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.
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.