- Disintegrated and fragmented rock materials due to the mechanism of weathering process.
- 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
- The movements of mass may range from slow to rapid, affecting shallow to deep columns of materials and include creep, flow, slide and fall.
- 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 unweathered materials.
- Mass movements are aided by gravity and no geomorphic agent like running water, glaciers, wind, waves and currents participate 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 over the slopes have their own resistance to disturbing forces and will yield only when force is greater than the shearing resistance of the materials.
- Several activating causes precede mass movements
- Removal of support from below to materials above through natural or artificial means
- Increase in gradient and height of slopes
- Overloading through addition of materials naturally or by artificial filling
- Removal of material or load from over the original slope surfaces
- Occurrence of earthquakes, explosions or machinery
- Heavy drawdown of water from lakes, reservoirs and rivers leading to slow outflow of water from under the slopes or river banks
- Indiscriminate removal of natural vegetation
- Mass movements can be grouped under three major classes
- Slow movements
- Rapid movements
- Movement of materials is extremely slow and imperceptible except through extended observation. Materials involved can be soil or rock debris.
- 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.
- 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.
- 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 earthflow.
- When slopes are steeper, even the bedrock especially of soft sedimentary rocks like shale or deeply weathered igneous rock may slide downslope.
- Another type in this category is 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 flows down along definite channels.
- It looks like a stream of mud within a valley
- 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
- A third type is the 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
- In Andes mountains of South America and the Rockies mountains of North America, there are a few volcanoes which erupted during the last decade and very devastating mudflows occurred down their slopes during the eruption as well as after eruption.
- These are known as relatively rapid and perceptible movements. The materials involved are relatively dry.
- 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 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.
- Landslide scars over steep slopes. Slides occur as planar failures along discontinuities like bedding planes that dip steeply.
- 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