lahar

^{ALASKA VOLCANO OBSERVATORY GLOSSARY}

An Indonesian term for a debris flow containing angular clasts of volcanic material. For the purposes of this report, a lahar is any type of sedimentwater mixture originating on or from the volcano. Most lahars move rapidly down the slopes of a volcano as channelized flows and deliver large amounts of sediment to the rivers and streams that drain the volcano. The flow velocity of some lahars may be as high as 20 to 40 meters per second and sediment concentrations of greater than 750,000 parts per million are not uncommon. Large volume lahars can travel great distances if they have an appreciable clay content (greater than 3 to 5 percent), remain confined to a stream channel, and do not significantly gain sediment while losing water. Thus, they may affect areas many tens to hundreds of kilometers downstream from a volcano.

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^{GLOSSARY OF VOLCANIC TERMS}

The Indonesian term for a debris flow or a mudflow originating on a volcano (Harris, 2000, p. 1301). Lahars are generally composed of volcanic materials, but can contain significant amounts of non-volcanic materials derived from erosion during flow. Most volcanologists prefer this term to be used for the process and not the sedimentary deposits that it forms, but unfortunately, this distinction has been largely ignored in the geological literature. Many lahars are composed of sand and coarser materials, and thus, can be distinguished from "mudflows" which predominantly contain silt- or clay-sized grains (Rodolfo, 2000, p. 973).

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^{USGS PHOTO GLOSSARY OF VOLCANIC TERMS}

Lahar is an Indonesian word for a rapidly flowing mixture of rock debris and water that originates on the slopes of a volcano. Lahars are also referred to as volcanic mudflows or debris flows. They form in a variety of ways, chiefly by the rapid melting of snow and ice by pyroclastic flows, intense rainfall on loose volcanic rock deposits, breakout of a lake dammed by volcanic deposits, and as a consequence of debris avalanches.

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Photograph by J.N. Marso on 14 August 1989

A small lahar triggered by rainfall rushes down the Nima II River near the town of El Palmar in Guatemala in the image shown here. The lahar developed on the slopes of Santiaguito volcano.