Geology And Geophysics

What is a Lahar



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It is a common misconception that people are killed by lava and ash as a result of a volcanic eruption. In fact, lava is made up of liquid rock that has been superheated to a temperature of up to 1250 degrees centigrade, and is a relatively slow moving and an easily avoided hazard.

The biggest killer as a result of volcanic activity does not always accompany an eruption and can be a devastating force.

It can travel at over 100 miles per hour, moves in any direction over land and sea, rips up trees, demolishes buildings, and kills whole communities in one terrible fell swoop.

The deadly force in question is known as a lahar, which is Indonesian for a"volcanic mudflow" It is specifically applied to the deadly, water saturated volcanic debris, (known as pyroclastic debris) that flows from a volcano and follows the line of a river valley.

There are two types of lahars known as cohesive and non-cohesive. Cohesive lahars have at least three to five percent of clay content which inhibits the loss of material, rather bonding it together and allowing a constancy in texture and flow over a greater distance. A non cohesive lahar typically contains about one to five percent of clay which will become diluted as it deposits sand and rock - ultimately outrunning the sediment.

Looking very much like wet concrete, the mixture of water, rock debris, sand and silt that cascades down the side of a volcano, can travel huge distances.

The eruption of Cotopaxi in Equador in 1877 produced a flow that travelled more than 320 km down the valley at an average speed of 27km per hour (Macdonald, 1972).

Sometimes fast moving lahars will surge over the top of valley walls when mounting a bend, often traversing any obstacles encountered. Lahars pushing down into narrow or blocked valleys, can thicken and fill the space up to a height of 100m or more. (Crandell, 1971).

The Whangaehu valley in New Zealand has one of the most active lahar channels in the world and the people living there know only too well never to underestimate the power and speed of this indomitable force, which destroyed the Tangiwai Rail Bridge in 1953 - at the precise moment a train was also crossing!

Because of the speed that lahar can travel, monitoring equipment is now used 14km upstream of the bridge to ensure that advance warning can be adequately given if it ever happens again.

Sometimes Lahars will occur with no warning whatsoever. For example, in 1998 at the Casita volcano in Nicaragua, over 2,500 lost their lives when 200,000 cubic metres of rock and loose soil on a steep section of the volcano, collapsed with no warning.

Within a few seconds, the lahar roared towards the towns of El Porvenir and Rolando Rodriguez, accompanied by a sound of thunder and earthquake like ground tremors. Within three minutes both towns were totally covered with mud and only a handful of people survived to tell the tale.

Lahars can vary in temperature, depending on how hot or cold the rocks are that it carries. The maximum temperature reached is 100 degrees Centigrade, or the boiling point of water. The material is fast flowing in its initial stage, and can set like concrete when it cools.

Its initial flow can erode rocks and vegetation from the side of a volcano and along the river valley it enters. It can also gather up water from melting snow and ice and the river it plunges into. For this reason lahars can easily grow up to 10 times in size, but as it moves away from the volcano it will begin to shed its heavy sediment load, and it's bulk will decrease.

Although eruptions can trigger lahars by melting the snow and ice or ejecting water from a crater lake. Lahars are mostly initiated by heavy rainfall during or after an eruption. Rainwater can easily weaken loosen volcanic rock and soil on hillsides and coupled with precipitation, landslides are easily triggered by a combination of events such as eruptions, earthquakes, or gravitational forces.

Lahars almost always occur on or near volcanoes that erupt explosively. This has the effect of creating tall, steep cones that are either snow covered or hold a crater lake and is made up of weak consolidated rock debris that is easily weakened by hot hydrothermal fluids.

One of the most common places to find lahars is in in Iceland where eruptions of fluid basalt lava frequently occur beneath huge glaciers.

When looking at measures to protect life and property in future occurrences, a particular reference is made to case studies occurring in Indonesia and New Zealand.

Measures that bear looking at include siphoning water from the crater lake, effective warning systems, and maps based on geological, pedological and botanical study which help to show regions that may be in potential danger.

Although it may be impossible to predict with any accuracy, when these disasters will occur, by learning all we can from the occurrence of recent lahar activity, will help to give invaluable information to people, so that they can be better informed and can make positive steps in ensuring an adequate contingency plan for the future to ensure their safety and safe stowage of personal belongings.

Sources: The Global Geospatial Magazine, October 2007. Lidar in Lahar Mapping.

Bulletin of Engineering Geology and the Environment. Lahars as major geological hazards Volume 13, Number 1, June 1976.Springer Berlin / Heidelberg

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