Mass movements and landslides

Detail

1. What is mass movement?

• Mass movement (also called mass wasting): rock debris moves down a slope because gravity pulls it directly.
• Air, water or ice do not carry the debris.

• The debris may carry water or ice along with it.

• It is not erosion.

• Erosion needs a geomorphic agent, a moving medium such as running water, glaciers, wind, waves or groundwater, that picks up material and carries it away.

• In mass movement no agent does the carrying. Gravity alone moves the mass, even though material still shifts from one place to another.

• Weathering (rocks breaking down where they lie) helps mass movement but is not needed for it.

• Mass movement is still most active on weathered slopes, because weathered material is loose and weak.

• Range: from slow to rapid, and from shallow to deep. The main kinds are creep, flow, slide and fall.

2. Conditions that favour mass movement

Factor	Why it helps movement
Weak, unconsolidated material (loose, not cemented)	Grains are poorly held together
Thinly bedded rocks	Many weak planes between layers
Faults (cracks along which rocks have moved)	Broken, crushed rock zones
Steeply dipping beds (rock layers tilted at a steep angle)	Layers can slide along their own surfaces
Vertical cliffs and steep slopes	Gravity pulls harder along the slope
Heavy, torrential rain	Adds weight and makes material slippery
Scarce vegetation	No roots to bind the soil

3. Activating causes (triggers)

Favourable conditions make a slope weak. A trigger sets it moving.

1. Removal of support from below. Rivers, waves, glaciers or people cut away the base of a slope (for example, road cutting at the foot of a hill).
2. Slopes made steeper and higher. Undercutting by rivers or excavation does this.
3. Overloading by added material or fill. People dump debris, or build on the slope.

4. Overloading by heavy rain, with saturation and lubrication. - Rainwater fills the pores in the soil and adds weight. - Water also reduces friction between grains and layers, so they slip easily.

5. Removal of load from the original slope or surface above it. The balance of the slope changes.

6. Earthquakes, explosions or machinery. Shaking loosens material.
7. Excessive natural seepage. Water flows out through the slope and weakens it from inside.

8. Heavy drawdown of lakes, reservoirs and rivers. - The water level drops fast. - Water trapped in the banks now pushes outward, and nothing holds the banks back. - The banks slump.

9. Indiscriminate removal of vegetation. Root binding is lost and runoff increases.

4. Three forms of mass movement

• Heave: soil is pushed up by frost growth (ice forming inside the soil) and other causes, then settles back down a little further downslope.
• Flow: the material moves like a thick fluid.
• Slide: the material moves as a block or mass along a surface.

5. Slow movements

• Soil creep
• A very slow movement of soil that you cannot see as it happens.
• It is seen on gentle, soil-covered slopes everywhere.

• Signs: tilted telephone poles, fences and trees, and bent walls.

• Solifluction ("soil flow")

• A slow downslope flow of saturated soil.
• It is typical of cold regions (sub-Arctic, tundra and high mountains).
• How it happens: in summer the top layer thaws → melt water cannot drain into the permafrost below (ground that stays frozen for at least 2 years) → the soaked top layer flows slowly over the frozen layer.

6. Rapid flows

• Earth flow: clay-rich or silty soil, soaked with water, flows down a low-angle terrace or hillside.
• Mudflow
• Soil and debris mix thickly with water after heavy rain on bare slopes.
• The mixture moves fast along stream channels.
• Lahar: a mudflow made of volcanic ash and debris mixed with water.

• Armero, Colombia (13 November 1985): the eruption of Nevado del Ruiz melted its ice cap. Lahars rushed down the river valleys and buried the town of Armero, killing about 23,000 people.

• Debris avalanche

• A very rapid flow of wet debris down steep, narrow channels.
• Typical of humid regions with or without vegetation.
• It moves much faster than a mudflow.
• NCERT exercise: a debris avalanche is a rapid flow type of mass movement.

7. Landslides

• Landslides: relatively rapid and perceptible (you can see them move) movements of relatively dry material.
• The type of landslide depends on:
• the discontinuities in the rock (joints, faults, bedding planes);
• the degree of weathering;
• the steepness of the slope.

Type	What happens	Key marker
Slump	One or more units of debris slip down with a backward rotation relative to the slope (NCERT Fig. 5.4)	Curved, spoon-shaped slip surface; the top of the block tilts back
Debris slide	Earth debris rolls or slides down rapidly	No backward rotation
Debris fall	Debris falls almost freely from a vertical or overhanging face	Nearly free fall of loose debris
Rockslide	Rock masses slide down bedding, joint or fault surfaces	Deep; very fast and destructive on steep slopes; a planar failure (sliding along a flat plane) on steeply dipping beds
Rock fall	Rock blocks fall freely, keeping away from the slope	Only the surface layers of rock are affected, unlike deep rockslides

• NCERT Fig. 5.5: landslide scars in the Shiwalik (Siwalik) Himalaya, near the Sarada (Sharda/Kali) river, on the India–Nepal border, Uttar Pradesh.
• The Class 9 NCERT text also covers avalanches: large masses of snow and ice sliding down mountain slopes, a related gravity hazard in snow-covered mountains.

