Mass Movement (Slides, Slumps, Rockfalls)

Definition of Mass Movement

Mass movement is the downhill movement of soil, rock, and other materials under the influence of gravity. It commonly occurs on slopes and coastal cliffs where materials become unstable and move downwards. Unlike erosion or deposition, mass movement specifically refers to the movement of material without the direct action of water or wind transporting it away. This means mass movement is a distinct process where gravity alone causes material to move down a slope.

Types of Mass Movement

Slides

Slides happen when a large block of soil or rock moves rapidly down a slope along a flat, clearly defined surface. The movement is usually sudden and fast, often triggered by heavy rainfall or human activity. The material remains largely intact as it moves.

Slumps

Slumps involve a rotational movement where a section of soil or rock moves down a curved slip plane. This causes the material to tilt backwards as it moves, creating a step-like feature on the slope. Slumps are slower than slides and often occur after the ground becomes saturated with water.

Rockfalls

Rockfalls occur when fragments of rock break off from a cliff or steep slope and fall freely through the air. This is a very rapid process and often happens after weathering weakens the rock. Rockfalls can create scree slopes at the base of cliffs.

For instance, on the Jurassic Coast in Dorset, rockfalls are common where weathered limestone cliffs break apart and fall onto the beach below.

Causes of Mass Movement

Saturation by Water: When heavy rain or waves saturate soil and rock, the material becomes heavier and weaker. Water fills the spaces between particles, reducing friction and making slopes more likely to fail.

Weathering: Processes like freeze-thaw weathering weaken rock by causing cracks to widen. Over time, this reduces the strength of the rock, making it easier for pieces to break off or for soil to slip.

Steep Slopes: The steeper the slope, the greater the force of gravity pulling material downwards. Steep cliffs, such as those along parts of the Holderness coast, are more prone to mass movement.

Human Activity: Construction, deforestation, or removing vegetation can destabilise slopes by removing roots that bind soil or by adding weight to the top of cliffs. For example, building roads or houses near cliff edges can increase the risk of slides or slumps.

For example, after heavy rainfall, a saturated cliff with a steep slope and weakened rock is much more likely to experience a slump or slide.

Impacts on Coastal Landscapes

Mass movement changes the shape and stability of cliffs by removing material and causing sections to collapse. This leads to cliff retreat, where the coastline moves inland over time.

These changes can affect habitats by destroying nesting sites for birds or altering plant communities. Human structures such as coastal roads, homes, and tourist facilities can be damaged or lost due to mass movement.

For example, the rapid cliff retreat at the Holderness coast has caused the loss of farmland and threatened villages like Mappleton.

Learning Example

Imagine a cliff slope that is saturated after heavy rain. The soil weighs 2000 kg, and the force of gravity pulling it down the slope is calculated by multiplying mass by gravitational acceleration (9.8 $\mathrm{m\,s^{-2}}$).

Force = mass $\times$ gravity = $2000 \times 9.8 = 19600\,\mathrm{N}$

This large force increases the chance of the soil slipping down the slope, especially if the slope is steep and the soil is weakened by weathering.