The rocks debris is transferred down the slopes due to gravity. It also suggests that the debris is not influenced by Air, water, or Ice to move from place to place. But the debris moves the water, ice, and Air.
The mass movements also called slope movement or mass wasting are both slow and fast and it influences the shallow to deep columns of material and works creep, flows, slide and fall. The gravitational attraction applies its force to the bedrock and also the products of weathering.
The Weathering is not a basic requirement for Mass Movement but it greatly helps it. The un-weathered material is inactive for slope movements whereas weathered material is active for slope movement.
Mass Wasting does not come under Erosion as it does not have any influence due to geomorphic agents such as waves, currents, glaciers, water, wind, etc as it is only guided by gravity.
Materials over the slopes have their own resistance towards the disturbing forces and will accept or allow only when the forces are greater than their shearing resistance.
The weak unconsolidated rocks, thinly bedded rocks, faults, steeply sliding beds, vertical cliffs, perpendicular slopes, plentiful rainfall or snowfall, torrential showers, lack of vegetation are the factors that positively influence mass wasting.
Factors that Precede Mass Movements
- Removal of support from underneath to the rocks above by natural or artificial ways.
- The rise in inclination and height of slopes.
- Overloading by the increasing of material due to natural or unnatural ways.
- Overloading by huge rainfall, lubricating the slopes, and fullness(saturation).
- Removal of materials from the slope.
- By earthquakes, explosion, etc.
- Excessive Natural drainage.
- Heavy drawdown of water from the lakes, rivers leads to slow outflow of water from under the slopes or river banks
- Random removal of natural vegetation.
The forms of Mass Movement are Heave, flow, and slide and its types are Slow Movements and Rapid Movement.
Creep is a type of slow movement which occurs on moderately steep and soil-covered slopes. This movement is extremely slow and gradual, only can be observed if viewed for a very long time.
Creep involves soil or rock debris. Due to this movement, the telephone lines lean downslope from their linear alignment, fence post lean, etc.
Types of Creep depends upon the type of material:
- Soil Creep
- Talus Creep
- Rock Creep
- Rock Glacier Creep
It includes gradual and slow, downslope flowing soil mass or fine-grained rock debris immersed or lubricated with water.
This process usually occurs in the moist temperate regions where the surface melting of deeply frozen ground and long-continued rain happens repeatedly.
When the upper parts get overfilled and while the lower parts are impenetrable to water percolation, flowing happens in the upper portions.
Rapid movements are most common in humid climatic regions and happen over gentle to steep hills/slopes.
The flow of water-saturated clayey or silty materials down low-angle terraces or hillsides is known as earthflow.
Earthflow – Sheet or stream of soil and rock materials saturated with water and flowing downslope by force of gravity and it shows that it is an intermediate stage between creep and mudflow.
Usually, the materials slump creating step-like terraces and leaving curves or bow-shaped called arcuate scarps at their heads and an accumulation bulge at the toe.
If the slopes are more perpendicular, even the bedrock primarily of soft sedimentary rocks such as shale or strongly weathered igneous rock may slide downslope.
Another type of rapid movement is Mudflow. In the inadequacy of vegetation cover and by massive rainfall, thick layers of weathered matter get soaked with water and both slowly or rapidly flow down by certain channels.
It seems like a river of mud in a valley. If the mudflows arise out of channels over the area at the base of a mountain or mountain range such as piedmont or plains, it can become very dangerous by engulfing or covering the roads, bridges, and houses.
Mudflows happen regularly on the slopes of exploding or newly exploded volcanoes. Volcanic ash, dust, and other particles transform into the mud because of heavy Showers and flow down as tongues or streams of mud producing large destruction to humans.
Debris avalanche is another type of rapid mass movement, which is more a feature of humid areas with or without vegetation cover and happens in narrow tracks on steep slopes. This debris avalanche can be much quicker than the mudflow.
A debris avalanche is comparable to a snow avalanche.
Landslides are nearly rapid movements and the material involved is relatively dry.
The size and shape of the loosened mass depend on the nature of discontinuities in the rock, the level of weathering, and the inclination of the hill.
Depending on the nature of the movement of materials different types are classified.
It is the slipping of one or several sections of rock debris with a backward rotation with regard to the slope where the movement occurs.
The rapid rolling or sliding of rock debris without backward rotation.
It is almost a free fall of rock debris from a vertical or overhanging surface.
Sliding of a single rock, masses down bedding, joint or fault surfaces, and over steep slopes, is rapid and destructive. These slides happen as the planar collapses along discontinuities like bedding planes.
It is the free-falling of rock masses over any steep slope running off from the slope. It happens from the surface layers of the rock face, an event that separates it from the rock slide. It concerns the materials up to a certain depth.
Mass Movement in India
Debris avalanches and landslides are frequent in the Himalayas.
The reasons are the Himalayas are tectonically active.
- They are mostly made 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 mostly made of very hard rocks. But still debris avalanches and landslides occur but not as frequently as in the Himalayas.