The various kinds of down- hill movement of coherent masses of rock- debris occurring under the pull of gravity are collectively known as “mass movement” or “Mass- Wasting”. It constitutes an important process in denudation of the continental surface.
Types of Mass- movement
The first attempts at classification of the various types of mass- wasting. He recognized four major classes of Mass- wasting as follows:-
Mass- movements occur through ‘flowage’ and ‘sliding’. Which may be so slow that the movement is imperceptible or too rapid like catastrophic slumping and rock falls.
- Soil creep:-it is the most widespread slow down- hill movement of regolith and soil under the influence of gravity. it usually occurs on gentle to medium gradient slopes where the weathered mantle is deep and is particularly common when it is also fine-grained. Here the down-hill movement of regolith is imperceptibly slow.
Heating and cooling of the soil, growth of frost needles, alternate drying and wetting of the soil trampling and burrowing by animals and vibrations of earthquakes all produce some disturbance of the soil and regolith. As gravity always exert a downward pull on very such rearrangement. The particles are urged slowly downslope. Soil creep may operate in unconsolidated materials on slope less than appox 35o.
- Talus creep:-while talus creeps is the down-hill movement of pieces of bed-rock ranging in size from tiny chips to sizeable blocks, which rest at a particular angle of repose.
- Rock creep:-rock creep is a movement of jointed blocks, partly due to soil creep and partly as a result of sliding.
- Rock glacier creep:-rock glacier creep is a movement of streams of boulders with little soil and only interstitial ice.
- Solifluction:-the slow down-hill flowage of water saturated regolith is known as solifluction. It occurs in cold lands where there is an annual cycle of freeze-thaw i.e. in frigid climates. During the winter the ground is frozen and partially thaws out in the summer months. When thawing occurs it affects only upper part of the regolith which have been forced up and deranged by frost-heaving while the deeper part stays solidly frozen. Surplus water then cannot drain downward and the thawed layer gets fully saturated with water. This water- saturated regolith layer flows almost imperceptibly downhill. The moving soil and debris takes forms like sheet, lobate or tongue as it moves.
rapid flowage is related largely to increasing supplies of water on steeper slopes.
- Earth flow:- in regions of humid climate, a mass of water- saturated soil regolith or weak clay or shale layers may more down a steep slope during a period of a few hours in the form of an earthflow. This is also known as soilflow . The water –saturated material flows sluggishly to from a bulging toe. Some high mountain areas in which bed rocks is very weak and easily weathered are afflicted with large earthflow. The upper part of the earthflow undergoes a subsiding motion with backward rotation of the down-sinking mass, where as the interior and lower parts of the mass move by slow flowage and a toe or lobe of extruded flowage material may be formed. The shortness of the toes which develop downslope is due to the loss of water by drainage and infiltration which eliminates any elements of liquidity in the flow mechanism and tends to extinguish rapid movement. The flowing mass leaves a steep scarp in the rear. Earthflowage occurs only on fairly steep slope but large scale flowage of saturated clay may take place on slope of less than 1o .
- Mudflow :-mudflows are fluid mass of regolith moving in surges down stream channels. Thay are characteristic of steep scantily vegetated slope on which heavy rainfull initiates movement in a thick layer of weathered material. They are mostly found in drier regions. They differ from earthflow in that (1)they move more rapidly and have a high water cotent in comparison to the earthflow(2)they are confined to channels.
They are also known as mud streams. They occur where sufficient water is concentrated in the regolith at the head of a valley to overcome the internal cohesion and they move at rates as rapid as a river. The weight of water is more important brcause floe starts when the weight component parallel to the slope exceeds the internal cohesion of the mud the friction at its base.
- Debris avalanche:- this is the most rapid form of rapid flowage. In humid regions soil is developed on steep slope which are commonly covered with vegetation and tendto be stable. Mass movement here occurs due to some natural disturbance and sliding as well as flow may be involved. Sometime the materials are so massive and the movement so swift that they arereported as avalanches.they are like snow avalanches except that rock debris instead of snow makes up the bulk of their mass. Besides water is an important factor in debris-avalanches.
The term landslide is used widely in a general sense to mean any downslope movement of a mass of regolith or bedrock under the influence of gravity. Here the downslope movement of the mass is always perceptible and is said to involve mainly dry material.
They occur on steep slopes where hard and heavy rocks overlie softer or more easily lubricated materials such as clay or gypsum in which the bedding or joint-planes dip downward to wards the valley.
- Slump:- it involves the movement of the bedrock mass down a curved slip As the rock mass moves down it also rotates on a horizontal axis and therefore the upper surface of the block becomes tilted toward the cliff that remains. In such cases the mass of rock fails along two surface-one is the common surface of shear failure, usually curved outline and the other is the surface seperating one block from the other in the immediate neighbourhood. Usually slumping takes place as several small independent units which gives rise to a number of step like features.
- Debris slide:- this is a rapid downslope movement of a mass consisting of regolith opf varied texture. They mostly occur on steep slopes covered with scauty vegetion. The amount of water is usually small thus it is distinguished from debris avalanches . it involves a sliding or rolling motion but not the type of backward rotation as in the case of slumping.
- Debris fall:– In the case of debris falls material falls from a vertical or overhanging cliff.
- Rock slide :– it consists of a rock mass slipping down the slope which are usually the planes of weakness like bedding, joint or fault planes.
- Rock fall:-rockfall is the free falling or rolling of single masses of rock from a steep cliff and thus involves disintegration of the rock- mass.
it is a downward movement of the rock mass without any horizontal motion. It takes place mostly due to slow removal of the materials beneath the surface due to chemical action of ground water or when weak plastic materials are overlain by heavy rockmasses.
Subsidence may be seen in the collapsed areas of surface due to the underground enlargement of caverns in limestone.
The following factors attribute to the process of mass wasting:
- Structural characteristics of a region.
- Composition of the rocks in a particular region.
- Climate and vegetation cover.