What is mass movement?
Mass movement (mass wasting) is the downslope movement of weathered rock, soil and other material under the direct pull of gravity. It differs from erosion because a transporting agent such as a river or wind is not needed. Weathering first loosens the material, and gravity then pulls it downhill.
Key idea
Movement occurs when the downslope force of gravity becomes greater than the friction and cohesion holding the material in place. Water is the most common trigger because it adds weight and reduces friction.
Factors and types
Movement is encouraged by a steep slope, heavy rainfall, weak or clay-rich rock, and the removal of vegetation whose roots normally bind the soil. Human actions such as cutting slopes for roads and clearing hill forests increase the risk.
- Soil creep — very slow, grain-by-grain movement; tilts fences, poles and trees.
- Landslide/rockfall — sudden, rapid sliding or falling of rock and soil down a steep slope.
- Mudflow — saturated soil flows quickly like a thick liquid after heavy rain.
- Slumping — a block of material rotates and slips along a curved surface.
Example
The 1993 Highland Towers collapse near Kuala Lumpur, and repeated landslides along hill roads such as those to Cameron Highlands, show how heavy rain on cut, deforested slopes can be deadly.
Effects
Mass movement can bury homes, block roads and rivers, damage farmland and cause loss of life. Blocked rivers may cause flooding upstream. Slopes can be made safer by terracing, building retaining walls and drains, planting cover crops, and controlling hillside development.
Remember
- Gravity is the driving force; water is the main trigger.
- Creep is slow; landslides and mudflows are rapid.
- Vegetation and good drainage reduce the risk.