Mass Movements
Pure Gravitational Mechanics · Heave, Flow & Slide · Creep & Solifluction · Rapid Flows & 5 Landslide Types (28 Micro-Realms)
The Direct Downslope Pull of Gravity (NCERT Chapter 6)
Mass Movements transfer the mass of rock debris down the slopes under the direct influence of gravity. That means air, water, or ice do not carry debris with them from place to place; on the other hand, the debris too may carry with it air, water, or ice—which is why mass movements do not come under erosion. Operating through Heave, Flow, and Slide, they range from imperceptible Creep and Solifluction to catastrophic Mudflows, Avalanches, and Landslides (Slump, Debris Slide, Rockslide, Rockfall).
I. Gravitational Mechanics, Non-Erosion Rule & Activating Causes (NCERT)
Direct Gravity Control
- Transfers mass of rock debris downslope
- Under direct influence of gravity
- Gravity acts on all matter (NCERT)
Why Mass Movement ≠ Erosion
WATER / AIR
CARRY DEBRIS
- Air, water, ice do NOT carry debris
- Instead, debris carries air/water/ice!
- Hence excluded from erosion (NCERT)
Weathering Not Mandatory
- Weathering is NOT a pre-requisite
- Though it aids mass movements
- Very active over weathered slopes
Shear Stress vs. Resistance
(Fs < 1.0 → SLIP)
- Weak unconsolidated materials
- Thinly bedded rocks & faults
- Steeply dipping beds & vertical cliffs
Dual Role of Water
- 1) Adds overburden weight to slope
- 2) Lubricates internal slip planes
- Pore pressure pushes grains apart
8 Activating Causes (NCERT)
- 1) Removal of support from below
- 2) Increase in gradient/height & 3) Load
- 4) Heavy rain, 5) Quakes/blasts, 6) Flora loss
Heave, Flow & Slide
- 3 basic forms of movement (NCERT)
- Heave: Frost/wetting soil expansion
- Flow (viscous) & Slide (shear plane)
II. Slow Mass Movements: Creep & Solifluction (NCERT)
Creep (Imperceptible Slow)
- Occurs on moderately steep, soil-covered slopes
- Extremely slow & imperceptible
- Noticed only by long-term observation
Tilted Poles & Trees
- Fence posts & telephone poles lean
- Retaining walls bulge & crack
- Pistol-butt curved tree trunks
4 Types of Creep (NCERT)
- Soil creep & Talus creep
- Rock creep & Rock-glacier creep
- Classified by material involved
Solifluction (Slow Flow)
- Slow downslope flowing soil mass
- Saturated with water or lubricating ice
- Unable to percolate deeper (NCERT)
Where Solifluction Occurs
TEMPERATE
TUNDRA
- 1) Periglacial frozen ground (Gelifluction)
- 2) Moist temperate areas where
- Heavy rain saturates thawed top layer
Terracettes (Sheep Tracks)
- Parallel miniature step-like ledges
- On steep grassy creeping hillsides
- Formed by soil heave & hoof tread
III. Rapid Mass Movements: Earthflow, Mudflow & Debris Avalanche (NCERT)
1. Earthflow (Humid Slopes)
- Water-saturated clayey or silty materials
- Down low-angle terraces or hillsides
- Leaves step-like scarps at head (NCERT)
2. Mudflows (Bare Channels)
- Absence of vegetation cover + heavy rain
- Thick layers of weathered materials
- Flow rapidly down definite channels
Volcanic Mudflows (Lahars)
- Erupting or recently erupted volcanoes
- Volcanic ash, dust & fragments turn
- Into destructive mud tongues with rain
3. Debris Avalanche
- Characteristic of humid regions
- With or without vegetation in narrow tracks
- Much faster than mudflow (NCERT)
IV. Landslides (5 Types) & Himalayas vs. Western Ghats (NCERT)
Landslides Definition
- Rapid & perceptible movements
- Materials involved are relatively dry
- Controlled by rock discontinuities
1. Slump (Backward Rotation)
- Slipping of one or several units
- With a BACKWARD ROTATION (NCERT)
- Along a curved concave slip plane
2. Debris Slide (No Rotation)
- Rapid rolling or sliding of earth debris
- WITHOUT backward rotation of mass
- Planar translational slip (NCERT)
3. Debris Fall (Vertical)
- Nearly a free fall of earth debris
- From a vertical or overhanging face
- River-undercut alluvial bluffs
4. Rockslide (Bedding/Joint)
- Sliding of individual rock masses
- Down bedding, joint or fault surfaces
- Very fast & destructive on steep slopes
5. Rockfall (Superficial Free)
- Free falling of rock blocks
- Over any steep slope keeping distance
- Occurs from superficial layers (NCERT)
Rockslide vs. Rockfall
SUBSTANTIAL DEPTH
SUPERFICIAL
- Rockslide affects materials to
- Substantial depth along dip planes
- Rockfall detaches superficial blocks
Himalayan Vulnerability (3)
- 1) Tectonically active & earthquake-prone
- 2) Mostly unconsolidated sedimentary rock
- 3) Steep slopes + heavy rain (Joshimath/Malpa)
Western Ghats & Nilgiris
- Tectonically stable & hard rocks, YET:
- Mechanical unloading + high $\Delta T$
- + Heavy short-span rain (Wayanad/Malin)
Anthropogenic Slope Cuts
- Unscientific road-widening toe cuts
- Tunnel blasting & dam reservoirs
- Deforestation & blocked hill drains
Slope Mitigation Engineering
- Retaining walls & wire-mesh Gabions
- Rock bolting, shotcrete & weep-holes
- Bio-engineering vetiver/bamboo roots
Core Distinctions
In erosion, a mobile geomorphic agent (water, wind, ice) acquires and carries rock debris. In mass movements, mass moves downslope under the direct influence of gravity alone—air, water, or ice do not carry the debris; rather, the debris carries air, water, or ice with it.
2. Slump vs. Debris Slide vs. Rockslide vs. Rockfall (NCERT Prelims Matrix)• Slump: Slipping of units with a backward rotation with respect to the slope.
• Debris Slide: Rapid rolling/sliding of earth debris without backward rotation.
• Rockslide: Sliding of rock masses down bedding, joint, or fault surfaces (affects substantial depth).
• Rockfall: Free falling of rock blocks from superficial layers of a steep face while keeping itself away from the slope.
Himalayas: Tectonically active, made of unconsolidated sedimentary rocks, and very steep slopes. Nilgiris / Western Ghats: Tectonically stable and made of very hard rocks, yet experience frequent direct rockfalls and landslides due to steep cliffs/escarpments, mechanical exfoliation from temperature changes, and heavy rainfall over short periods.
Mass Movements Matrix
NCERT Chapter 6 Classification