What is a lahar? 


Lahar is an Indonesian word meaning "mudflow."

A lahar is a form of a mudslide, triggered most often by a volcanic eruption, which flows down the mountain.  They are also called “debris avalanches.” As the lahar travels down the mountain, other debris such as rocks, logs, and trees become part of the fast moving mudslide.  Secondary lahars, forming after the volcanic eruption, also include volcanic ash.  There are two types of lahar flows:  turbulent and laminar flow.  Turbulent lahars occur when there is 20 to 60% sediment in the lahar.  Laminar flow lahars have more than 80% of sediment, and they flow very smoothly, but also move much faster.  They can even float cars, houses and bridges!

A volcano does not always have to erupt for a lahar to form.  Sometimes heavy rains falling on old, loose volcanic ash or a crater dam failure can also cause a lahar.  Even with a small volcanic eruption, a large lahar can travel quickly down a mountain, up to 100 km (62 miles)/hour. 

These lahars are dangerous to people and property when they flow (via gravity) into valley areas near the volcanic mountains. As lahars quickly and silently travel to the valleys, they can cause severe damage, crushing houses and even destroying entire towns.  If people cannot evacuate fast enough, then they will also be killed by the lahar. 

There have been many life-destroying lahars in recent history.  In 1985, a lahar in Nevado del Ruiz, Columbia destroyed the entire city, killing 25,000 people.  In 1991, the Filipino volcano Mount Pinatubo erupted, with lahars formed within 30 minutes of volcanic eruption.  Over 500 lives were lost.  A major lahar also happened in the United States in 1980 at Mt. Saint Helens in Oregon, killing 57 people. In 2002, a lahar from the African Mount Nyiragongo volcanic eruption killed at least 75 people.  While there has not been a recent lahar from Mt. Rainier in Washington State, over 200,000 lives are at risk from a future Mt. Rainier lahar.

       Mt. St. Helen's lahar 1980      Lahar damage, Columbia 1985