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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.
Turbulent lahars occur when there is 20 to 60% sediment in the lahar. Laminar flow lahars have more than 80% of sediment, and they flow very smoothly, but also move much faster. They can even float cars, houses and bridges! A volcano does not always have to erupt for a lahar to form.Sometimes heavy rains falling on old, loose volcanic ash or a crater dam failure can also cause a lahar. Even with a small volcanic eruption, a large lahar can travel quickly down a mountain, up to 100 km (62 miles)/hour.s the lahar travels down the mountain, other debris such as rocks, logs, and trees become part of the fast moving mudslide. Secondary lahars, forming after the volcanic eruption, also include volcanic ash. There are two types of lahar flows: turbulent and laminar flow.