meaning of mass wasting in geography
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 initial resistance is caused by the friction between the materials and the underlying rock mass.
Water plays an important role in helping the materials overcome this resistance.
Most of the movements are slow amounting to a few centimeters per year but sometimes they can be sudden and rapid.
7 factors that affect or influence nature and speed of movement in mass wasting;
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 experiences massive landslides, especially where the slope is steep.
Light rain penetrates slowly into materials and may take a 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.
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.
Earthquakes and volcanic eruptions cause the vibration of the earth which often trigger off the widespread movement of the materials such as landslides.
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.
Here, vegetation increases the rate of water intake into the rock materials beneath, causing the soil to be saturated
types of mass wasting
Slow Mass Wasting
this is Slow but steady movement of soil or loose rock debris down slope
- Alternate heating and cooling causing expansion and contraction of particles causing them to change their positions.
- Alternate wetting and drying of soil whereby when it‟s wet its compact and when dry the particles are loosened and tend to move away from each other.
- Trampling and burrowing of animals.
- External forces e.g. shaking by earthquakes, explosives, heavy vehicles, etc.
- Ploughing down hill
- Freezing of soil water causing it to expand which lifts particles at right angles to the slope in a process called heaving.
Movement of saturated soil, gravel and weathered rock down a moderate slope.Common in mountainous and very cold climates
process of solifluction
Slow and gentle movement of the mass of broken rock particles which accumulate at the base of cliffs (scree) downhill.
Slow movement of individual rocks which lie on clay at a very low speed down slope in the presence of moisture
rapid mass wasting
Type of mass wasting involving large amounts of weathered material moving suddenly and fast down the slope
Movement of oversaturated weathered material informs of liquid downslope.It occurs mainly in dry areas after heavy rains.
o Erosion occurs on the weak rocks at the base of a cliff undercutting the weak rock.
o The overlying rocks break off causing the overlying rocks to slide down hill rotating around a curved plane.
Sudden downhill movement of accumulated rock debris and other loose material downhill as a whole
Sudden free fall of debris from a vertical or hanging cliff to the base of the slope.
Sliding down of masses of rock a steep slope along a bending plane, joint of fault.
Falling or rolling of individual rocks or boulders down a steep slope or a cliff.Most rapid of all mass wasting.
Sudden slipping and falling of a large mass of snow, ice, and loose rock materials down a mountainside
minimizing mass wasting hazards
all forms of mass wasting can be very destructive
the hazard of mass wasting tend to be predictable and largely avoidable
evidence of past events help identify areas prone to slope failure
mass wasting and its effects can be minimized through engineering which involves:
- reducing slope
- increasing drainage
- constructing barriers