5.8.1. 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.

Landslides refer to quick, noticeable movements of earth materials down a slope. Unlike slow mass movements, landslides occur rapidly, often after a heavy rainstorm or earthquake. The type of landslide depends on factors such as how dry the materials are, how the rocks are structured, their weathering state, and the angle of the slope. As a result, we categorize landslides into types based on how the materials move.

Imagine a stack of blocks on a sloped table. If someone shakes the table (akin to an earthquake), the blocks might tumble down quickly. This is like a landslide—it's rapid and can change the landscape significantly.

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.

Landslides come in various forms, each defined by how the materials move: 1. Slump: The land slides down with a backward twist, creating a curved motion. 2. Debris slide: Here, materials slide down rapidly without that twist. 3. Debris fall: This occurs when earth debris drops straight down from a cliff or steep area. 4. Rockslide: This involves larger rock pieces sliding down the slope along natural fractures or weaknesses in the rock.

Think of a tray full of marbles placed at an angle. If you shake the tray, the marbles may either tumble in a clump (slump), roll down quickly (debris slide), fall off the edge (debris fall), or slide down in singular chunks (rockslide), depending on how they are arranged and the angle of the tray.

In our country, debris avalanches and landslides occur very frequently in the Himalayas. There are many reasons for this. One, the 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 Tamilnadu, Karnataka, Kerala and the Western Ghats along the west coast are relatively tectonically stable and are mostly made up of very hard rocks; but, still, debris avalanches and landslides occur though not as frequently as in the Himalayas, in these hills.

Regions like the Himalayas experience frequent landslides primarily due to their geological activity and steep slopes. The tectonic activity means that the earth is constantly shifting in those areas, often leading to instability. In contrast, the Nilgiris and Western Ghats are more stable but still prone to landslides, especially where steep cliffs and heavy rain are present.

Consider a pile of sand in a sandbox. If you wet the sand and then quickly push on the pile, it can collapse easily due to gravity, resembling a landslide. Similarly, the steep slopes and unstable geological makeup in places like the Himalayas lead to their frequent collapsing.

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