardened lava, tephra, pumice and ash. Unlike shield volcanoes, stratovolcanoes are characterized by a steep profile with a summit crater and periodic intervals of explosive eruptions and effusive eruptions, although some have collapsed summit craters called calderas. The lava flowing from stratovolcanoes typically cools and hardens before spreading far, due to high viscosity. The magma forming this lava is often felsic, having high-to-intermediate levels of silica, with lesser amounts of less-viscous mafic magma. Extensive felsic lava flows are uncommon, but have travelled as far as 15 km (9.3 mi).

Nevado del Ruiz Volcanic mountain in Colombia

The Nevado del Ruiz, also known as La Mesa de Herveo is a volcano located on the border of the departments of Caldas and Tolima in Colombia, about 129 kilometers (80 mi) west of the capital city Bogotá. It is a stratovolcano composed of many layers of lava alternating with hardened volcanic ash and other pyroclastic rocks. Volcanic activity at Nevado del Ruiz began about two million years ago, since the Early Pleistocene or Late Pliocene, with three major eruptive periods. The current volcanic cone formed during the present eruptive period, which began 150 thousand years ago.

Glacier Peak Stratovolcano in Washington

Glacier Peak or Dakobed is the most isolated of the five major stratovolcanoes of the Cascade Volcanic Arc in the U.S state of Washington. Located in the Glacier Peak Wilderness in Mount Baker–Snoqualmie National Forest, the volcano is visible from the west in Seattle, and from the north in the higher areas of eastern suburbs of Vancouver such as Coquitlam, New Westminster and Port Coquitlam. The volcano is the fourth tallest peak in Washington state, and not as much is known about it compared to other volcanoes in the area. Local Native Americans have recognized Glacier Peak and other Washington volcanoes in their histories and stories. When American explorers reached the region, they learned basic information about surrounding landforms, but did not initially understand that Glacier Peak was a volcano. Positioned in Snohomish County, the volcano is only 70 miles (110 km) northeast of Downtown Seattle. From locations in northern Seattle and northward, Glacier Peak is closer than the more famous Mount Rainier, but as Glacier Peak is set farther into the Cascades and almost 4,000 feet (1,200 m) shorter, it is much less noticeable than Mount Rainier.

Mount Pinatubo active stratovolcano on the island of Luzon in the Philippines

Mount Pinatubo is an active stratovolcano in the Zambales Mountains, located on the tripoint boundary of the Philippine provinces of Zambales, Tarlac and Pampanga, all in Central Luzon on the northern island of Luzon. Its eruptive history was unknown to most before the pre-eruption volcanic activities of 1991, just before June. Pinatubo was heavily eroded, inconspicuous and obscured from view. It was covered with dense forests which supported a population of several thousand indigenous Aetas.

1980 eruption of Mount St. Helens Major volcanic eruption in Washington state, US

The 1980 eruption of Mount St. Helens was a series of volcanic explosions and pyroclastic flows from Mount St. Helens in Skamania County, U.S. state of Washington, that began on March 27, 1980. It initiated as a series of phreatic blasts from the summit then escalated on May 18, 1980, as a major explosive eruption. The eruption, which had a Volcanic Explosivity Index of 5, was the most significant to occur in the contiguous 48 U.S. states since the much smaller 1915 eruption of Lassen Peak in California. It has often been declared the most disastrous volcanic eruption in U.S. history. The eruption was preceded by a two-month series of earthquakes and steam-venting episodes, caused by an injection of magma at shallow depth below the volcano that created a large bulge and a fracture system on the mountain's north slope.

Spirit Lake (Washington) Lake in Skamania County, Washington, USA

Spirit Lake is a lake north of Mount St. Helens in Washington State. The lake was a popular tourist destination for many years until the 1980 eruption of Mount St. Helens. Prior to 1980, there were six camps on the shore of Spirit Lake: a Boy Scout camp, a Girl Scout camp, two YMCA camps, Harmony Fall Lodge, and another for the general public. There was also a number of lodges catering to visitors, including Spirit Lake Lodge and Mt. St. Helens Lodge; the latter was inhabited by Harry R. Truman, who became one of the volcano's victims.

Eruption column

An eruption column or eruption plume is a cloud of super-heated ash and tephra suspended in gases emitted during an explosive volcanic eruption. The volcanic materials form a vertical column or plume that may rise many kilometers into the air above the vent of the volcano. In the most explosive eruptions, the eruption column may rise over 40 km (25 mi), penetrating the stratosphere. Stratospheric injection of aerosols by volcanoes is a major cause of short-term climate change.

Armero tragedy November 1985 volcanic eruption in Colombia

The Armero tragedy was one of the major consequences of the eruption of the Nevado del Ruiz stratovolcano in Tolima, Colombia, on November 13, 1985. After 69 years of dormancy, the volcano's eruption caught nearby towns unaware, even though the government had received warnings from multiple volcanological organizations to evacuate the area after the detection of volcanic activity two months earlier.

Mudflow

A mudflow or mud flow is a form of mass wasting involving "very rapid to extremely rapid surging flow" of debris that has become partially or fully liquified by the addition of significant amounts of water to the source material.

