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Lahars move rapidly down valleys like rivers of concrete.

If you are on the floor of a valley that leads from a volcano and become aware of an oncoming lahar, seek high ground.

Lahar is an Indonesian term that describes a hot or cold mixture of water and rock fragments that flows down the slopes of a volcano and typically enters a river valley. Small seasonal events are sometimes referred to as "debris flows", especially in the Cascades. Lahars generally occur on or near stratovolcanoes, such as those of the Aleutian volcanic arc in Alaska and the Cascade Range in the Western U.S.

A moving lahar looks like a roiling slurry of wet concrete, and as it rushes downstream, the size, speed, and amount of material carried can constantly change. The initial flow may be relatively small, but a lahar may grow in volume as it entrains and incorporates anything in its path – rocks, soil, vegetation, and even buildings and bridges. The flowing slurry may consume additional water through melting of snow and ice or by engulfing river or lake water. Voluminous lahars commonly grow to more than 10 times their initial size as they move downslope. In steep areas, lahars can exceed speeds of 200 km/hr (120 mi/hr), but as they move farther away from a volcano and decelerate in lowland areas, they eventually begin to deposit some of the load and decrease in size.

Lahars can occur with or without a volcanic eruption.

Eruptions may trigger lahars by melting snow and ice or by ejecting water from a crater lake. Pyroclastic flows can generate lahars when extremely hot, flowing rock debris erodes, mixes with, and melts snow and ice as it travel rapidly down steep slopes.

Lahars can also be formed when high-volume or long-duration rainfall occurs during or after an eruption. On steep slopes, rainwater can easily erode and transport fine-grained, loose volcanic sediment and form a slurry, especially if vegetation has not had time to grow back on recent volcanic deposits.

Some of the largest lahars begin as landslides of wet, hydrothermally altered rock on the steep flanks of volcanoes. These types of collapse and resultant lahars are natural events during a stratovolcano's life history and can occur long after it stops erupting.

Lake breakout floods that occur without an eruption can also lead to lahars. They commonly occur after a stream becomes blocked by a volcanic landslide or pyroclastic flow that forms a natural dam. The most frequent cause of a lake breakout is the overflow of water across a newly formed natural dam, followed by rapid erosion of the loose rock debris. By further erosion and entrainment of sediment and water, the initial flood can transform into a slurry and increase in volume as it races downvalley.

Lahars and excess sediment cause serious economic and environmental damage to river valleys and flood plains.

Large lahars can crush, abrade, bury, or carry away almost anything in their paths. Buildings and valuable land may be partially or completely buried. By destroying bridges and roads, lahars can also trap people in areas vulnerable to other hazardous volcanic activity, especially if the lahars leave fresh deposits that are too deep, too soft, or too hot to cross.

Over a period of weeks to years after a volcanic eruption, the erosion and transportation of loose volcanic deposits can lead to severe flooding in areas far downstream from a volcano. Intense rainfall easily erodes loose sediment on steep slopes to produce lahars that travel onto flood plains and bury entire towns and valuable agricultural land. These rainfall-induced lahars can wreak havoc on rivers and streams, sometimes depositing so much sediment that chronic flooding also becomes a problem.

Seasonal debris flows commonly occur on glacier-clad volcanoes.

Ice-clad volcanoes are common locations for the initiation of small debris flows, owing to their steep slopes, abundant loose rocks and surface water, and diurnal, seasonal, and long-term variability of surface water flow. These events have some seasonality—one population of events occurs after the snowpack has diminished during mid-to late summer, and the other is triggered by early-season intense rains of winter before the snowpack has accumulated. The Cascades Volcano Observatory offers additional information about these events.

Research addresses lahar trigger mechanisms, flow dynamics, forecasting, and improved warning systems.

To understand lahars, scientists observe and quantify what happens when these events occur naturally, derive equations to describe lahar behavior, and perform controlled experiments in a 310-foot (95-m) long debris flow flume. The results help us to understand flow dynamics and develop improved technologies for mitigating the destructive effects of lahars and other debris flows.

Because of the danger presented by lahars, scientists are ready to deploy lahar-detection systems when eruptions are imminent. Officials near Mount Rainier in Washington State maintain a permanent lahar-detection system and accompanying public notification system.

Find out more about lahar safety in the .
Additional information about lahars