Mass movement events occur when slope stability is reduced and gravity causes material to move downhill. This topic is central to IB Geography, particularly within Option D: Geophysical Hazards. While gravity is always acting on slopes, mass movement only occurs when resisting forces are weakened or driving forces increase. A combination of physical and human factors usually triggers these events.

One of the most important triggers of mass movement is water. Heavy or prolonged rainfall increases the weight of slope material, making it more likely to move downhill. Water also reduces friction between particles by acting as a lubricant. When soils become saturated, pore water pressure increases, lowering slope stability and triggering landslides, mudflows, or slumping. Rapid snowmelt can have a similar effect in cold environments.

Slope angle is another key factor. Steeper slopes are more vulnerable because gravity exerts a greater downslope force. While gentle slopes can still experience mass movement, especially slumping or soil creep, steep slopes are more likely to experience sudden and rapid events such as rockfalls and landslides. In IB Geography, slope angle is often linked to relief and tectonic uplift.

The geology and structure of the slope strongly influence mass movement. Permeable rock layers overlying impermeable layers are particularly unstable because water accumulates at the boundary, creating a potential slip plane. Rock type also matters; weak or unconsolidated materials such as clay are more prone to failure than hard, resistant rock. Faults, joints, and bedding planes create weaknesses that allow movement to occur more easily.

Weathering increases the likelihood of mass movement by weakening rock and soil over time. Freeze–thaw weathering breaks rock apart in cold environments, increasing the risk of rockfalls. Chemical weathering weakens rock structure in warm, wet climates. These processes gradually reduce slope stability until a trigger event occurs.

In IB Geography, human activity is recognised as a major trigger of mass movement. Deforestation removes vegetation that binds soil together, increasing erosion and instability. Construction, road building, and mining can undercut slopes or add weight to them, reducing stability. Poor drainage systems can increase water infiltration and saturation, triggering slope failure.

Earthquakes are another important trigger. Ground shaking reduces friction and cohesion within slope materials, causing landslides even on slopes that were previously stable. Earthquake-triggered landslides often cause more damage than the earthquake itself, especially in mountainous regions.

Overall, mass movement events are triggered when factors such as water, slope angle, geology, weathering, earthquakes, and human activity reduce slope stability. In IB Geography, strong answers recognise that mass movement is rarely caused by a single factor but by the interaction of multiple triggers.

RevisionDojo helps IB Geography students understand mass movement triggers clearly, linking physical processes and human actions into confident, exam-ready explanations.