Mass Movements – Geomorphic Processes

Mass movements are typically associated with the greatest loss of life, whether natural or man-made.

These movements of mineral material are often composed of rock fragments ranging from mud to boulders, however not all types of mass movements are necessarily composed of rock.

That means, air, water or ice do 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 unweathered materials.

Mass movements do not come under erosion though there is a shift (aided by gravity) of materials from one place to another.

Heave (heaving up of soils due to frost growth and other causes), flow and slide are the three forms of movements.

Slow mass Movements

Slow-moving mass movements happen when a large mass of a slope suddenly falls a short distance. This may occur on moderately steep, soil-covered slopes where creep causes a loosening of the overlying material and overbalance occurs. The 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 downslope 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 downslope flowing soil mass or fine-grained rock debris saturated or lubricated with water.

Rapid mass Movements

Rapid mass movements are common features of hilly or mountainous terrains, and depending on the type of movements or processes involved these give rise to characteristic deposits.

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 downslope.

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.