About

• These movements transfer the mass of rock debris down the slopes under the direct influence of gravity.
• That means, air, water or ice does not carry debris with them from place to place but on the other hand the debris may carry with it air, water or ice.
• Gravity exerts its force on all matter, both bedrock and the products of weathering.
• So, weathering is not a pre-requisite for mass movement though it aids mass movements.
• Mass movements are very active over weathered slopes rather than over un- weathered materials.
• Mass movements do not come under erosion though there is a shift (aided by gravity) of materials from one place to another.

Slow mass Movements

• Creep is one type under this category which can occur on moderately steep, soil covered slopes.
• Movement of materials is extremely slow and imperceptible except through extended observation.
• Materials involved can be soil or rock debris.
• fence posts, telephone poles lean down slope from their vertical position that is due to the creep effect.
• Depending upon the type of material involved, several types of creep viz., soil creep, talus creep, rock creep, rock-glacier creep etc., can be identified.
• Also included in this group is solifluction which involves slow down slope flowing soil mass or fine grained rock debris saturated or lubricated with water.
• This process is quite common in moist temperate areas where surface melting of deeply frozen ground and long continued rain respectively, occur frequently.
• When the upper portions get saturated and when the lower parts are impervious to water percolation, flowing occurs in the upper parts.

Rapid Movements

• These movements are mostly prevalent in humid climatic regions and occur over gentle to steep slopes.
• Movement of water-saturated clayey or silty earth materials down low-angle terraces or hillsides is known as earth flow.
• Quite often, the materials slump making step- like terraces and leaving arcuate 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 down slope.

Mudflow–

• 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 emerge 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.
• Debris avalanche, which is more characteristic of humid regions 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.

Landslides –

• These are relatively rapid and perceptible movements. The materials involved are relatively dry.
• The size and shape of the detached mass depends on the nature of discontinuities in the rock, the degree of weathering and the steepness of the slope.
• Depending upon the type of movement of materials several types are identified in this category.
• Slump is slipping of one or several units of rock debris with a backward rotation with respect to the slope over which the movement takes place
• Rapid rolling or sliding of earth debris without backward rotation of mass is known as debris slide.
• Debris fall is nearly a free fall of earth debris from a vertical or overhanging face.
• Sliding of individual rock masses down bedding, joint or fault surfaces is rockslide.
• Over steep slopes, rock sliding is very fast and destructive.
• Rock fall is free falling of rock blocks over any steep slope keeping itself away from the slope.
• Rock falls occur from the superficial layers of the rock face, an occurrence that distinguishes it from rockslide which affects materials up to a substantial depth.
• In our country, debris avalanches and landslides occur very frequently in the Himalayas.

There are many reasons for this-

• Himalayas are tectonically active.
• They are mostly made up of sedimentary rocks and unconsolidated and semi-consolidated deposits.
• The slopes are very steep.
• Compared to the Himalayas, the Nilgiris bordering Tamil nadu, Karnataka, Kerala and the Western Ghats along the west coast are relatively tectonically stable and are mostly made up of very hard rocks;
• Many slopes are steeper with almost vertical cliffs and escarpments in the Western Ghats and Nilgiris.
• Mechanical weathering due to temperature changes and ranges is pronounced.
• They receive heavy amounts of rainfall over short periods
• The coarser materials get deposited first and finer ones later.
• By deposition depressions get filled up.
• The same erosional agents viz., running water, glaciers, wind, waves and groundwater act as aggradational or depositional agents also.