A lahar is a type of mudflow composed of pyroclastic material and water that flows down from a volcano, along a river valley. The word lahar is from the Javanese language of Indonesia.
Lahar is like concrete, its wet and muddy when it’s moving and hardens when stopped. Lahars can be huge. The Osceola lahar produced 5,600 years ago by Mount Rainier in Washington produced a ton of mud 460 ft deep in the White River canyon and extends over an area of over 130 sq mi for a total volume of 0.55 cubic miles.
Lahars usually travel down valleys. They have a wide range of velocities from 1 m/s to 40 m/s. The speed of a lahar depends on the channel width, channel slope, volume of the flow, and grain size composition. Lahars can travel long distances. Some lahars have traveled hundreds of kilometers from their source. The deposits of a lahar that traveled 60 km from its source at Mount Rainier can be found near the large city of Seattle, Washington. The lahar's origin at Mount Rainier helped make that volcano a decade volcano.
Lahars have been known to transport very large boulders. At Mount Pinatubo, boulders measuring 1.5 m long were not uncommon in lahar deposits. The lahars from Nevado del Ruiz transported a boulder that was 208 cubic meters, 300 m downstream.
When a lahar travels down valley, the high point of the lahar is usually marked by the mudline it leaves on trees, valley walls, and buildings. This mudline marks the upper limit of how high a lahar will go. This upper limit is important because it defines how high people must go to be out of danger from the lahar. The small eruption of Nevado del Ruiz in 1987 produced large lahars that destroyed the city of Armero. Unfortunately, the 30,000 people who lost their lives might have been saved had they established an appropriate line of communication and evacuated to higher ground. If you live in the mountains you should be safe but if the mountain is volcanic I can’t believe you still live there.