Geology And Geophysics

Mudslides and Debris Slides Called Lahars



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Lahar is an Indonesian word meaning mudflow or debris flow from the slope of a volcano.

According to the United States Geologic Survey a lahar is a mudflow and/or debris flow along the slope of a volcano. In order to be classified as a lahar the flow must contain a large content of volcanic rock. The rock content gives the flow the muscle needed to gain momentum and pick up and carry large boulders, topple trees and add them to the flow, and destroy buildings, bridges and other structures in its path. The lahar is an extremely destructive force.

Although it must originate from the slope of a volcano, it does not necessarily follow that it be connected to a volcanic eruption. Lahars may be triggered by volcanic activity, by landslides of wet debris such as volcanic rocks, trees and other debris. They may start due to heavy rainfall eroding volcanic debris, sudden melting of large amounts of snow or near a volcanic vent from a pyroclastic flow. They can also be triggered by a sudden break of a glacier, a crater lake, or the breaking of a volcanic dam.

As the lahar becomes more fluid larger debris and rocks will settle to the bottom of the flow creating what is called a hyper-concentrated steam flow. Lahars gain energy from the force of gravity as they flow.

Lahars may be very small consisting of a few centimeters or they may be massive flowing long distances. They have been known to wash over an entire valley consuming whatever is in its path. The speed of the flow may start slow and gain speeds of 87 kilometers per hour.

Examples of Lahars

One example of glacial outburst flood are those that happen on Mount Rainier in Washington State and Mount Hood in Oregon. The outburst floods on Mount Rainier happen when a sudden flow of water from the base of a glacier or inside glacial ice are released from melt or pressure. These sudden flows of water can pick up ash deposits, mud and rock as it flows downward.

Mount St. Helens eruption on May 18, 1980 caused one of the largest recorded landslide-debris avalanches of all times along the north flank. Some of the debris crossed and mixed with Spirit Lake, while the largest slide met with the North Fork of the Toutle River. The resulting combination of Lahar types from the mud, debris and pyroclastic material covered an area of 24 square miles. The mixture left deposits averaging 150 ft. totaling an immense volume of about 0.7 cubic miles.

The Nevado del Ruiz lahar of November 13, 1985 destroyed Armero, Columbia killing 23,000 people. Tremors and other disturbances from the volcano had been occurring since 1984. Within a year of the eruption and resulting lahars the USGS put together a team and portable observatory that could travel to any place in the world when a volcano showed signs of becoming active.

Lahars have been experienced along the slopes of volcanoes all over the world. They may develop due to volcanic activity or they may be caused by the loosening of debris, soil volcanic ash and rocks left by older volcanic eruptions. Thanks to the United States Geological Survey warnings of eruptions and the study of Lahars are making it possible to predict at least some of these destructive forces and their causes.

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