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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.
Introduction. Lahars are mudflows that originate on the slopes of a volcano. The mudflows contain so much rock debris they look and feel like flowing wet concrete. They are triggered by a variety of events including pyroclastic flows, rainstorms and the collapse of debris dams.he mudflow began when a small pyroclastic occurred on Nevada del Ruiz, a volcano about 30 miles away. The mudflow was 9 to 15 feet high when it struck the village late at night as the people in the village sleeping. Twenty-two thousand people were killed by the lahar.