Mass movement Geography A level 2027 spec

Mass movement

Mass movement is when gravitational force exceeds the force of resistance. 

“The downslope movement of rock fragments and soil under the influence of gravity”

It occurs when shear stress exceeds the shear strength of the slope.

Heaves

Solifluction and soil creep

Soil creep

Extremely slow movement, at 1 cm per year

It affects topsoil

Requires moisture to facilitate movement but the soil is not saturated.

Occurs in temperate environments such as the UK

Occurs on very gentle slopes (5 degrees)

Leaves behind little evidence, primarily in the form of terracettes and damage to infrastructure, tilting trees

Solifluction

Solifluction has very slow movement, at a few centimeters per year

It is most common in tundra and alpine biomes. It depends on the aspect of the slope, which way it is facing

It is driven by freeze thaw cycles, which causes water to be trapped, soil cohesion lost and then followed by movement

Solifluction causes lobes and terraces

Flows

Flows are medium fast movements. They have a lot of water content and internal derangement occurs (they lose composition). They leave a scar at the top, from where they came from and a track, which is the route they travelled, and a toe, at the final resting place where they reach. This is compared to slides with no internal derangement

Mudflow

When there is enough saturated water that it mixes with the fine grained soil and causes liquid movement. It is a fast form of mass movement.

Debrisflow

Debrisflow is mostly made up of regolith, variable water content but generally there is some to lubricate the rocks

Earthflow

Regolith and broken down rocks, flowing down slopes of more than 10-15 degrees

Saturated with water, but not as much as mudflow.

Slide

A slide is a sudden movement of land en masse with no internal derangement. The break is often clean, and leaves an exposed scarp. A slide is distinct from a fall as the material remains in contact with the slope for the entire fall. In a fall, the material loses contact with the slope.

Planar slides

The material slides down along a pre-exsisting line of geological weakness, such as a bedding plane or fault line. Typically fast.

There is limited derangement

Rotational slides (slump)

Rather than following a natural rock joint the rotational slide occurs across uniform material. 

This leaves an exposed scarp, rotated terraces, and a toe of the slump. It is slower than a planar slide. 

It is common near to coasts and cliffs. There is more erosion at the bottom of the cliff. This means that “basal undercutting occurs” and the cliff becomes top heavy. Once the support is removed the cliff is forced to rotate downwards. 

Falls

A fall occurs when gravity acts directly on an unloosened block. It is the fastest of all mass movement type, the rock does not maintain contact with the slope. Occurs on slopes greater than 40 degrees. Falls are driven by mechanical failure rather than from high water content. For example if slope is weakened by freeze thaw, if there is undercutting.

This leaves a scree slope known as talus, and a scar, where the rock used to be attached.