Lahars are mud flows resulting from volcanic activity, and they can pose the deadliest threat of all arising from a volcanic eruption.

What are Lahars?

Lahars are a type of muddy debris-laden stream that flows rapidly down the slopes of a volcano and into river valleys. (The word comes from an Indonesian word that can also mean what we call lava). They are formed when there is volcanic activity in the presence of a lot of water. They are often produced if there is heavy rainfall or storm during or just after a volcanic eruption, and they can also be produced if the volcano is covered in ice or snow, such as a glacier, or if there is a lake crater at the top of the volcano. They can also be caused by landslides.

As the lahar forms, water mixes with the volcanic ash and debris and results in a fast moving, deadly stream, which may or may not be hot. As the lahar travels down the side of the volcano it can become much bigger as more debris and water are collected by the stream. Any snow or ice is melted and incorporated, and if the lahar reaches a river it can overrun the river altogether. As it continues on its course a lahar will eventually slow and gradually lose its load of debris and sediments, but before it reaches this stage it can wipe out virtually anything in its path.

Lahars can be extremely fast moving, and can contain hot ash, rock fragments and debris. Their speed makes them particularly deadly because they can be literally impossible to out-run. A large lahar running down a steep slope will be much faster than a small lahar running down a gentle slope. The fastest lahars can reach speeds of tens of yards per second and can be hundreds of yards wide.

Examples of Lahars

One famous example of a deadly lahar was that produced after the eruption of Mount Pinatubo in the Philippines in 1991. In this case the volcanic eruption occurred during a hurricane. The resulting lahar of hot mud, ash and debris, buried about 700 people and destroyed most of the buildings in its path. Secondary lahars are still occurring there today when rainfall mixes with deposits of ash.

An even more disastrous lahar occurred during the eruption of the Nevado del Ruiz volcano in Columbia in 1985. This volcano was covered in snow and ice and the eruption coincided with a storm. During the eruption the mixture of lava and hot toxic gases formed what is known as a pyroclastic flow, which is like a hot avalanche flowing down the side of the volcano. This lahar sped down the mountain at speeds of up to 60 miles per hour. Mixing with the snow, ice and rain, the pyroclastic flow became a deadly lahar that raced down the river valley destroying the town of Armero and killing three quarters of its inhabitants. Over 23,000 people were killed in Armero and in villages up to six miles from the foot of the volcano. This lahar was powerful enough to sweep away bridges and burst a dam, and it filled an entire ground floor of a hospital with mud.

The largest lahars of all have occurred in Iceland, where volcanos and glaciers form a common, and violent, combination. Fortunately, the population density is low and these lahars have not claimed as many lives as those in more populous areas, where lahars are one of the most deadly features of volcanic eruptions.

In the public imagination, lava is most often thought of as the greatest danger, and the sight of molten lava flowing down the side of a volcano is a terrifying one. Lahars may not strike terror into the imagination in the same way, but a fast-moving lahar of debris-laden mud is far more deadly and kills far more people than lava flows.