Lahar

A lahar /ˈlɑːhɑr/ is a type of mudflow or debris flow composed of a slurry of pyroclastic material, rocky debris, and water. The material flows down from a volcano, typically along a river valley. Lahars are extremely destructive: they can flow tens of meters per second, be 140 metres (460 ft) deep, and destroy any structures in their path. Notable lahars include those at Nevado del Ruiz and Mount Pinatubo, each of which killed thousands of people.

• http://en.wikipedia.org/wiki/Lahar