8. Landslide-prone areas of India

• How large the problem is
• About 0.42 million km², or 12.6% of India's land area (not counting snow-covered areas), is prone to landslides [2].
• Split by region [2]:
  • North-East Himalaya, including the Darjeeling and Sikkim Himalaya: 0.18 million km²
  • North-West Himalaya (Uttarakhand, Himachal Pradesh, Jammu & Kashmir): 0.14 million km²
  • Western Ghats and Konkan hills (Tamil Nadu, Kerala, Karnataka, Goa, Maharashtra): 0.09 million km²
  • Eastern Ghats, Araku area of Andhra Pradesh: 0.01 million km²

Feature	Himalayas	Western Ghats and Nilgiris
Tectonic state	Tectonically active: the Indian plate still pushes north, so the region is young and has frequent earthquakes	Tectonically stable (an old block)
Rock	Mostly sedimentary rocks and unconsolidated or semi-consolidated deposits	Hard rock, mainly crystalline rock
Slope	Very steep	Near-vertical cliffs and escarpments (steep cliff faces)
Main triggers	Earthquakes, monsoon rain, river undercutting	Strong mechanical weathering caused by temperature changes; heavy rain in short spells
Common types	Debris avalanches and landslides, very frequent	Landslides, with frequent direct rock falls

• Human causes (Class 9 NCERT):
• deforestation;
• mining and quarrying;
• road building (cutting the foot of slopes);
• unplanned construction on hillsides;
• poor drainage, which lets water seep into slopes.

9. Mapping and monitoring

• GSI – National Landslide Susceptibility Mapping (NLSM)
• The Geological Survey of India started NLSM in FS 2014-15 [2].
• Its aim is a single, seamless database for the whole country showing how likely each area is to have landslides [2].

• Landslide susceptibility means how likely an area is to have a landslide, judged from its slope, rock, soil, land use and past slides.

• ISRO-NRSC – Landslide Atlas of India (2023)

• It records about 80,000 landslides mapped by NRSC/ISRO between 1998 and 2022 [3].
• It covers 17 states and 2 Union Territories in the Himalayas and Western Ghats [3].
• It ranks 147 districts by how exposed people and property are to landslides [3].
• Rudraprayag and Tehri Garhwal (both in Uttarakhand) are among the most exposed districts.
• It has three types of inventory (record of landslides) [3]:
  • seasonal: the 2014 and 2017 rainy seasons;
  • event-based: Kedarnath, the Kerala floods, the Sikkim earthquake and landslides that blocked valleys;
  • route-wise: tourist and pilgrimage routes.
• The atlas uses satellite images from Resourcesat, Cartosat, Sentinel, Pleiades and WorldView, checked in the field [3].

10. Recent cases

Event	Place	Type / cause
Kedarnath (June 2013)	Rudraprayag district, Uttarakhand	Extreme rain and the burst of Chorabari lake caused debris flows and landslides along the Mandakini valley
Malin (30 July 2014)	Ambegaon taluka, Pune district, Maharashtra (Western Ghats)	Heavy rain caused a hill slope to fail and bury the village
Joshimath (2023)	Chamoli district, Uttarakhand	Land subsidence (the ground slowly sinks); the town stands on old landslide debris and is under heavy construction load
Wayanad (30 July 2024)	Meppadi grama panchayat, Wayanad, Kerala: Chooralmala, Mundakkai, Attamalai, Vellarimala [4][5]	Several rain-triggered landslides after torrential rain on 29–30 July 2024 [4]

• Landslide management (early warning, zoning and NDMA guidelines) is covered in natural-hazards-disasters.

Prelims Hooks

• Mass movement is not erosion, because no geomorphic agent carries the material. Gravity acts directly.
• Weathering aids mass movement but is not a prerequisite for it.
• Tilted poles, fences and trees → soil creep.
• Saturated soil flowing over permafrost → solifluction, a cold-region process.
• Lahar = volcanic mudflow. Example: Armero, Colombia, 1985 (Nevado del Ruiz).
• Debris avalanche = a rapid flow, not a slow movement and not a landslide (NCERT exercise trap).
• Slump has a backward rotation. A debris slide has none.
• Rockslide affects material deep in the slope. Rock fall affects only the surface layers.
• About 12.6% (0.42 million km²) of India's land is landslide-prone. The North-East Himalaya has the largest share (0.18 million km²) [2].
• The Landslide Atlas of India (2023) by ISRO-NRSC covers about 80,000 landslides (1998–2022) and ranks 147 districts [3]. Rudraprayag is among the most exposed.

Mains Points

• Himalayas vs Western Ghats. The Himalayas get landslides because the region is young and still tectonically active, with soft sedimentary rocks and steep slopes. The Western Ghats are stable hard rock, but their near-vertical escarpments, mechanical weathering and short bursts of intense monsoon rain cause failures and frequent rock falls. Wayanad (2024) and Malin (2014) show that "stable" regions are not safe.
• Human-made triggers make natural weakness worse: slopes cut for roads and hydropower projects, deforestation, quarrying, rapid reservoir drawdown, and unplanned buildings that load slopes. Examples are the pilgrimage and highway corridors of Uttarakhand and the construction load behind Joshimath's subsidence (2023).
• Policy response. Use GSI's NLSM susceptibility maps (from 2014-15) [2] and the NRSC Landslide Atlas district rankings [3] for land-use zoning, building codes for hill towns, routing roads away from slide zones, and early warning based on rainfall thresholds (GS-III, disaster management).
• Climate link. More frequent extreme rain events make saturation and lubrication more likely, and saturation and lubrication are the most common trigger. Landslide risk in the monsoon hills is therefore rising.