Lahars are unconsolidated sediments left when pyroclastic material (primarily volcanic ash) and water flows down from a volcano. They are distinct from a pyroclastic flow of hot ash by the presence of a significant amount of water. Note that a lahar does not need to be produced by an active volcanic eruption. For example, an eruption can produce a large amount of ash, blocking an outflow channel from the crater. Over time, the crater can fill with water from accumulated rainfall, and the lahar can form when the blockage is abruptly released.

Some lahars are huge. Mount Rainier in Washington produced a lahar about 5600 years ago that is nearly 500 feet thick and covers over 100 square miles, with a total volume of just over 1/2 cubic mile.

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