External Landform Processes
Senior 2 · Geography and Environment
External Landform Processes
Lesson 3 of 3
22 min
On the night of 2–3 May 2023, after days of rain, hillsides in Rubavu, Rutsiro, Ngororero, Nyabihu and Karongi gave way. Mud and rock swept over homes and roads, and at least 130 people died. The governor of the Western Province explained that the soil was already soaked from earlier rain. The movement of whole slopes downhill under gravity is called mass wasting, and in a country of steep, crowded hills it is one of the deadliest natural hazards.
Mass wasting (mass movement) is the downhill movement of weathered rock and soil under the pull of gravity, often helped by water. Unlike erosion, the material is not carried by an agent such as a river; it moves as a mass.
| Speed | Form | Description |
|---|---|---|
| Slow | Soil creep | Very slow movement of soil down a slope, seen in leaning trees, fences and walls and in small ridges (terracettes). |
| Slow | Solifluction | Waterlogged soil flowing slowly downhill, usually over frozen ground on cold mountains. |
| Rapid | Landslide | A large mass of soil and rock sliding suddenly down a slope along a surface of weakness. |
| Rapid | Slump | A block of soil slides down with a rotating, backward-tilting movement along a curved surface. |
| Rapid | Mudflow | Soil so full of water that it flows like a thick liquid, often along valleys. |
| Rapid | Rockfall | Rock fragments falling freely from a cliff or steep road cut. |
Link each cause to the evidence.
A disaster needs both a hazard (the landslide) and people in its way. Reducing either reduces the deaths.
Tip
Exam technique. Separate triggers (heavy rain, earthquakes) from underlying causes (steep slopes, deep weathering, deforestation). An answer that lists both, with a Rwandan example, shows understanding rather than memory.
Solution: the downhill movement of weathered rock and soil under gravity.
Solution: soil creep.
Solution: water adds weight to the soil and reduces friction between particles, so the slope can no longer hold.
Solution: any three of: relocate families from high-risk zones; terraces and drainage on slopes; plant trees; retaining walls on road cuts; early warnings.
Summary
Sources and credits
The rainfall figures for May 2023 come from the IFRC's final report on the Rwanda floods (reported facts, not copied text). The worked example and practice items are written by Ganzaa.
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
