course hero what is a lahar?

What is a lahar?

The word lahar is a general term used to describe a flowing mixture of water and pyroclastic debris; it does not specifically refer to a particular rheology or sediment concentration.

What is the speed of a lahar?

On steep slopes, lahar speeds can exceed 200 kilometres per hour (120 mph). With the potential to flow distances of more than 300 kilometres (190 mi), a lahar can cause catastrophic destruction in its path.

What triggers a lahar?

Several factors can trigger lahars. Although usually associated with volcanic activity, lahars can occur without volcanic activity if the necessary conditions trigger the collapse and flow of mud from existing deposits of volcanic ash. Therefore, glaciers and snow can melt during hot weather can cause lahars.

How dangerous are lahars to humans?

Lahars have been responsible for 17% of volcano-related deaths between 1783 and 1997. 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.

What is a lahar flow?

The word lahar is a general term used to describe a flowing mixture of water and pyroclastic debris; it does not specifically refer to a particular rheology or sediment concentration. Lahars can exist as normal stream flows (sediment concentration of less than 30%), hyper-concentrated stream flows (sediment concentration between 30-60%), or debris flows (sediment concentration exceeding 60%). Indeed, the rheology and subsequent behaviour of a lahar flow may vary in space and time within a single event, owing to changes in sediment supply and water supply. Lahars may be 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.

How fast can a lahar go?

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 200 kilometres per hour (120 mph). With the potential to flow distances of more than 300 kilometres (190 mi), a lahar can cause catastrophic destruction in its path.

What happened to the town of Armero?

Main article: Armero tragedy. 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 ...

How many people died in the 1985 Armero eruption?

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. 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 been responsible for 17% of volcano-related deaths between 1783 and 1997.

What causes a lahar?

Lahars have several possible causes: 1 Snow and glaciers can be melted by lava or pyroclastic surges during an eruption. 2 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. 3 A flood caused by a glacier, lake breakout, or heavy rainfalls can generate lahars, also called glacier run or jökulhlaup. 4 Water from a crater lake can combine with volcanic material in an eruption. 5 Heavy rainfall can mobilize unconsolidated pyroclastic deposits.

How deep is the Lahar?

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 ( 1⁄2 cu mi). 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 with time, and can be further thinned by rain, producing a quicksand -like mixture that can remain fluidized for weeks and complicates search and rescue.

What can cause a Lahar avalanche?

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 18.64 mph (30.0 km/h), causing devastating results.

What are the Lahars?

Lahars are an all too common phenomenon of stratovolcanoes. One recent lahar disaster has many frightening parallels to Mount Rainier. Like Rainier, Nevado del Ruiz in Colombia had glacial caps, high relief and a long history of lahar activity. On September 13, 1985, Nevado del Ruiz released a lahar triggered by a glacial outburst from a small eruption the previous day. This lahar traveled at a speed of 10-30 km/hr down the Azufrado River valley (Milet and others, 1991:12) and buried the town of Armero and approximately 24,000 of its residence before they knew what was happening (Milet and others, 1991:68).

What is a Lahar volcano?

Lahar (also called debris flow) is an Indonesian word used by geologists to describe a mudflow or a water-saturated debris flow on a volcano. Technically, any flow that is not saturated should be referred to as a debris avalanche; however, lahar is the term most often used to describe any type of debris or mud flow on a volcano. Lahars are powerful forces capable of moving great quantities of debris - house-size boulders, trees, etc. for long distances in a short amount of time. They look and behave like flowing concrete and destroy or incorporate virtually everything in their path. The result of lahars is usually a deposit of sediment that can range anywhere from a few yards to hundreds of yards thick (Hoblitt and others, 1995:4). Topography greatly influences the direction of lahars, so developed regions located at the base of volcano valleys are in extreme danger. In addition, any reservoirs in the path of lahars could add to the threat. The debris carried in the lahars could quickly fill the reservoirs and release the previously contained water (Hoblitt and others, 1995:4).

How do Lahars form?

Lahars develop from volcanic eruptions, glacial melting, and sector collapse but can also develop from earthquakes, steam explosions, and intense rainstorms (Hoblitt and others, 1995:4). All these possibilities exist in profusion on Mount Rainier’s steep slopes. Of all the risks associated with this volcano, the threat of lahars is certainly the greatest. Rainier’s history, as well as current eruptions elsewhere, can prove how destructive these lahars can be.

What is a lahar in geology?

