IB Geography D.1.4 Classification of Mass Movement Types Notes

Mass Movement Begins When Downslope Force Exceeds Slope Strength

1. Factor of safety compares resisting strength with driving stress, and values below one indicate that the slope cannot remain stable under existing conditions.
2. Progressive weakening can move a slope toward failure through weathering, repeated wetting, root decay or slow crack growth before any visible collapse.
3. Threshold behaviour explains why a small final trigger can produce rapid movement after long-term changes have already reduced slope stability.
4. Gravity creates a downslope shear stress, while friction, cohesion and root reinforcement help the slope resist movement.
5. Failure begins when driving stress becomes greater than the shear strength of the slope material.
6. Steeper slopes and added weight increase driving stress, while weathering, saturation and excavation can reduce resistance.
7. A trigger initiates movement, but pre-existing geology and slope condition determine whether the trigger causes failure.

Mass movements form a continuum from falling solid rock to slow creep and water-rich flows.

Falls, Slides, Flows and Creep Move in Different Ways

1. Movement surface distinguishes slides from flows because a slide retains a recognisable failure plane while a flow deforms throughout its moving mass.
2. Material type matters because rock, coarse debris and fine earth respond differently to water content, slope angle and confinement.
3. Transformation can increase runout when a coherent slide breaks apart, mixes with water and enters a channel as a debris flow.
4. Fall occurs when detached rock or debris travels through the air by free fall, bouncing or rolling from a steep face.
5. Slide occurs when material moves as a coherent mass along a distinct failure surface.
6. A rotational slide follows a curved surface and tilts blocks backward, while a translational slide moves along a more planar surface.
7. Flow occurs when water-rich earth or debris deforms internally and moves like a viscous fluid.
8. Lateral spread extends coherent ground sideways above a weak or liquefied layer, often on gentle slopes.
9. Creep is an extremely slow, persistent deformation of soil or rock that bends trees, fences and retaining walls.

Classification Uses Six Dimensions Rather than One Label

1. Cause and trigger should be recorded separately because geology may prepare a slope for decades before rainfall, shaking or excavation initiates movement.
2. Onset and duration determine warning potential because accelerating creep may be monitored while a rockfall can begin and end within seconds.
3. Extent and frequency guide land-use decisions because repeated small failures may justify routine maintenance while a rare large failure may require exclusion.
4. Cause identifies the geological, climatic, seismic or human processes that prepared and triggered the failure.
5. Liquidity places the movement on a continuum from dry, coherent rock to saturated material behaving as a flow.
6. Speed of onset distinguishes sudden failures with little warning from gradually accelerating movement.
7. Duration records whether movement lasts seconds, hours, seasons or many years.
8. Extent describes the area, depth and travel distance of the failure and its deposit.
9. Frequency describes how often similar movement recurs at the site or within the landscape.
10. Two mass movements with the same speed may still differ in cause, water content, duration, extent and recurrence.

Water Can Increase Weight and Reduce Friction

1. Infiltration sequence begins when water enters pores, raises pore-water pressure and lowers effective stress between grains or along a failure surface.
2. Drainage contrast means permeable layers may transmit water toward an impermeable boundary where pressure accumulates and sliding becomes more likely.
3. Antecedent rainfall matters because a storm falling on an already saturated slope can cross a failure threshold more quickly than the same storm after a dry period.
4. Rainfall and snowmelt add water to pores and fractures within slope material.
5. Higher pore-water pressure pushes particles apart and reduces the effective friction holding them in place.
6. Saturation also adds mass, so gravity exerts a larger downslope force.
7. Surface runoff can erode the foot of a slope and remove support from material above.
8. A flow may accelerate and travel farther when water mixes with loose sediment in a confined channel.

Water can turn loose slope material into a faster flow by adding weight and reducing internal resistance.

Earthquakes and Human Activity Can Trigger Prepared Slopes

1. Seismic acceleration briefly increases downslope force while repeated shaking weakens joints and can reduce the strength of saturated sediment.
2. Toe removal destabilises a slope because excavation or river erosion removes support from its lower section and steepens the remaining profile.
3. Reservoir influence can change both loading and groundwater pressure, so rapid filling or drawdown may alter stability around valley sides.
4. Seismic shaking applies rapid stress and can weaken fractured rock, liquefy sediment or dislodge material on steep slopes.
5. The 2008 Sichuan earthquake triggered thousands of landslides across steep terrain and blocked some river valleys.

Sichuan shows how one earthquake can generate a regional landslide field where steep relief and fractured rock are widespread.

1. Road cuts and mining remove support from the slope toe or create steeper artificial faces.
2. Buildings, spoil and reservoirs load a slope, while leaking pipes and irrigation alter its water balance.
3. Deforestation removes root reinforcement and interception, although the effect depends on soil depth, species and time since clearance.

Cutting into a slope for transport can reduce support and expose weak material unless drainage and stabilisation are maintained.

A Classification Profile Supports Hazard Comparison

1. Runout links classification to exposure because faster, wetter flows usually travel farther along channels than slow coherent slides.
2. Monitoring choice follows movement type because inclinometers suit gradual deformation while rainfall thresholds and acoustic sensors may support rapid-flow warnings.
3. Management choice should match process because drainage may lower pore pressure, rock bolts may stabilise blocks and exclusion zones may be safer where failure cannot be prevented.
4. A rockfall is mainly solid, sudden, short-lived and local, but repeated weathering can make it frequent along a cliff.
5. A debris flow is water-rich, rapid and short-lived, but it may extend for kilometres along a channel.
6. Soil creep is relatively solid, very slow, long-lasting and widespread across a slope.
7. A rotational slide may be sudden or progressive depending on rainfall, groundwater and movement along its curved failure surface.
8. This multi-dimensional profile explains warning time, likely travel path and the type of monitoring or protection required.