What is a Lahar?

Mary McMahon
Mary McMahon

A lahar is a slurry of volcanic material and water. Despite the fact that lahars can be very thick, like concrete, they can move incredibly quickly, and they are often deadly. Because a lahar can occur on a volcano which is not actively erupting, it is a significant risk, since one may occur with minimal warning. Geologists have studied lahars in an attempt to be able to predict their path so that people can evacuate to safety quickly when it becomes apparent that a lahar is happening.

A volcano does not need to erupt in order to form a lahar.
A volcano does not need to erupt in order to form a lahar.

The word is borrowed from the Javanese language used in Indonesia. In Javanese, lahar means lava, or lava flow, and geologists adopted the term to refer specifically to flows of volcanic rock and water. In some regions of the world, the term “lahar” also refers to a dry landslide of volcanic material, which can be no less deadly. A lahar will always flow down slope, following the path of least resistance, which makes predicting the paths of potential lahars much easier.

The precise contents of a lahar vary, depending on the volcano. Typically, it contains pyroclastic material, meaning rocks and debris of volcanic origin, along with debris picked up as the lahar travels. The water may come from rain, snow, packed ice, or a diverted river, and a lahar can be cool or hot. Hot lahars are usually caused by active volcanoes melting packed snow, while cool lahars are caused by torrential rains which loosen enough material to form a landslide.

A lahar can build up quite a head of steam, making it rather devastating. Lahars have leveled and buried entire towns, along with neighboring forests and fields. When the lahar finally stops, it will dry and harden much like concrete, making the ground underneath useless until fresh dirt is deposited on top. A lahar will also leave large chunks of debris like uprooted trees and chunks of rock as it travels. These pieces can be used as clues to find historic lahars.

With an ever-growing human population, lahars have become a matter of grave concern. Many populations settle around volcanoes and former volcanic sites, since the soil is rich, making it ideal for farming. Unfortunately, bad weather could cause a lahar at any time, putting these populations at great risk. Since a lahar cannot be outrun, it may be difficult to get to safety, especially in a heavily congested area. Many people who live around active volcanoes are already aware of the inherent risk in their choice of living environment, but people who live on and near dormant volcanoes may not be aware of the risk of lahars.

Mary McMahon
Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a wiseGEEK researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

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Discussion Comments


I remember when Mount Pinatubo erupted back in 1991, there was a lot of damage caused by lahars. I was in middle school at the time, and my geology teacher spent some time showing us the news reports and giving us updates about it.

The lahars caused so much damage. A typhoon happened right around the time of the eruption, and all the rain made the ash wet and heavy. There were so many mudslides because of this, and houses got buried under hundreds of feet of lahar material.

It really scared me out of ever wanting to live in a tropical area filled with volcanoes. Many islands are actually made of volcanoes, and those are the ones I want to avoid.


@wavy58 - I have heard of lahars preserving trees from centuries ago, so I’m sure they could also preserve towns. It would be difficult to unearth them, but the way that scientists discovered the preserved trees was through another eruption.

Lahars were produced by an eruption hundreds of years after the trees were preserved, and these old trees could be seen floating by with the rocks. Some were in pieces because they had been ripped up in the beginning, but others were preserved upright.

I have always had an interest in volcanoes, and I’m fascinated by the way they can freeze a moment in time. It is tragic and sad, but it is also awe-inspiring.


Since a lahar will become as hard as concrete over time, is it possible to unearth a village that was buried by a lahar deposit, or is the material just to tough to penetrate without destroying the artifacts?

I know that towns and societies have been buried by volcanic ash, but were any buried by lahar? I guess the only way we could know that is if they were able to be excavated.

I have an awesome respect for volcanoes, but I’ve never wanted to visit an active one. I have no problem peering down into the mouth of a long extinct one, because I am sure that no ash, lahar, or lava will be taking me by surprise.


I would never settle anywhere near a place with volcanic activity! I think people who do this are either really brave or in denial of the risks.

There are other places with fertile soil. You don’t have to live near a volcano in order to grow a good garden.

Lahars are just one of the many dangers. I’ve always been more afraid of bright orange hot lava, but a landslide is no joke. Material that heavy could be just as destructive, but instead of melting or burning, you would simply be crushed or buried.

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