Explosive eruption Type of volcanic eruption in which lava is violently expelled

In volcanology, an explosive eruption is a volcanic eruption of the most violent type. A notable example is the 1980 eruption of Mount St. Helens. Such eruptions result when sufficient gas has dissolved under pressure within a viscous magma such that expelled lava violently froths into volcanic ash when pressure is suddenly lowered at the vent. Sometimes a lava plug will block the conduit to the summit, and when this occurs, eruptions are more violent. Explosive eruptions can send rocks, dust, gas and pyroclastic material up to 20 km (12 mi) into the atmosphere at a rate of up to 100,000 tonnes per second, traveling at several hundred meters per second. This cloud may then collapse, creating a fast-moving pyroclastic flow of hot volcanic matter.

Debris flow

Debris flows are geological phenomena in which water-laden masses of soil and fragmented rock rush down mountainsides, funnel into stream channels, entrain objects in their paths, and form thick, muddy deposits on valley floors. They generally have bulk densities comparable to those of rock avalanches and other types of landslides, but owing to widespread sediment liquefaction caused by high pore-fluid pressures, they can flow almost as fluidly as water. Debris flows descending steep channels commonly attain speeds that surpass 10 m/s (36 km/h), although some large flows can reach speeds that are much greater. Debris flows with volumes ranging up to about 100,000 cubic meters occur frequently in mountainous regions worldwide. The largest prehistoric flows have had volumes exceeding 1 billion cubic meters. As a result of their high sediment concentrations and mobility, debris flows can be very destructive.

Prediction of volcanic activity

Prediction of volcanic eruption, or volcanic eruption forecasting, is an interdisciplinary monitoring and research effort to predict the time and severity of a volcano's eruption. Of particular importance is the prediction of hazardous eruptions that could lead to catastrophic loss of life, property, and disruption of human activities.

Cascade Volcanoes Chain of stratovolcanoes in western North America

The Cascade Volcanoes are a number of volcanoes in a volcanic arc in western North America, extending from southwestern British Columbia through Washington and Oregon to Northern California, a distance of well over 700 miles (1,100 km). The arc formed due to subduction along the Cascadia subduction zone. Although taking its name from the Cascade Range, this term is a geologic grouping rather than a geographic one, and the Cascade Volcanoes extend north into the Coast Mountains, past the Fraser River which is the northward limit of the Cascade Range proper.

Mount Rainier Stratovolcano in the U.S. state of Washington

Mount Rainier, also known as Tahoma or Tacoma, is a large active stratovolcano in Cascadia located 59 miles (95 km) south-southeast of Seattle, in Mount Rainier National Park. With a summit elevation of 14,411 ft (4,392 m), it is the highest mountain in the U.S. state of Washington, and of the Cascade Range of the Pacific Northwest, the most topographically prominent mountain in the contiguous United States, and the tallest in the Cascade Volcanic Arc.

Lake Pinatubo summit crater lake of Mt. Pinatubo

Lake Pinatubo is the summit crater lake of Mount Pinatubo formed after its climactic eruption on June 15, 1991. The lake is located in Botolan, Zambales, near the boundaries of Pampanga and Tarlac provinces in the Philippines. It is about 90 km (56 mi) northwest of the capital city of Manila. While one paper by researchers from Japan suggested a depth of 600 m (2,000 ft), more detailed research suggests that 95–115 m (312–377 ft) is more accurate.

The Mount Rainier Volcano Lahar Warning System consists of two separate components, operating in tandem: Acoustic Flow Monitors (AFM) and the All Hazard Alert Broadcast (AHAB) sirens. The AFM system was developed by the United States Geological Survey (USGS) in 1998 and is now maintained by Pierce County Emergency Management. The purpose of the warning system is to assist in the evacuation of residents in the river valleys around Mount Rainier, a volcano in Washington, in the event of a lahar. Pierce County works in partnership with the USGS, the Pacific Northwest Seismic Network (PNSN), Washington Military Department's Emergency Management Division, and South Sound 9-1-1 to monitor and operate the system.

1991 eruption of Mount Pinatubo Volcanic eruption in the Philippines in 1991

The 1991 eruption of Mount Pinatubo was a significant volcanic eruption in the Luzon Volcanic Arc of the Central Luzon region of the Philippines. Eruptive activity began on April 2 as a series of phreatic explosions from a fissure that opened on the north side of Mount Pinatubo. Seismographs were set up and began monitoring the volcano for earthquakes. In late May, the number of seismic events under the volcano fluctuated from day-to-day. Beginning June 6, a swarm of progressively shallower earthquakes accompanied by inflationary tilt on the upper east flank of the mountain, culminated in the extrusion of a small lava dome.

Volcanic hazards

A volcanic hazard is the probability that a volcanic eruption or related geophysical event will occur in a given geographic area and within a specified window of time. The risk that can be associated with a volcanic hazard depends on the proximity and vulnerability of an asset or a population of people near to where a volcanic event might occur.

The Bridge for Kids is a proposed bridge across the Carbon River in Orting, Washington, about a mile upstream of where it joins the Puyallup River. It would provide an emergency evacuation route for school children to escape a future lahar flow from Mount Rainier, consisting of an up to 10-meter (33 ft) high flood of mud, rock and boulders. As of 2016, the $40 million bridge was still in the planning phase.

References

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  15. PD-icon.svg This article incorporates  public domain material from the United States Geological Survey document: Schuster, Robert L.; Highland, Lynn M. (2001). "Socioeconomic and Environmental Impacts of Landslides in the Western Hemisphere". Open-File Report 01-0276. Retrieved June 11, 2010.