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7. Mass Wasting Outline 7.1 Overview 7.2 Types of Mass Wasting 7.3 Controlling Factors 7.1 Overview Earth’s surface is marked in general by hills and valleys, the slopes of which vary from gently inclined to precipitous. Slopes can be mantled with a thick cover of regolith and trees or they can be bare bedrock. Regardless, the presence of slopes is the norm and it is difficult to find a spot where one can stand and not see at least a small slope or two. Although these slopes appear to be stable to the naked eye, in fact many are not. Those that are not stable may experience imperceptible downslope movements that can only be measured over years or decades, or they may experience sudden movements that sweep the surface cover away in a roaring debris flow or rock slide. All of these movements, whether extremely slow or very fast, are called mass movements or mass wasting. The term “landslide” has a similar connotation, but it is a general term with no precise geologic meaning and we will avoid its use in MINE2200. Mass wasting is just one of the many types of erosion that contributes to the development of landforms. As land rises and weathering occurs, the bedrock weakens and regolith gradually forms. The spectacular cliffs, peaks and valleys of mountain ranges result from the combination of uplift and erosion – uplift creates differences in elevation and mass wasting transfers rock and debris downslope to the valleys floors where streams and alpine glaciers can carry the detritus away to lower elevations. Although there may be many intermediate resting places along the way, the sediment is transported eventually to its ultimate destination, the sea. This journey can take millions of years. One of the more devastating mass wasting events in recorded history is the Yungay slide that occurred in Peru in 1970. A strong offshore earthquake struck on May 31, and the vibrations caused a huge mass of ice and rock near the summit of 6700 m Nevado Huascaran to break free (Figure 7.1). The ice and rock fell about 1 km before it hit the lower slopes of the mountain. The material pulverized on impact and began to flow quickly downward, fluidized by melted ice and trapped air. As the debris roared downward along a previously eroded gorge, more and more rock and mud were picked up. When the rapidly flowing material reached a bend in the valley below the mountain, a portion of the mass jumped a 200-300 m high bedrock ridge above Yungay, burying the entire town in an instant. The smaller village of Ranrahica further downslope was also destroyed before the material reached the main valley floor and came to a stop on the other side. The debris had a total final volume of 50-100 million m 3 and travelled more than 16 km from its source, over a vertical drop of 4 km. The average speed was 280 km/hr, with a peak speed of as much as 1000 km/hr. About 18,000 people were killed, including nearly every one of the 14,000 inhabitants of Yungay. Geological studies after the 1970 disaster showed that Yungay had been built on the debris of previous mass movements, one of which might have been even bigger than the 1970 event.
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