Key Terms: Desert Systems & Processes (AQA A Level Geography): Revision Note

Sources of energy and sediment - key terms

Aeolian transport – The movement of sediment by wind, often from outside the desert margins, contributing to sediment supply.

Exogenous rivers – Large, permanent rivers that originate outside of the desert and maintain their flow despite crossing arid land due to high discharge (e.g. the Nile).

Harmattan winds – Powerful, dust-laden winds blowing from the Sahara over West Africa between November and April.

Insolation – The amount of solar radiation received by a surface; high in deserts due to lack of cloud cover.

Mass movement – The downslope movement of rock and soil due to gravity, contributing sediment to desert systems.

Precipitation runoff – Water flow over the surface following intense but infrequent storms; a key energy input for erosion and sediment transport.

Sediment budget – A calculation of the balance between sediment added to and removed from a desert system.

Sediment cell – A subsystem of sediment movement within the desert involving inputs (sources), transfers, stores (sinks), and outputs.

Sediment sources – Origins of desert sediment such as weathered rock, river deposits, wind-borne material, and erosion of exposed surfaces.

Weathering processes - key terms

Block disintegration – A process where jointed rocks (e.g. limestone) break into blocks due to natural weaknesses.

Crystal growth – A chemical weathering process in which salts left by evaporated saline water expand in rock pores, causing disintegration.

Freeze–thaw weathering – Occurs in semi-arid regions when water enters cracks, freezes, expands, and eventually breaks the rock apart.

Granular disintegration – A weathering process where individual mineral grains in rock break apart, common in rocks with differently coloured minerals.

Hydration – A type of chemical weathering in which minerals expand when absorbing water, leading to stress and fracturing of rock.

Pressure exfoliation – Also known as pressure release; occurs when overlying material is removed and the exposed rock fractures and peels off in layers.

Thermal fracture – Also called onion-skin weathering; daily temperature changes cause expansion and contraction of rock, leading to cracking.

Role of wind in hot deserts - key terms

Abrasion (by wind) – Sand carried by the wind wears away rock surfaces like sandpaper, usually within a metre of the ground.

Attrition (by wind) – Sediment particles collide during transport, breaking into smaller, more rounded pieces.

Creep – A method of aeolian transport where larger particles roll along the surface due to wind drag.

Deflation – The removal of fine, loose sediment by wind, often forming hollows and leaving behind coarser material as desert pavement.

Desert pavement – A hardened desert surface formed after deflation removes finer material, leaving tightly packed coarse particles.

Deposition (by wind) – Occurs when wind velocity falls below the threshold to carry particles, allowing sediment to settle.

Saltation – A transport process where medium-sized particles bounce close to the ground in response to wind energy.

Suspension – The transport of fine particles (like dust) high into the atmosphere by strong winds; can lead to dust storms.

Role of water in hot deserts - key terms

Alluvial plains – Lowland areas where sediment is deposited by rivers or flash floods from surrounding highlands.

Bank erosion – The removal of sediment from riverbanks by flowing water.

Corrosion (by water) – Chemical erosion caused by the dissolving action of acidic water on certain rock types.

Ephemeral rivers – Temporary rivers (wadis, arroyos) that flow only after rainfall or snowmelt and are powerful agents of erosion.

Gully erosion – Severe erosion caused by concentrated runoff carving deep channels in the landscape.

Hydraulic action – Erosion by the force of water itself breaking away rock or sediment.

Overland flow – Water flowing over the land surface due to low infiltration rates, common in deserts due to compacted soils.

Rill erosion – Minor erosion where water creates small channels in the soil surface.

Sheetwash – A form of runoff where water flows in thin, even sheets across the land surface during heavy rainfall events.

Splash erosion – Erosion caused by raindrops hitting bare soil, loosening and displacing particles.

Traction (by water) – A form of river transport where large sediment rolls along the riverbed.