Definition of mass movement/Mass Wasting

Mass movement also known as mass wasting is referred to the slow or rapid movement of weathered materials (rocks, sand, soil or mud) down the slope and mountain under the direct force of gravity. Mass movement can take place slowly, gradually or rapidly, hence affecting shallow to deep columns of materials depending on the gradient of the slope. However, these movements transfer the mass of rock debris down the slopes under the direct influence of gravity. This means that air, water or ice dont usually transport debris along with them from place to place but on the other hand the debris may carry with it air, water or ice.

Gravity places its force on all materials, both bedrock and the products of weathering. However, weathering is not a necessity for mass movement though it aids mass movements. Mass movements are very effective on weathered slopes compared to unweathered materials.

Mass movements are influenced by gravity and no geomorphic processes like running water, glaciers, wind, waves and currents contribute in the process of mass movements. That means mass movements do not come under erosion though there is a shift (aided by gravity) of materials from one place to another. Materials on a sloppy hill have their own resistance to disturbing forces and will respond only when the force is greater than the shearing resistance of the materials. Weak unconsolidated materials, faults, thinly bedded rocks, steeply dipping beds, abundant precipitation, vertical cliffs or steep slopes, and torrential rains and scarcity of vegetation for instance favour mass movements. Mass movement or mass wasting occurs on both terrestrial and submarine slopes, and has been seen onEarth lands and on other planets likeMars,Venus, and Jupiter’s moon.

Types of Mass Movement

There are several kinds of mass movement namely:

1. Soil flows
2. Mudflows
3. Soil creep
4. Landslides
5. Rock falls

Soil flows- soil flow happens when the soil receives heavy amount of rainfall turning the soil into semi liquid state. During this process, the liquid state of the soil allows it to flow downwards as soil or mud flow. However, the materials slump making step-like terraces and leaving actuate scarps at their heads and an accumulation bulge at the toe. When slopes are steeper, even the bedrock especially of soft sedimentary rocks like shale or deeply weathered igneous rock may slide downslope. These movements are very common in humid climatic regions and occur over gentle to

Mudflows– In the absence of vegetation cover and with heavy rainfall, thick layers of weathered materials get saturated with water and either slowly or rapidly flow down along definite channels. It looks like a stream of mud within a valley. When the mudflows comes out of channels onto the piedmont or plains, they can be very destructive engulfing roads, bridges and houses. Mudflows occur frequently on the slopes of erupting or recently erupted volcanoes. Volcanic ash, dust and other fragments turn into mud due to heavy rains and flow down as tongues or streams of mud causing great destruction to human habitations. This can may also occur due to volcanic activity. Mudflows are common in tropical regions where there is deep weathering of soils and abundant rainfall.

Soil creeps. Soil creep refers to the slow downward movement of weathered materials or soil materials down a gentle slope. The most powerful force is water which now acts as the main trigger which enables the rock materials to roll or creep over others. The factors that can encourage soil creep are heating and cooling, wetting and drying of the soil in alteration.

Landslides– landslide takes place when large quantities of loosened surface rocksand soil suddenly slide down a steep slope such as a cliff face, a valley side or an embankment. Landslides are cause by the lubricating action of water and the pull of gravity. This type of mass wasting is most rapid of all and very destructive. Slides occur as planar failures along discontinuities like bedding planes that dip steeply. The actions that help to produce a landslide are the undercutting of the base of a steep slope by a river or sea and by human action. Landslide are often triggered off by an earthquake or by prolonged erosion.

The materials involved are relatively dry. The size and shape of the separated mass depends on the nature of discontinuities in the rock, the degree of weathering and the steepness of the slope. Several types of landslides are identified depending on the movement of material involved. slum- slipping of one or several mass of rock debris with a backward rotation with respect to the slope over which the movement takes place. Debris slide- rapid rolling or sliding of earth debris without backward rotation of mass, this is nearly a free fall of earth debris from vertical or overhanging face.

Rockfalls– these are referred to a mass of rock which fall from a steep cliff; they involve the sliding of individual rock masses down bedding, joint or fault surfaces. Rockfalls occur from the superficial layers of the rock face, an occurrence that differentiates it from rockslide which affects materials up to a substantial depth. When this happens over a steep slopes, it is very fast and destructive, and when the sequences continue for over a long period of time, the broken rocks collect at the base of the slope in a mound.

Debris avalanche– these are turbulent clouds of debris mixed with air that rush down steep slopes in this case, they move relatively slowly. This type of mass movement is more common in the humid regions, it can take place with or without vegetation cover and occurs in narrow tracks on steep slopes. This debris avalanche can be much faster than the mudflow. Debris avalanche is similar to snow avalanche.

Causes of mass movement

1. The removal of support from below to materials above through natural or artificial means
2. Increase in gradient and height of slopes- height of slopes increase mass movement. When the slope is overly steep, gravity will pull the material downward, causing a mass movement. The angle of repose can also influence how fast the material will move, ifa slope is extremely steep and the gravitational force is greater than that of the resisting force, the angle of repose has not been met and the slope is likely to fail.
3. Weathering- chemical weathering produces weaker minerals while physical weathering breaks rocks apart.
4. Overloading due to heavy rainfall, saturation and lubrication of slope material- An overabundance of water allows the soil to be very mobile. Water actually lubricates the soil and contributes to additional weight, thus too much of it reduces cohesion.
5. Excessive natural seepage earthquakes or explosions – Earthquakes, explosions and machinery are also common causes of mass movement. As the ground shakes, due to the energy released during the earthquake or explosion, portions of the hillside or mountain can loosen up and slide downslope.
6. Heavy drawdown of water from lakes, reservoirs and rivers leading to slow outflow of water from under the slopes or river banks- excess water can wash away small particles soil that help keep the hillsides intact.
7. Indiscriminate removal of natural vegetation or scarcity of vegetation- an area with veryscanty vegetation is also prone to mass movement, this is because vegetation helps to hold the soil in place, which prevents it from washing down the hillside. When vegetation is removed, that anchor is lost and soil can be easily roll away. Overloading through addition of materials naturally or by artificial filling

Effects of mass movement

1. Once a very large portion of soil is removed, it is very hard for it to be replaced and regain back it nutrients.
2. Landslides can be a more severe cause of the soil erosion problem. After a heavy downpour, the entire hillsides saturated with water can slide downward, causing serious structural damage to buildings, homes, and agricultural land use.
3. Mudflow and debris falls can damage human settlements that are located at the foot of the mountains, this may also affect the transport systems on the lower slope regions.