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Chapter 10: Mass Movement Mass movement • Loose, weathered material (regolith) • Moves downslope under the influence of gravity • Movement can be either: slow – soil creep or rapid – landslides, bog bursts, mudflows, avalanches Chapter 10: Mass Movement Factors affecting mass movement • Slope (gradient) of the land surface • Water content • Human activity • Plants and trees • Tectonic activity Chapter 10: Mass Movement Mass movement types • Soil creep • Slump-rotational slide • Earthflow • Rock fall • Debris avalanche • Mudflow • Bog burst • Avalanche • Landslide Chapter 10: Mass Movement Processes of mass movement 1. Soil creep • Very slow movement • Underlying rock becomes heavily saturated following periods of heavy rainfall • Water can no longer pass through the rock • Soil particles expand and later contract when they dry out • Causes soil particles to creep slowly downhill under the influence of gravity Chapter 10: Mass Movement 2. Slump-rotational slide • Loosely consolidated rock layers moves down slope • Influence of gravity • Moderate speed • May be an isolated occurrence or they may occur in large areas • Often associated with areas which have been affected by human activities • E.g. during the construction of roads where the land has been too steepened and has been left unstable • Also linked to coasts and river banks where, due to erosion, slopes have been undercut • Also associated with landscapes following heavy rain Chapter 10: Mass Movement 3. Earthflow • Water saturated material (fine sand, silt and clay) flows downslope • Influence of gravity • Moderate speed • Generally occur on hilly slopes • Associated with heavy rain • Speed at which they travel varies from several cm per year to hundreds of metres per day • More common in humid areas • Tend to have a lower water content than mudflows Chapter 10: Mass Movement 4. Rockfall • Rocks fall downslope • Influence of gravity • Rapid speed • Occur following periods of heavy rain and frost weathering • Strong earthquakes may also cause rock falls to occur. • Caused by either a biological or a climatic event changing rock stability Chapter 10: Mass Movement 5. Debris avalanche • Movement of rocks, soil and debris mixed with ice or water or both • Rapid movement • Material transported liquefies • Rapid speed • Can move further than the foot of a slope due to their rapid speed Chapter 10: Mass Movement 6. Mudflow • Saturated regolith flows rapidly downslope • Influence of gravity • Associated with steep slopes and deep soils • Fast speed • Soil becomes waterlogged and saturated if there is an impermeable rock layer beneath • Can be of speeds in excess of 100 km per hour • Can occur as a result of volcanic activity – known as lahars • Volcano with snow on its top erupts, the melting snow and ice cause a mudflow Chapter 10: Mass Movement Mudflow case study • 15 June 1991, Mount Pinatubo, Philippines • Volcanic eruption deposited more than 5 cubic kilometers of volcanic ash and rock fragments on the volcano slopes • Heavy rains washed this material down into the surrounding lowlands in giant, fast-moving lahars • The next four rainy season’s lahars carried about half of the deposits off the volcano, causing even more destruction in the lowlands Chapter 10: Mass Movement 7. Bogburst • An area of bog moves downslope • Influence of gravity • Slow to fast speed • Tend to occur mostly during autumn and winter due to high precipitation levels • Periods of very heavy rain after long periods of very dry weather seem to be associated with the occurrence of bog bursts • Human activity is also linked to the occurrence of bog bursts, e.g. peat cutting, draining of land for agriculture and the construction of wind farms Chapter 10: Mass Movement Bogburst case study • Derrybrien, Slieve Aughty Mountains, Co. Galway • 16 October 2003: bog had been affected by a dry spell during the summer and when heavy rain fell, the peat became saturated • Resulted in local roads being closed • Lough Cutra river system was polluted • Thousands of fish killed when some 70 acres of bogland flowed down river Chapter 10: Mass Movement Avalanche • Very rapid movement of snow downslope • May occur due to either natural events or human activity • Fast speed • Usually occur on mountain slopes • Occur when the weight of the snow is too much for the slope to hold Chapter 10: Mass Movement Major causes of avalanches may include the following: 1. Weather – they are most likely to occur after a heavy snowstorm 2. Snowfall – recent snowfall exerts increased pressure on existing snow deposits. 3. Temperature – warmer temperatures experienced over several days may cause upper layers of snow to melt and weaken 4. Wind direction – snow may be packed unevenly on the leeward side of a mountain due to prevailing wind direction 5. Slope angle – most avalanches occur on slopes of between 30 and 45 degrees 6. Slope orientation – most avalanches occur on slopes facing north, north-east and east 7. Human activities – deforestation and tourist activity may lead to the occurrence of avalanches Chapter 10: Mass Movement 9. Landslide • Slope stability changes from stable to unstable • Causes may be loss of vegetation, erosion of slope, weakening of slope, earthquake activity, volcanic eruptions and human activities • Fast speed Chapter 10: Mass Movement Landslide case study • February 2010 Maierato, Calabria, southern Italy • Entire hillside moved downslope • Occurred following a period of heavy rain • Over 2300 residents were evacuated from their homes Chapter 10: Mass Movement Human activities can impact on the operation of surface processes • Impact of overgrazing • Impact of overcropping • Impact of deforestation Chapter 10: Mass Movement Sahel case study • Areas that are most affected by desertification are regions/areas that are on the edges of deserts • Sahel Region lies to the Southern end of the Sahara Desert in Africa • Covers an area of approximately 1000 km in total • Stretches from the Atlantic Ocean coast to the Red Sea coast and is located at the southern end of the Sahara desert • Desert is growing at a rapid rate • Experienced an increase in population in recent years Chapter 10: Mass Movement Sahel case study (continued) • High birth rate and immigration rates are also high • High population growth increases the demand for food • Increased demand for food and for land to cultivate • Growth of farming and cultivation has led to soil becoming exhausted and exposed to erosion • Increase in the number of animals required for food has also resulted in overgrazing • Cattle are also seen as a sign of wealth in the region. • Energy needs are satisfied by cutting down trees Chapter 10: Mass Movement Sahel case study (continued) • Vegetation, along with the trees, bind the soil together • When removed, the soil becomes weak and dry • When extra land is needed, a practice of ‘slash and burn’ has been used and has increased the barrenness of the region • The Sahel Region has experienced long periods of drought • Also experiences spots of unpredictable heavy rainfall • This type of rain can damage crops • When land experiences desertification, fertility is reduced and this has a direct effect on food supply; it may lead to famines