Pyroclastic material is dispersed by a volcanic eruption and, if combined with large amounts of rain, it can create a viscous liquid that combines with mud and creates a flow of debris down the side of a volcano. Lahars are similar to avalanches in that they start when the material is disturbed and they move very fast, destroying or carrying away anything in their path. Earthquakes that occur close to volcanoes can also trigger lahars, because loose volcanic material can be dislodged, creating a collapse of material.
Lahars are considered dangerous because they can devastate any settlements that are unlucky enough to be in their path. They range in size, some being relatively small and others enormous. After the 1985 eruption of a volcano called Nevado del Ruiz in Mexico, a lahar decimated the nearby city of Amero. When the lahar stopped, the city was buried beneath 16 feet of lahar deposits and about 23,000 people were killed.
Lahar deposits consist largely of volcanic materials, rocks and mud. However, lahars will pick up and drag objects that are in their path, so often objects like trees will be present in lahar deposits. A moving lahar has the consistency of concrete before it has set; it is a thick, coarse liquid. When the lahar slows down and eventually stops, the mixture of mud, rocks and other pyroclastic materials rapidly hardens until the lahar is solid. Once the lahar has hardened, everything it contains becomes encapsulated within the concrete-like material.
Lahar deposits can be found close to many volcanoes around the world. Indonesia has numerous volcanoes, and volcanic mud flows that happened hundreds of years ago have left lahar deposits nearby. In Washington state, towns in the Puyallup River Valley are built atop 500-year-old lahar deposits originating at Mount Rainier. At Popocatepetl volcano in Mexico, recent lahar deposits from 1997 and 2001 have been studied after plants were found growing from them.