Geomorphological Processes (AQA A-Level Geography): Revision Notes

Geomorphological processes

Hot desert landscapes are constantly shaped by various geomorphological processes. As these systems develop over time and move towards dynamic equilibrium, several processes work together to change the characteristics of desert landforms. These processes are driven by energy inputs and operate through weathering, mass movement, erosion, transportation, and deposition.

Weathering

Weathering is the breaking down of rock material in its original location (in situ). Unlike erosion, weathering does not involve moving the broken material away. Instead, it produces loose, broken material called regolith.

Weathering in hot deserts can be divided into three main types:

• Mechanical weathering: The physical breakdown of rock through processes like thermal expansion and contraction, freeze-thaw action, and salt crystallisation
• Chemical weathering: The decay of rock through chemical reactions, such as oxidation and hydration
• Organic weathering: The breakdown of rock caused by biological activity, though this is limited in hot deserts due to sparse vegetation

Mass movement

Mass movement describes how material moves downhill under the force of gravity. Rainfall can also assist this process, but in hot deserts mass movement is limited because:

• Moisture levels are very low
• There is little vegetation to destabilise slopes
• Limited soil development means less material available to move

Where mass movement does occur in hot deserts, it is dominated by two main processes:

Rock falls

Rock falls happen when small blocks of rock become detached from an exposed cliff face and fall freely to the base of the cliff. This commonly occurs in steep, solid rock cliffs.

Vertical jointing is often a key feature in rocks where rock falls occur. When vertical joints (cracks) run through the rock, and the cliff is underlain by a less resistant rock layer, the structure becomes unstable. The weaker rock underneath is eroded away, leaving the harder rock above unsupported. Eventually, blocks of rock break away along the joints and fall to the cliff base.

Rock slides

Rock slides occur when there is a failure throughout the entire rock mass. Rather than individual blocks falling, the material collapses en masse (as a whole body) instead of as separate pieces.

Both rock falls and rock slides create steep slopes with coarse, angular debris (called talus or scree) accumulating at the base. This debris collects on gentle slopes below and at the foot of steep cliffs.

Erosion

Erosion refers to processes that wear away the land surface through mechanical action. The key agents of erosion include glaciers, rivers, wind, and ocean waves and currents. In hot deserts, only wind and water perform this role.

Erosion causes denudation, which means the degradation and wearing down of the Earth's surface.

Transportation

Transportation is the process of moving material from where erosion occurred to where it will be deposited. In hot deserts, the two main agents of transportation are moving water and wind.

The material being transported is called the load. Water generally transports its load through four main processes:

Traction

Larger stones and boulders are rolled along the channel bed by the movement of water flowing downstream. This process requires high levels of energy and discharge (the volume of water flowing).

Saltation

Smaller stones and pebbles are bounced along the channel bed in a leaping motion. The water needs high discharge and energy levels to pick up (thrust) small particles into the water column. However, the flow is not fast enough to keep particles off the bed, so they fall back down after moving a short distance downstream.

Particles landing on the bed can dislodge other particles, pushing them up into the moving water. This creates a repeating bouncing action as material moves downstream.

Suspension

The smallest particles (generally silt and clay) are carried along within the moving water column by the flow itself. The turbulence of moving water allows rivers to pick up this fine load and carry it downstream. These finest particles are only deposited when the water becomes almost stationary.

Solution

Dissolved minerals are transported within the mass of moving water itself. These minerals remain dissolved and invisible as they move with the water flow.

Deposition

Since moving water and wind are not continuously present in hot desert environments, depositional features are very common. Both water and wind deposit their load when they experience a reduction in the amount of material they can carry. This happens through:

• A reduction in energy levels
• A reduction in capacity (the maximum amount of material that can be transported)

With a loss of energy, both water and wind deposit their load. A common cause of deposition in hot deserts is evaporation. Water and streams simply dry up, leaving their load behind on the desert surface.