Activating causes of mass movement
Topic: Geomorphic Processes: Weathering, Mass Movement, Erosion and Soil Formation · NCERT: Class 11, Ch 5 "Geomorphic Processes"
Meaning
Activating causes of mass movement are the triggers that set a slope moving downhill. Weak rock, steep slopes and thin vegetation only make a slope unstable. A trigger is the event that finally makes it give way. The main triggers are:
- loss of support from below;
- slopes made steeper or higher;
- overloading by fill, or by heavy rain that soaks (saturates) and lubricates the soil;
- removal of load from the slope;
- shaking by earthquakes, explosions or machinery;
- heavy seepage (water slowly soaking through the ground);
- rapid drawdown (fast lowering of water in a reservoir);
- clearing of vegetation.
These causes matter because many of them come from human actions such as road cutting, blasting and cutting down forests. So they can be managed.
Example
On 30 July 2024, very heavy rain soaked the steep slopes of Wayanad in the Western Ghats of Kerala. The wet, heavy soil lost grip on the rock below it, and deadly landslides followed. Saturation and lubrication by rain were the trigger.
Don't confuse with
- Favourable conditions for mass movement: these are features already present in the slope, such as loose material, faults, steep cliffs and little vegetation. They make a slope ready to fail. An activating cause is the event that actually sets it moving.