Mass wasting is the downslope movement of rock materials due to the influence of gravity. When such movement occurs after the materials have been lubricated by rainwater from melting snow it is referred to as MASS MOVEMENT.
The force involved in this process is gravity but it is only able to exert its influence when the materials overcome their initial resistance to movement. This initials resistance is caused by the friction between the materials and underlying rock mass. Water plays an important role in helping the materials to overcome this resistance. Most of the movements are slow amounting to a few centimeters per year but sometimes they can be sudden and rapid.
The nature and speed of movement in mass wasting are influenced by the following 7 factors;
- The nature and weight of materials. If the layer of the weathered rock is very deep or thinly bedded, it will result in rapid mass wasting. Thin beds increase the tendency for movement as there are more bedding planes over which movement can occur. Massive rock overlying weak rocks such as clay or shale can slide more easily than if they were overlying sand. Large rocks are likely to be overcome by gravity more easily than fine weathered materials
- Amount of water. The more saturated the material is the more likely it is to move. A mass of materials saturated with water moves more easily than a dry mass. This is because water increases the weight of the materials and at the same time reduces the cohesion between particles of materials between the mass. Water also acts as a lubricant along the bedding plane thus facilitating movements.
- The angle of the slope. The steeper the slope, the faster the movement. This is because gravity becomes stronger with increasing the angle of the slope. On a gentle slope, the movement is slower but is the slowest on a plain which is almost flat.
- The climate of the area. The amount and nature of rainfall received in the area determine the amount of movement that will occur. The area which receives heavy rainfall experience massive landslides, especially where the slope is steep. Light rain penetrates slowly into materials and may take long to saturate it or may not saturate it at all. The resultant movement of the materials will therefore be slow. Alternate freezing and thawing encourage mass wasting as well. In a dry climate, materials may be loose but they lack the added advantage of the water. the resultant movement is therefore likely to be slow
- Vegetation covers the area. Plants such as grasses, shrubs, and large trees, help to hold rock materials together, thus reducing their movement on the earth’s surface. Bare surfaces are more likely to experience mass wasting than surfaces that have vegetation cover. The presence of dense vegetation cover in the wet regions increases the rate of water intake (infiltration) into the soil and to the rock beneath the surface. This can speed up the saturation of the rock materials and trigger off mass wasting.
- Human activities. People affect the stability of the earth’s surface through various activities such as cultivation, building, grazing animals, mining, clearing vegetation, and road construction. Some of these activities result in the direct movement of materials, while others create favorable conditions for other factors to exert their influence. Vibration from moving trains and vehicles as well as tremors caused by explosions may shake the ground causing some materials to move downslope.
- Tectonic movement. Earthquake and volcanic eruption cause the vibration of the earth which often trigger off the widespread movement of the materials such as landslide.
The factors discussed above do not work in isolation. One factor can facilitate the effect of another factor and movement accelerated when they operate in combination. For example, heavy rainfall which is falling on deeply weathered rock materials on the steep slope may cause massive landslides even if the surface is covered in thick vegetation. Here, vegetation increases the rate of water intake into the rock materials beneath, causing the soil to be saturated