For field geologists who need to interpret the origin of a layer of rock from from its field characteristics, a lahar may be defined as a debris flow composed of a significant component of volcanic materials (>25%) (Fisher and Schmincke, 1984), a descriptive definition that can be applied in the field from observations of deposits without requiring a judgement about synchroneity of volcanism. Many volcanologists prefer to define a lahar as caused by a volcanic eruption. But in ancient deposits, it is not always possible to determine if the lahar was caused directly by eruption or by remobilisation of volcanic rubble long after an eruption.

What are the three main types of Lahars?

Three major categories of lahars by origin are, (1) those formed by the direct and immediate result of eruptions through crater lakes, snow or ice, and heavy rains falling during or immediately after an eruption on abundant unstable loose material.

How is Lahar caused?

But in ancient deposits, it is not always possible to determine if the lahar was caused directly by eruption or by remobilisation of volcanic rubble long after an eruption. For geologists who study modern deposits, lahars may be defined in terms of visible characteristics of witnessed flows. The following definitions of lahar come ...

How do lahars form?

Common non-volcanic processes by which lahars and other debris flows form are by heavy rains falling upon loose debris or by loose debris becoming saturated with water from melting snow, glaciers or heavy rains (Osterkamp et al., 1986). Water-saturated material can move downhill like wet concrete when its internal strength is exceeded.

How does a lahar affect a flood?

During downstream movement in water-rich environments, lahars may progressively mix with water and transform to hyperconcentrated flood flows. Hyperconcentrated flood flows lack the strength and cohesion of lahars, but can carry high sediment loads with fragments supported both by turbulence and particle interactions (Pierson and Scott, 1985; Scott, 1988; Smith, 1986). Such floods can move as far as 250 km or more down valleys, and because of their high loads, will impact an entire river system such as occurred at Mount St. Helens in 1980.

Where was Lahar produced?

Approximately 5,600 years ago, an immense lahar was produced by Mount Rainier in the US state of Washington, which covered an area of 130 sq mi and had a volume of about 0.55 cu mi.

How big can a lahar be?

Large lahars can be up to 460 feet deep, covering up to 190 miles, and reaching speeds of up to 60 mph. With such enormous flow and speed, they are capable of destroying anything in their path. Examples of lahars include those experienced at Nevado del Ruiz and Mount Pinatubo. Approximately 5,600 years ago, an immense lahar was produced by Mount Rainier in the US state of Washington, which covered an area of 130 sq mi and had a volume of about 0.55 cu mi.

What is the purpose of the Lahar siren?

Lahar warning sirens have been established by the United States Geological Survey (USGS) in Pierce County, Washington, to notify people of imminent debris flows from Mount Rainier in case of eruption. Similarly, lahar warning systems have been created near Mount Ruapehu by the New Zealand Department of Conservation (DOC). In March 2007, officials were praised for alerting the public of an impending lahar. Warning systems have also been installed near Mount Pinatubo, which have been active since 1991 to monitor and warn against lahars.

Where did the Lahar volcano flow?

The lahar from the volcano flowed through the Abacan River and Sapang Balen Creek, causing destruction in Pampanga, Angeles City, as well as neighboring towns. The mudflow was about 20 ft high. Benjamin Elisha Sawe November 11 2019 in Environment.

Which mountain has the highest potential for lahars?

Numerous mountain areas around the world are likely to experience lahars. Certain areas have been noted to have the highest potential, including Mount Ruapehu (New Zealand), Mount Rainier (United States), and Galunggung and Merapi (Indonesia).

Can glaciers cause Lahars?

Therefore, glaciers and snow can melt during hot weather can cause lahars. Similarly, earthquakes that occur near a volcano can also cause materials to become loose and collapse, resulting in a lahar avalanche. Heavy rainfall also has the potential to cause lahars, especially on unconsolidated pyroclastic deposits.

What is the source of Lahar material?

Weak or easily erodible volcanic rock layers for source of lahar material

How much volcanic debris is in Lahars?

Dilute pure lahars, contain 20% to about 60% volcanic debris by volume

Lahars can occur with or without a volcanic eruption

Volcanic mudflows (lahars and debris flows) occur more commonly after a landscape has been covered by loose volcanic material. Sign on the slopes of Rabaul, Papua New Guinea.

Lahars and excess sediment cause serious economic and environmental damage to river valleys and flood plains

Large lahars can crush, abrade, bury, or carry away almost anything in their paths. Buildings and valuable land may be partially or completely buried.

Seasonal debris flows commonly occur on glacier-clad volcanoes

Bridge destroyed by lahar in North Fork Toutle River during eruption of Mount St. Helens, May 18,1980.

Research addresses lahar trigger mechanisms, flow dynamics, forecasting, and improved warning systems

To understand lahars, scientists observe and quantify what happens when these events occur naturally, derive equations to describe lahar behavior, and perform controlled experiments in a 310-foot (95-m) long debris flow flume.