Mass movement is defined as the movement of Earth material by the force of gravity. In other words, it is the detachment of rock materials and their down-slope transportation. According to R.J Chorley:
”Mass movement is the detachment and downslope transport of soil and rock material under the influence of gravity. The sliding and flowing of these materials are due to their position and to gravitational force but the mass movement is accelerated by the presence of water ice and air”.
Mass movement plays a key role in the breakdown of rocks. After the loosening of a joint block from the face of a granite escarpment, the force of gravity pulls the block downslope at a great speed.
Types of Mass movement:
There are four basic types of mass movement:
- Creep movement (Slow motion of Soil).
- Flow movement (Flow of weathered bedrocks).
- Slide movement (Landslide or rockslide).
- Fall Movement (Rolling of pieces of rocks).
(1). Creep Movement:
It is the slowest down-slope movement of the soil layer. According to F.J Monkhouse: ”The most common type of mass movement is known as creep which may be almost indiscernible especially when it proceeds under a turf mat. Gradually, however, it becomes apparent as posts or fences are first tilted and then displaced downhill even growing trees may be moved, a ribbed or stepped pattern develops across the slope”. In this type of movement, the angular rock debris creeps down a slope, sometimes, known as talus.
(2). Flow Movement:
It is another type of slow mass movement. In this movement clays containing excessive amounts of water serve as a plastic mass moving as a thick viscous fluid. This flow is also called solifluction or sludging. In this case, a section of soil or weathered bedrock lying at a steep angle on a hillside is lubricated by rains and then starts flowing downslope.
(3). Slide Movement:
These movements include landslides and rock slides. A landslide is defined as a rapid sliding of large masses of rock without the flow of materials. The landslide moves faster than rockslides and roar downslope with a thunderous sound and great force. Sometimes, millions of tons of rock debris is involved in the landslides which can bury an entire town within minutes. In 1970, Huascaran (Peru) landslide witnessed the movement of 50 million cubic meter of depris. Main causes of landslides are:
- Earthquakes and Earth tremors.
- Over-steepening of slope through a cutting by a river can initiate a collapse.
- Cutting of the mountainsides by people.
- Torrential rains can cause landslides.
- Weathering can loosen pieces of bedrock.
(4). Fall Movement:
Freefall or downslope rolling of pieces of rocks loosened by weathering is known as fall movements. As a result of such movements, the debris is collected at the base to form a talus cone. In this case, the slope angle is from 34 to 36 degrees. This accumulation is called a screen slope.