A lahar is a mudflow/landslide that typically occurs after a major volcanic eruption. It is a slurry of pyroclastic material and water that ravages down from a volcano, usually along a river valley. In many cases, lahar is even more destructive than the actual volcanic eruption.
The origins of the term
The term “lahar” is borrowed from the Javanese language, which literally means “lava” or “lava flow”. In Hindi, the term means “wave.” In geology, the term specifically refers to surge of volcanic rock and water. The term is alo adopted to refer to a dry landslide of volcanic material, which is as destructive and deadly.
Causes of lahar
Lahars have a number of possible causes. A flood due to heavy rainfall or lake breakout can release a lahar. Glaciers and snow melted as a result of a pyroclastic flow during a volcanic eruption can also trigger lahar. In particular, while lahars are usually associated with the aftermaths of volcanic activities, this natural phenomenon can happen even without current volcanic activities. Lahars can occur as long as there are geologic conditions that result in the movement and collapse of mud from existing ash deposits.
Characteristics
Lahars flow down slope, following the trail of least resistance. This makes forecasting potential lahar paths much easier. Geologists have been studying the behavior of lahars to predict the path of the flow so that people can be evacuated to safe places quickly when there is impending lahar.
A lahar has the consistency of a concrete. It is fluid when moving, but solid when stopped. Although lahars are very thick, they can move with incredible speed, often destroying anything in its path. Some lahars move at more than 40 mph, easily outpacing those who try to outrun them.
Lahars can be huge. For example, the Osceola lahar deposited some 5,600 years ago by Mount Rainier made a wall of mud 1460 feet (40 meters) deep in the White River canyon. It extended over an area of more than 130 square miles (330 square kilometers) and had a total volume of 0.55 cubic miles (2.3 cubic kilometers).
Composition
The precise composition of lahars depends on the volcano. Lahar typically contains pyroclastic materials (the rocks and debris from the volcano) and water (rain, packed ice, snow, or a diverted river). Along with the volcanic materials and water are debris picked up as it travels.
Destructive nature
Lahars can be very destructive and fatal. The 1985 Nevado del Ruiz eruption in Colombia caused lahars that killed around 23,000 people. Lahars in the 1991 eruption of Mout Pinatubo in the Philippines killed 1500 people. In 1953, New Zealand’s Tangiwai disaster killed 151 people.
