Mass movement can be defined as the movement of weathered materials (regolith) on slope under the force of gravity. It can also be referred to as the movement of rock materials from one place to another under influence of gravity.
FACTORS THAT AFFECT MASS MOVEMENT
These are factors that affect mass movement
- Gradient of the slope.
- Human activities.
- Nature and weight of materials.
- Pressure of the lubricating moisture.
- Presence of vegetation.
Gradient of the slope
The gradient of the slope is one of the most important factor that affect mass movement. For example, rock material move faster in steep slopes or hilly areas than in gentle slopes.
The activities of man like construction, farming, grazing, running on the slope of mountains or highlands to promote or reduce the movement of rock materials.
Nature and Weight of materials
It is noted that the loose rock materials tend to move faster than tightly held in material i.e. the heavier the weight of the material the slower the movement.
Pressure of lubricating moisture
The pressure of lubricating moisture like rain, water, ice tends to produce, promote or increase the movement of rock materials down the slope.
Presence of vegetation
The pressure of vegetation can either increase or decrease/reduce the movement of rock materials.
TYPES OF MASS MOVEMENT
There are two main types of mass movement
- Slow movement
- Fast movement
They are more effective in area of deep chemical weathering. There are three most important types which are
- Soil creep
- Talus creep
- Soil flow
It is a slope almost unnoticeable but continuous movement of weathered material down a slope under the influence of gravity. It operates on a gentle slope and speed of the movement will be as slow as 1cm per year. This movement is aided by water which act as a lubricant which enables rock materials to creek over each other. Alternating, melting and drying, heating and cooling of soil e. t. c. are other factor that influences soil creep.
Soil creep can be noticed when fence and trees bend towards the direction of movement of the soil in a gentle slope, electric and telegraph pole are tilted, vertical rocks layers are covered down slow leaning of tree trunks down the slope, bulging of walls and fences, soil accumulated behind walls which may collapse as a result of the soil erosion.
This is less pervasive than soil creep. Talus creep is the movement of angular rocks down moderately through steep slopes. Large talus sheets move in mass especially in mountains region, where frequency thawing action is frequent.
This is a slightly faster movement usually average of five cm and one meter a year on moderate slopes. It occurs on temperate and Polar Regions. During the winter the surface layer whole ground is frozen. In the summer the surface layer thaws while the ground is still frozen. The saturated top soil may now begin to move on active layer over the frozen sub-soil
FAST MOVEMENT TYPES
Landslide is the most significant for of the fast movement types. This takes place when large quantities of loosened surface rocks and soil suddenly slide down a steep slope such as cliff, a valley slide or an embarkment.
Landslide are caused by the lubricating action of water at fall of gravity. The actions that help to produce a land are the under cutting of the base of a steep slope a river or sea and human action.
This is the most likely to occur where rocks have bedding planes which slip towards valley. The bedding plane provide the ship surface on which the saturated overlying rock layer easily slight over the under laying stable crust. Rock slides occur in over steepened slopes e.g. scarps, wall cutting.
This is the most rapid of all mass movement and occurs on very steep slopes which may even be vertical the movement is spontaneous once the rock is detached. It falls vertically down slope to form talus or screen at the put of the slope. This detachment is caused by weathering agents such as freeze the action of wave pounding on sick leaves, earthquake or pressure release.
Earth flow and mud flow
This two are also considered as rapid movement when the regolith on slopes of five degree to fifteen degree become saturated with water, it begins to flow down- hill.
Mud flow occurs in firmly steep slopes. The material is made up of semi-liquid mud at times with gravel and boulder. Large volumes of unconsolidated materials, super saturated becomes fluid begins to flow down- hill. It is common in arid and semi-arid regions.
Effects of Mass Movement
- It can lead to loss of farmlands.
- It can also result to displacement of settlement.
- It can cause disruption of transportation network.
- It can lead to loss of soil fertility.
Differences between Erosion and Mass Movement
- Erosion is the gradual movement of top soil while mass movement is the movement of loose rock material down slope.
- Agents of erosion are running water, wind, waves and glaciers while that of mass movement entails the force of gravity.
Erosion involves scratching, polishing and pushing and plucking of loosed rock surfaces while mass movement involved creeping, sloping, sliding and falling.