The big idea: Mass movement is the downhill movement of rock, soil and debris under gravity.
It happens when the driving force (gravity pulling material downslope) becomes greater than the resisting force (friction and the strength holding material in place). When that balance tips, the slope fails and material moves.
Mass movements are a key geophysical hazard in Option D — they range from imperceptible soil creep to a deadly debris flow or rock fall.
Key terms for this micro
- Mass movement — the downslope movement of rock, soil and debris under gravity.
- Slope stability — how resistant a slope is to failing; high friction and strong material = stable.
- Slip surface — the curved or flat plane along which material breaks away and moves.
- Speed of onset — how quickly a hazard strikes; creep is slow, a flow or fall is almost instant.
- Primary hazard — the original event (e.g. an earthquake); a secondary hazard is one it triggers (e.g. a rock fall).
- Trigger — the final factor (heavy rain, an earthquake, undercutting) that tips a slope into failure.
Driving force vs resisting force: A slope fails when gravity wins. Anything that adds weight or water, or removes support or strength, pushes the slope toward failure.
Water is the big one — it adds weight, lubricates the slip surface and reduces friction between grains.
How this is tested: Paper 1 Option D opens with a data-response. You may State or Identify a value or a type off a classification or triangular graph — for example reading a slope-steepness percentage at a plotted point, or naming the type that moves most slowly (soil creep).
You are also asked to read evidence of a mass movement off a diagram — its effect on the human landscape (cracked roads, tilted poles) or the physical landscape (slump scars, dead trees).
| Type | Speed | Water content | What it looks like |
|---|---|---|---|
| Soil creep | Very slow (mm/yr) | Low | Soil edges downhill — tilted poles, curved tree trunks, terracettes |
| Slump | Slow to moderate | High | A block rotates along a curved slip surface, leaving a back scar |
| Slide | Fast | Moderate | Rock or soil slides as a block down a flat slip plane |
| Flow (mud/debris) | Fast to very fast | Very high | Saturated soil and rock flow like a fluid downslope |
| Rock fall | Very fast | Low (often dry) | Blocks detach from a cliff and fall or bounce to the base |
Reading the classification
- Speed runs from very slow creep to very fast flows and falls — read the slowest type as soil creep.
- Water content rises from a dry rock fall to a saturated debris flow — more water usually means faster, more fluid movement.
- Slope steepness is read straight off the axis at the plotted point — quote the percentage value.
Evidence of mass movement on the landscape
- Human landscape — cracked or uneven roads, tilted telegraph poles, cracked house walls, broken fences and pipes.
- Physical landscape — a curved slump back-scar, hummocky ground, surface cracks in the soil, dead or tilted trees.
Read the axis, then the point: On a triangular or classification graph, follow the gridline from the plotted point to the labelled axis and quote the value with no rounding error. For a 'which moves slowest?' read, go to the slow end of the speed axis — that is soil creep.
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Whether a slope creeps slowly or fails in seconds depends on a mix of physical factors (which set how unstable the slope is) and a trigger (which tips it over the edge). Examiners want the mechanism — name a factor, then explain how it speeds the movement or weakens the slope.
Physical factors that speed mass movement
- Water content — heavy or prolonged rain saturates soil, adds weight and lubricates the slip surface, so material moves faster.
- Gradient (relief) — a steeper slope gives gravity a bigger pull, so material accelerates downhill.
- Sediment size and structure — loose, fine or layered material has less internal strength and fails more easily.
- Geology — weak, jointed or clay-rich rock fails faster than solid, well-cemented rock.
- Vegetation — bare slopes have no roots binding the soil, so material moves more freely.
Human activities that destabilise a slope
- Undercutting — cutting into the base of a slope for a road or quarry removes the support holding material against gravity.
- Building on steep slopes — adds weight to the top of the slope, increasing the driving force.
- Deforestation — removing trees takes away the roots that bound the soil, so it loosens and fails.
- Mining and waste heaps — piling loose spoil on a slope adds an unstable, saturated load.
- Disturbing drainage — leaking pipes or blocked drainage soak the slope, raising its water content.
Primary and secondary hazards: A mass movement is often a secondary hazard — one triggered by another event. A rock fall is a secondary hazard when heavy rain or an earthquake shakes a cliff and dislodges blocks that were already loose. The earthquake is the primary hazard; the fall it sets off is the secondary one.
Real triggered mass movements: Nevado del Ruiz (Colombia, 1985) — a small eruption melted the volcano's icecap, sending a lahar (volcanic mud flow) into the town of Armero, killing over 20,000 people. The eruption was the primary hazard; the flow it triggered was the deadly secondary one.
The 2010 Haiti earthquake and the 2011 Tohoku earthquake (Japan) both shook hillsides and cliffs, triggering landslides and rock falls that blocked roads and destroyed buildings — secondary hazards of the quakes.
How this is tested — the structured cluster: Paper 1 Option D tests this micro through short structured questions, not a single essay. The recurring tasks are: Outline a physical factor and how it speeds a movement, Outline a human activity that destabilises a slope, and Outline when a rock fall counts as a secondary hazard.
Each is worth 2 marks: 1 for a valid point, 1 for developing it — so always add the mechanism.
Vargas debris flows, Venezuela (1999): After days of torrential rain, the steep, deforested coastal slopes of Vargas state in Venezuela failed in huge debris flows. Saturated soil and rock flowed off the mountains onto the towns below, killing tens of thousands.
Why it was so deadly: steep gradient + saturated soil + bare slopes + dense settlement on the debris fans — every factor that speeds a mass movement was present at once.
Always give the mechanism: Outline [2] is never just a name. Factor → how it works → effect on the slope. Heavy rain → saturates and lubricates the soil → material moves faster. Half the marks are for the development.