Name: Date: 20/ / . Geography Form 3: Mass Movements Definition: The movement of soil and rock debris down a slope, due to gravity and the lubricating action of water. This movement is both fast and slow and the degree of movement is influenced by: 1. The angle of the slope 2. The nature of the material 3. The structure of the rocks Mass wasting is induced by: 1. Steep slopes 2. Heavy rainfall 3. Human interference with natural vegetation Slow Movement: Soil Creep ֎ Define what is soil creep. ________________________________________________________________________________________________________________________________ ________________________________________________________________________________________________________________________________ What are some indicators of soil creep? ________________________________________________________________________________________________________________________________ ________________________________________________________________________________________________________________________________ Factors affecting soil creep In temperate environments soil can creep downhill at between 1mm and 2mm per year, but in tropical regions it is quicker, perhaps 3-6mm. In cold semi-arid areas, it is even faster. Soil creep is caused by: The alternate wetting and drying of the soil The alternate freezing and thawing of the soil The expansion and contraction in soil is the main cause of soil creep. The wetter the soil is, the faster it is able to flow. Therefore, soil creep is much more active in wet areas of the world. In regions of seasonal wetting and drying the soil contracts when dry and expands when wet; wet soil moves more easily. Rain-wash and rain splash can also move soil downslope. ֎ What is the difference between rain-wash and rain splash? _______________________________________________________________________________________________ _______________________________________________________________________________________________ Rapid Movement – Landslide Define what is a landslide. _______________________________________________________________________________________________ Ms. T. Thomas 1 _______________________________________________________________________________________________ Physical/Natural Causes of Landslides 1. Unconsolidated material on a slope: In 1998, the rains from Hurricane Mitch caused devastating mudflows from the unconsolidated ash slopes of the Casita volcano which killed 1900 people and destroyed the town of Posoltega in Nicaragua. 2. Slippery rocks like shale and clay: Shale and clay are both slippery, esp. when wet. Wet clay acts as a lubricant, so rock beds above it slip down more easily while shale is made of very thin layers, so it slides easily. 3. Bedding planes being roughly parallel to the slope surface: This makes it easy for material to slip downwards along the bedding plane. Gravity can exert its force easily on the rock beds. The effects are even greater if the rock layer below is impermeable. This keeps all the moisture in the top layer of the rock, so it becomes saturated quickly. 4. The base of a slope being undercut by river or coastal erosion: Rivers meander and undercut their valley sides. If the slope above is made of unconsolidated material or very wet material and becomes unsupported it is much more likely to collapse. Coastal erosion can produce a similar effect. 5. Earthquakes: 1977, a 7.7 earthquake shook parts of Peru. Ice and snow near the summit of Huascaran formed an avalanche which picked up rocks and boulders along the way. It hit the town of Yungay in the Rio Santo Valley killing and destroyed the town. 6. Volcanic eruptions: Volcanic ash and mudflow (lahars) move rapidly destroying and killing anything in their paths. 7. Heavy rainfall: all of the mentioned are made worse by the surface material becoming saturated, which makes it heavier and reduces friction. Water does two things; it adds weight to the material, making the slope less stable and it decreases friction which helps movement downslope. Human causes of landslides 1. Building on unstable slopes: _____________________________________________________________________________________ _______________________________________________________________________________________________________________________ _______________________________________________________________________________________________________________________ 2. Undercutting the base of a slope by road building: _____________________________________________________________ _______________________________________________________________________________________________________________________ _______________________________________________________________________________________________________________________ 3. Dumping waste material: _________________________________________________________________________________________ _______________________________________________________________________________________________________________________ _______________________________________________________________________________________________________________________ 4. Building dams: _____________________________________________________________________________________________________ _______________________________________________________________________________________________________________________ _______________________________________________________________________________________________________________________ ACTIVITY: MASS WASTING Ms. T. Thomas 2 1. What measures can be taken to minimize the effects of soil creep? 2. Discuss the causes and consequences of a particular landslide in the Caribbean region. 3. Why do you think that a landslide is more likely to occur in a less developed country versus that of a more developed county? Ms. T. Thomas 3
