Geography · Ch 11 — Geomorphic Processes
Landslides
Landslides
The diagram is a cross-section of a hillslope, drawn like a slice cut into the ground so you can see both the surface and what lies beneath it. The undisturbed part of the slope is shown with horizontal lines representing bedded rock strata — these are the intact layers that have not moved. Cutting into this is a curved, concave slip surface, shaped like a spoon or a shallow bowl, and it is along this surface that the movement happens.
Above this curved surface sits the slump body: a mass of rock debris that has detached from the slope and moved down. The key feature is the backward rotation. The mass has not simply slid straight down; instead, its top has tilted backward, toward the slope, so the whole block has rotated as it slipped. Within the displaced mass, curved hatched lines show the internal distortion of the material — the debris has been bent and sheared as it rotated, rather than staying rigid.
Two labels anchor the diagram. One is "Initial position of slope," shown as a dashed line with an upward arrow at the scarp. The dashed line marks where the slope surface originally was before failure, and the arrow at the scarp (the steep, freshly exposed head of the slump) indicates the direction in which that part of the slope has moved — upward relative to the displaced block, because the block has dropped away beneath it. The other label is "Slump body showing backward rotation of materials," which points directly at the rotated, hatched mass. …
The photograph shows a stretch of the Shiwalik (or Outer) Himalayan foothills where the river Sarada has cut through the terrain near the India–Nepal border in Uttar Pradesh. The hillslopes are steep and largely bare of vegetation, and running down them from the ridge crest are multiple pale, scar-like tracks. These are the landslide scars — the exposed surfaces left behind where masses of earth and rock have detached and moved downslope. The scars stand out against the darker, vegetated patches of slope that have not failed, so the eye reads the image as a series of light, roughly parallel streaks descending from the top of the ridge toward the valley floor.
What the diagram is meant to show is not a single landslide event but the cumulative evidence of many. Each scar marks a separate failure of the slope, and their repetition across the hillside tells you that this is a landscape where mass wasting is frequent and ongoing. The steepness of the slope is the key visual clue: the scars are long and straight, following the line of maximum slope, which is exactly the geometry you expect when gravity pulls material straight down a steep face. The bareness of the scarred areas also matters — once vegetation is stripped away by a slide, the exposed rock and soil are more vulnerable to further erosion, so a scar can widen or deepen over time. …
Landslides are among the fastest and most visible forms of mass movement. Unlike the slow, creeping downhill flow of soil, a landslide is a relatively rapid and perceptible movement of material down a slope. A key feature of landslides is that the materials involved are relatively dry — this is what separates them from flows, where water plays a central role.
The size and shape of the mass that breaks away and moves downhill is not random. It depends on three things: the nature of discontinuities in the rock (like cracks, joints, or bedding planes), the degree of weathering the rock has undergone, and the steepness of the slope. A heavily weathered, steep slope with many fractures will produce a very different landslide than a fresh, gentle one.
Because the movement of material can take several distinct forms, landslides are classified into different types. Each type is defined by how the material actually moves.
Slump is the slipping of one or several units of rock debris down a slope, but with a distinctive backward rotation. The block of material tilts backwards as it moves, relative to the slope surface it travels over. It is not a simple straight slide — the mass rotates as it slips.
Debris slide is the rapid rolling or sliding of earth debris down a slope, but crucially, without the backward rotation that characterises a slump. The material simply rolls or slides forward.
Debris fall is nearly a free fall of earth debris. This happens when material drops from a vertical or overhanging face — the debris is not sliding along a surface, it is falling through the air.
Rockslide is the sliding of individual rock masses down a surface such as a bedding plane, a joint, or a fault surface. When these slides occur over steep slopes, they are very fast and very destructive. Slides of this kind often happen as planar failures along discontinuities — for example, along bedding planes that dip steeply. This is why the angle of the rock layers matters so much.
Rock fall is the free falling of rock blocks over any steep slope, where the blocks keep themselves away from the slope as they fall. A distinguishing feature of rock falls is that they occur from the superficial layers of the rock face — only the surface material breaks off. This is what separates a rock fall from a rockslide, which affects material up to a substantial depth below the surface.
The single most important distinction to remember: rock fall strips only the superficial surface layer, while rockslide involves material from a substantial depth.
The textbook also raises two reflective questions worth thinking through. First, between the terms "mass wasting" and "mass movements," which is more appropriate? The answer lies in the fact that not all mass movements involve a wasting or lowering of the land surface — some movements simply redistribute material. Second, can solifluction be included under rapid flow movements? It can be argued both ways: solifluction is a flow of water-saturated material, but it is typically a slow process, not a rapid one, which is why it does not fit neatly into the category of rapid flows.
The discussion then turns to why landslides are so common in specific parts of India. In the Himalayas, debris avalanches and landslides occur very frequently, and there are clear reasons for this. The Himalayas are tectonically active — the region is still being uplifted and deformed. They are mostly made up of sedimentary rocks and unconsolidated and semi-consolidated deposits, which are weak and easily disturbed. And the slopes are very steep. These three factors together make the Himalayas highly prone to landslides.
But here is the puzzle: the Nilgiris (bordering Tamil Nadu, Karnataka, and Kerala) and the Western Ghats along the west coast are relatively tectonically stable and are mostly made up of very hard rocks. Yet debris avalanches and landslides still occur there, though not as frequently as in the Himalayas. Why? …
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. Many slopes are steeper with almost vertical cliffs and escarpments in the Western Ghats and Nilgiris. Mechanical weathering due to temperature changes and ranges is pronounced. They receive heavy amounts of …