What is a lahar?

A volcanic eruption can be one of the most astonishing natural phenomenons in the world, but also one of the most dangerous due to a variety of hazards.

Volcanic ash that is spewed thousands of miles into the air can pose a serious threat to air traffic and air quality, volcanic gas can have an impact on health, vegetation and infrastructure, and lava flows are slow-moving forces of destruction that can wipe away anything in their path.

Another one of the main threats that can arise from a volcanic eruption is a type of powerful mudflow called a lahar.

A lahar is an Indonesian term that describes a mixture of water and rock fragments that flow down the slopes of a volcano, according to the United States Geological Survey (USGS).

Lahars can be considered a cross between a landslide and mudslide, said Tom Pierson, a research hydrologist for the USGS.

Due to the mixture of water and rocks, lahars can be denser than water alone, have greater erosive power and can sometimes transport enormous boulders, according to AccuWeather Senior Meteorologist Jim Andrews.

The viscosity of these mudflows has often been compared to wet concrete and they can also be hot or cold, according to Pierson.

"The hot lahars are a little less common," Pierson said. "But whenever fresh magma is involved and mixes with water, a lot of that heat stays in the flow. They stay hot and steam for quite a long distance."

In November 2017, over 100,000 people were ordered to evacuate from the Indonesian island of Bali as Mount Agung started to erupt. One of the chief reasons was the threat of lahars, since the eruption occurred during Bali’s wet season, according to the Associated Press.

Volcanic mudflows also forced more than 75,000 residents in the Philippines to evacuate when Mount Mayon erupted in January, Reuters reported.

Lahars can form in several ways and don’t always occur during an eruption.

They can develop when pyroclastic flows melt existing layers of snow or ice such as on ice-clad volcanoes like Mount Rainier or Mount St. Helens in the Cascade mountain range.

However, a lahar can still develop even after an eruption occurs such as when heavy rainfall falls for a long period of time and erodes loose volcanic sediment, according to the USGS.

"Most volcanoes are characterized by steep slopes, often with deposits of loosely consolidated volcanic material (lava, ash, tephra/cinders), which are prone to fail under the relentless pull of gravity," Andrews said. He added that older rocks making up a volcano can also rot from internal heat, steam and volcanic gases to form clays. This allows older slopes to become increasingly weak and slide-prone with time.

Even areas that might seem far away from a volcano are not always typically safe from a lahar. In some of the notable cases in history, destructive lahars can travel dozens of miles and have reached speeds of over 50-60 mph when the slopes of a volcano are steep.

According to the USGS, past lahars at Mount St. Helens in Washington state have traveled anywhere from 30-60 miles and are said to be the greatest volcanic threat to valley communities.

There are warning systems in place to monitor for sudden development of lahars. The USGS utilizes solar-powered acoustic flow monitors (AFM) that are installed in rivers that are prone to lahars. These AFMs are specifically designed to detect ground movements from a lahar. The data is then transmitted to a volcano observatory where information can be analyzed and warning notifications can be disseminated.

While lahars are not a hazard that occur often, when they do strike they can be devastating, Pierson said.

"There are engineering methods that some governments and agencies can use to try to slow down lahars or redirect them away from valuable property and populated areas. But in general the best solution is for people to be aware of hazard zones around volcanoes," he said.

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