Sources of energy and sediment in hot deserts; weathering, mass movement, aeolian and fluvial processes; the landforms of erosion and deposition; and the origin of desert landscapes.

What this dot point is asking

AQA section 3.1.2 wants you to explain how hot desert landscapes develop: the sources of energy and sediment, the weathering and mass-movement processes that prepare material, the aeolian (wind) and fluvial (water) processes that shape the land, and the erosional and depositional landforms they produce. The surprise for many students is how much water matters in deserts despite the aridity.

Sources of energy and sediment

Desert landscapes are high-energy in pulses. The energy comes from intense daytime insolation (driving large diurnal temperature ranges and thermal stress), strong, persistent winds (often unobstructed by vegetation), and rare but intense rainfall that delivers huge erosive energy in a few hours. The sediment comes from weathered bedrock, material delivered by mass movement down steep slopes, and sediment carried in by episodic floods and wind from beyond the desert.

Weathering and mass movement

Weathering prepares loose material. Mechanical (physical) weathering dominates: thermal fracture (exfoliation and granular disintegration) as large daily temperature ranges stress rock, salt crystallisation in pores, and limited freeze-thaw at altitude. Chemical weathering is slow but not absent, leaving features such as desert varnish. Mass movement then delivers the weathered debris downslope as rockfall and scree, supplying the sediment that wind and water rework.

Aeolian processes and landforms

Wind erodes by deflation (removing loose sand and dust) and abrasion (sandblasting near the ground). Transport is by suspension (fine dust), saltation (sand grains bouncing) and surface creep (larger grains rolled).

Erosional landforms include deflation hollows (where wind scours out fines), ventifacts (faceted pebbles), yardangs (streamlined ridges aligned with the wind) and zeugen (mushroom rocks undercut by near-ground abrasion). Depositional landforms are the dunes: barchans (crescentic, horns downwind, unidirectional wind, limited sand), seif/linear dunes (winds from two directions), star dunes (multidirectional winds) and loess (fine dust deposited far downwind). Dunes migrate as sand erodes from the windward face and deposits on the steeper slip face.

Fluvial processes and landforms

Despite aridity, water shapes much of the desert surface. Rare, intense storms produce flash floods that, with little vegetation to slow them, generate high-energy, sediment-laden flows. These cut wadis (steep-sided, normally dry channels) and canyons. Where flow leaves a confined channel onto a plain and loses energy, it builds alluvial fans; coalescing fans form a bahada. Water draining to enclosed basins forms temporary lakes that evaporate to leave salt-crusted playas (salt flats). An inselberg (isolated steep-sided hill) and pediment (gentle rock slope at a mountain foot) reflect long-term backwearing under both processes.

Try this

Q1. Name the three ways wind transports desert sediment. [3 marks]

• Cue. Suspension (fine dust), saltation (bouncing sand) and surface creep (rolled coarser grains).

Q2. Explain how an alluvial fan forms. [4 marks]

• Cue. A flash flood leaves a confined channel onto a plain, loses energy and gradient, so it deposits its coarse load in a fan shape at the mountain foot.

Q3. Distinguish between a barchan and a star dune. [2 marks]

• Cue. A barchan is crescentic, forming under a unidirectional wind with limited sand; a star dune has multiple arms, forming under multidirectional winds.