Volcanic Hazards » Lahars
“Lahar” is an Indonesian word for a rapidly flowing mixture of rock debris and water originating on the slopes of a volcano. Lahars are also referred to as volcanic mudflows or debris flows (see http://volcanoes.usgs.gov/images/pglossary/lahar.php for more information). They form in a variety of ways:
- directly through the rapid melting of snow and ice by pyroclastic flows, or ejecting water from a crater lake (Mount St Helens or Eyjafjallajökull)
- directly by the breakout of a lake dammed by volcanic deposits or as consequence of debris avalanches
- Indirectly by intense rainfall during or after an eruption (Mount Pinatubo).
Mount St. Helens erupted often between 1980 and 1986. An explosive eruption on March 19, 1982, sent pumice and ash 9 miles (14 kilometers) into the air, and resulted in a lahar (the dark deposit on the snow) flowing from the crater into the North Fork Toutle River valley. Most of the flow went west down the Toutle River, eventually reaching the Cowlitz River, 50 miles (80 kilometers) downstream. (U.S. Geological Survey)
Lahars can:
- Destroy by direct impact
- Lead to increased deposition of sediment
- Block tributary streams
- Bury valleys and communities with debris