Lahars are one of the most dangerous by-products of a volcanic eruption due to the speed with which they can travel. A lahar is a mixture of pyroclastic material and rocks suspended in water and can flow like a mudslide. They can travel as fast as 50 miles per hour. The largest lahar flow known to geologists buried the ancient White River Canyon in 460 feet of volcanic mud.
For a lahar to form, a volcanic eruption needs to occur near a large body of water or other source of moisture to create the slurry. Melting snow or glaciers can provide the water for the lahar, or a pyroclastic flow may travel into a lake or other body of water. In some cases, torrential rainfall occurring near a volcanic eruption may provide enough water to trigger a lahar.
Mount Ranier in the United States, Mount Pinatubo in the Philippines and Mount Ruapehu in New Zealand are volcanoes that have produced lahars in the past due to their geographic features. In 1985 the volcano Nevado del Ruiz in Colombia set off a major lahar, killing 23,000 people. The town of Armero lost 20,000 of its 29.000 inhabitants as it was overwhelmed by the fast-moving flow of water and volcanic material.