Mass Movement


  • Mass Movement: The downslope movement of loose soil, mud, and rock under the influence of gravity.

  • Regolith: The loose layer of broken rock, soil, and debris covering the solid bedrock.

  • The Main Trigger: Heavy rainfall, which saturates the ground, making the regolith heavier and lubricating it so it slides easily.

  • Classification: Grouped by speed (Slow vs. Fast) and water content (Dry vs. Wet).



1. Soil Creep (Slow & Relatively Dry)

  • What it is: The slowest form of mass movement, moving down gentle slopes at less than 1 cm per year.


  • How it works: Rain causes soil particles to expand and move straight up. When the soil dries out, the particles contract and fall slightly further down the slope due to gravity.


  • Evidence (Look for these on a map or picture question):

    • Tilted fences, telegraph poles, and gravestones.

    • Trees with curved trunks (growing upwards to find light as the base moves).

    • Terracettes: Steplike ridges running across the hillside.



2. Landslides (Very Fast & Relatively Dry)

  • What it is: The rapid movement of a massive amount of dry regolith, rock, and soil straight down a steep slope or cliff face.


  • How it works: Often triggered by earthquakes, coastal erosion cutting away the base of a cliff, road building cutting into hillsides, or sudden heavy downpours.


  • Real-world impact: Destroys roads, crushes buildings instantly, and changes the coastline.


3. Mudslides (Very Fast & Extremely Wet)

  • What it is: A rapid, liquid-like surge of saturated soil, mud, and rock rushing down a steep slope or valley.


  • How it works: When heavy rainfall completely saturates a hillside, the soil loses its grip on the bedrock underneath. It turns into a thick liquid ("slurry") and rushes downhill like a river.


  • Real-world impact: Block roads, destroy crops, and can bury entire villages at the base of mountains.


4. Bog Bursts (Very Fast & Wet – Unique to Ireland! )

  • What it is: A specific type of mudslide that occurs in upland blanket bogs when a thick layer of peat becomes completely waterlogged and bursts downhill.


  • How it works: Heavy autumn rain fills the peat bog like a giant sponge. The water trapped under the surface cannot drain away. The pressure builds up until the bog "bursts," sending a torrent of black peat mud down the mountainside.


  • Irish Example: The famous Meenbog Bog Burst (Co. Donegal, 2020) where peat rolled down a hillside, knocking over wind turbines and polluting local rivers.