ABSTRACT
Any geomorphic process of sufficient magnitude that occurs suddenly and without warning is a danger to humans. Landslides, debris flows, rockfalls, and many other mass movements associated with hillslopes take their toll on human life. Most textbooks on geomorphology catalogue such disasters. A typical case is the Mount Huascarán debris avalanches. At 6,768 m, Mount Huascarán is Peru’s highest mountain. Its peaks are snow-and ice-covered. In 1962, some 2,000,000 m3 of ice avalanched from the mountain slopes and mixed with mud and water. The resulting debris avalanche, estimated to have had a volume of 10,000,000 m3, rushed down the Rio Shacsha valley at 100 km/hr carrying boulders weighing up to 2,000 tonnes. It killed 4,000 people, mainly in the town of Ranrahirca. Eight years later, on 31 May 1970, an earthquake of about magnitude 7.7 on the Richter scale, whose epicentre lay 30 km off the Peruvian coast where
the Nazca plate is being subducted, released another massive debris avalanche that started as a sliding mass about 1 km wide and 1.5 km long. The avalanche swept about 18 km to the village of Yungay at up to 320 km/hr, picking up glacial deposits en route where it crossed a glacial moraine. It bore boulders the size of houses. By the time it reached Yungay, it had picked up enough fine sediment and water to become a mudflow consisting of 50-100 million tonnes of water, mud, and rocks with a 1-km-wide front. Yungay and Ranrahirca were buried. Some 1,800 people died in Yungay and 17,000 in Ranrahirca.
