Mass movements

Mass movements move rock debris down slopes under the direct pull of gravity. No agent (air, water, ice) carries the debris; instead, the debris may carry air, water or ice with it.

• They range from slow to rapid and shallow to deep: creep, flow, slide and fall. The three forms of movement are heave, flow and slide.
• Gravity acts on all material, so weathering is not required, though it helps: mass movements are much more active on weathered slopes.
• Because no agent is involved, mass movements are not counted as erosion, even though material shifts.
• Material holds until the disturbing force exceeds its shearing resistance.

What favours mass movement: weak or loose materials, thinly bedded rocks, faults, steeply dipping beds, vertical cliffs or steep slopes, heavy and torrential rain, and little vegetation.

Activating causes:

• removal of support from below (natural or man-made);
• steeper and higher slopes;
• overloading by added material or by heavy rain that saturates and lubricates the slope;
• removal of material from the slope surface;
• earthquakes, explosions or machinery;
• excessive seepage, or a sudden drawdown of water in lakes, reservoirs and rivers;
• removal of natural vegetation.

Landslides are relatively rapid, visible movements of fairly dry material:

• Slump: one or more units of debris slip with a backward rotation.
• Debris slide: rapid rolling or sliding of debris without backward rotation.
• Debris fall: a near free fall of debris from a vertical or overhanging face.
• Rockslide: rock masses slide down bedding, joint or fault surfaces; very fast and destructive on steep slopes.
• Rock fall: rock blocks fall freely away from a steep slope, from the surface layers only (a rockslide goes deeper).

In India: debris avalanches and landslides are very common in the Himalayas, which are tectonically active, made largely of sedimentary and loose deposits, with very steep slopes. The Nilgiris and Western Ghats are tectonically stable and made of hard rock, yet slides and rock falls still happen there.

Example (new): cutting into the foot of a hillside to widen a mountain road removes the support for the slope above; if heavy rain then soaks the slope, both causes act together and a slide becomes likely.