Lahars...

I'd get away from that volcano if I were you.

Mother Nature's Trump Card

So...What are Lahars?

Lahars are semi-liquid mixtures of water, mud, and volcanic ash, and rock fragments all mixed together hurtling down a volcano's slope towards you at somewhere from a few to tens of meters per second. Lahars can be hot or cold and they can happen for quite a while after volcanos have stopped erupting. They can also be hundreds of meters wide, up to 140 meters deep, and carve their own path instead of merely following the path of normal water in its particular river basin, making them incredibly hard to predict.

Image Credit: https://volcanoes.usgs.gov/images/pglossary/lahar.php

Why should we worry?

Lahars are incredibly dangerous. When they are particularly large and hot, they can move faster than people can run, knock over or crush buildings, and/or simply envelop you in what is basically scorching hot, runny cement.The biggest lahars have been known to travel at up to 100 kilometers an hour for three hundred kilometers. These monsters of nature not only can kill thousands of people at a time, but they can literally wipe communities off the map. Lahars can have more long-reaching effects as well. When lahars lose energy and solidify, they can block the paths of rivers, creating droughts downstream and flooding upstream. This could also lead to plants, animals, and possibly people dying if they can't get enough or have too much water. In North Carolina, Lahars don't threaten us. However, their environmental effects and sediment deposition can, and obviously, the loss of life due to lahars is a terrible thing.
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The Very Muddy "Meat and Potatoes"

Where should we worry?

Lahars only form near deposits of volcanic ash, which generally means that they only form near volcanoes. Some volcanic mountains that have been deemed to have a special risk of lahar occurrence are Mount Rainier in Washington State U.S.A, Mount Ruapehu in New Zealand, and Galunggung in Indonesia. There have also been other severe lahars in Nevado del Ruiz, Colombia, and Mount Pinatubo in the Philippines, to name a few.

How do you know a Lahar is bad for you?

In general, unless a lahar is very small, they have the potential to kill people and carry away buildings. However, they are more dangerous when they are faster and/or larger (in depth and width). So, if you know how large the lahar is (or is estimated to be, because if lahars absorb more material they can become more than 10 times larger than they originally were) and how fast it is going/will be going, you can estimate how many people it could kill, how many buildings it is going to carry away, and the area that it is going to cover once it dries up. Lahars do not have a scale. This is why the destructive power of lahars is measured as two separate variables.

When and Where have Lahars happened?

Here are a few examples of when lahars have happened in the past--and these haven't been pretty.

1.) November 13th, 1985: Armero, Colombia. Earlier that day, for the first time in 69 years, the Nevado Del Ruiz volcano erupted. The eruption, however, was only the beginning. Heated Gas and rock moving down the mountain melted the glaciers that rested there, mixing with the water to form gigantic lahars that flowed down the mountain at 50-60 kilometers per hour. In all, the lahars spread out 74 kilometers from the mountain, killing around 23,000 people on the way.

2.) June 15th, 1991: Mount Pinatubo, Philippines. Mount Pinatubo had just erupted at the same time that a typhoon was going overhead. The resulting deluge and subsequent mixture of the stormwater with volcanic material lead to lahars that killed over 1,500 people and damaged several river systems to the point that they are so saturated with sediment that it will take them years to repair. Post-eruption lahars have displaced 50,000 people and buried 400 square kilometers of land through sediment deposition.

Lahars have also happened at Mt. St. Helens in 1980, the Santa Maria Volcano yearly, Galunggung in 1982, and Mount Ruapehu in 2007.

How do we save ourselves from certain muddy doom?

There is no way to prevent lahars from happening. As long as precipitation still falls and volcanoes still erupt, there will be lahars. However, there are a few methods you can use to either get away from a lahar or stop it in its tracks. Those methods are currently limited to an early warning system that consists of manned watchtowers, rain gauges that transmit their data via radio telemetry, and acoustic flow monitors that can detect the rumble of the lahars as they go by. Also, if you have no other way, you can always build a dike or a dam to hold the flow back, like they did in the Philippines after the 1991 eruption of Mount Pinatubo.

Image Credit: https://commons.wikimedia.org/wiki/File:3882jfDolores_Bacolor_Pampanga_River_Megadike_Roadsfvf_02.JPG

Here are the four basic methods of stopping or detecting a lahar such that you have enough time to get away:


  • Acoustic flow monitoring that detects the rumbling of the ground that lahars cause as they go by.
  • Manned Watchtowers
  • Radio Telemetry Rain Gauges that measure rainfall in areas susceptible to lahars so that scientists know whether or not to worry about the potential for lahars.
  • Building a large dam/dike that stops the flow of the lahar.

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