The big idea: A hot desert is shaped by three groups of processes. Deserts look 'dry', but water still does a lot of the shaping.
Three groups of processes
- Weathering — rock broken in place.
- Wind action (aeolian) — wind erodes, transports and deposits sand.
- Water action (fluvial) — rare but powerful flash floods.
The big Option C skill: Take a named landform and explain which process made it — and weigh wind against water in a markband essay.
Key terms for desert landforms
- Weathering — the breakdown of rock in place (no movement), by heat, salt or water.
- Aeolian (wind) action — wind erodes (abrasion, deflation), transports and deposits sand.
- Abrasion — wind-blown sand sandblasts rock, wearing it away (a wind-erosion process).
- Deflation — wind lifts and removes loose sand, lowering the surface into a hollow.
- Fluvial (water) action — running water from flash floods erodes and deposits, even in deserts.
- Aeolian deposition — wind drops its sand load, building sand dunes where it slows.
Wind vs water — the deal-breaker: Wind shapes the erosion features (rock pedestals, yardangs, deflation hollows) and builds dunes.
Water (rare flash floods) carves wadis and canyons and deposits alluvial fans and playas. Most desert landscapes are made by both — which is why the [10] essay asks you to weigh them.
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How this is tested: The figure here shows the three controls on which sand dune forms: the amount of sand, the wind, and the vegetation. The data-response asks you to read a value off it.
Short, precise command terms
- Identify [1] the dune type for a given mix of conditions.
- Estimate [1] a percentage off the relevant scale.
Quote the number: Read the figure carefully and quote the number — don't explain yet.
Read the key first. Each bar splits 100% across the three controls — which control is largest for each dune type?
Interactive diagram
Explore the labelled diagram, charts and maps for this topic in full study mode.
Using the figure: (a) identify the dune type whose formation is dominated by wind strength; (b) estimate the percentage contribution of sand supply to a transverse dune.
Model answer plan
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A sand dune forms by aeolian deposition: wind carries sand, hits an obstacle or slows down, and drops its load. The dune then migrates as sand is blown up the gentle windward slope and slips down the steep leeward slope. The type of dune depends on the sand supply, the wind, and how much vegetation anchors the sand.
How to read the sand-wind-vegetation dune graph
| Conditions on the graph | Dune type that forms | Why |
|---|---|---|
| Lots of sand, one steady wind direction | Transverse (ridges across the wind) | Plentiful sand piles into ridges at right angles to the wind |
| Limited sand, strong wind | Longitudinal / seif (long, parallel ridges) | Little sand is stretched into long ridges aligned with the wind |
| Moderate sand, winds from several directions | Star dune (radiating arms) | Shifting winds pile sand into a central peak with arms |
| Some sand + some vegetation to anchor it | Barchan / crescentic (horns point downwind) | Vegetation and an obstacle fix sand; horns trail downwind |
Reading the triangular graph: Each corner = 100% of one control (sand, wind, vegetation). Follow the gridlines from a plotted point back to each axis. To Estimate a value, read where the point sits on that axis (e.g. the lowest sand needed for transverse dunes is about 35%, accept 30-40%).
Using the dune graph: (a) identify the dune type that forms with a strong wind but only a small amount of sand; (b) estimate the lowest percentage of sand needed before transverse dunes form.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.
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Three families of process shape a desert, and each makes its own landforms. Name the process, then the landform it builds.
The three families
- Weathering — breaks rock in place.
- Wind erosion — then sandblasts and lowers the surface.
- Water erosion — rare flash floods carve and deposit.
Weathering processes in a hot desert (and what they do)
| Process | How it breaks rock |
|---|---|
| Exfoliation (thermal) | Huge day-night temperature swings expand and contract the rock surface, so outer layers peel off like onion skins |
| Granular / block disintegration | Different minerals expand by different amounts when heated, loosening grains or whole blocks |
| Salt crystal growth (haloclasty) | Salty water seeps into cracks; as it evaporates, growing salt crystals prise the rock apart |
| Chemical weathering (hydrolysis) | The little dew/water that is present reacts with minerals, slowly rotting and weakening the rock |
Wind-erosion landforms (aeolian)
- Rock pedestal / mushroom rock — abrasion is strongest near the ground (most sand is carried low), so the base is worn thin while the top survives.
- Yardang — abrasion grooves softer rock into long, streamlined ridges aligned with the wind.
- Deflation hollow — deflation lifts and removes loose sand, lowering the surface into a basin.
- Desert pavement — deflation blows the fines away, leaving a surface of stones too heavy to lift.
Water-formed landforms (fluvial)
- Wadi / canyon — a flash flood erodes a steep, dry valley that only holds water briefly.
- Alluvial fan — as a flooded channel leaves the mountains onto flat ground it slows and dumps a fan of sediment.
- Playa / salt lake — flood water collects in a basin and evaporates, leaving a flat salt crust.
- Mesa & butte — flat-topped uplands left when water (helped by weathering) strips the softer rock around a hard cap.
Always pair the landform with its process: Marks come from the process chain, not the name. Wind abrasion is strongest near the ground -> it wears the base of a rock thin -> a mushroom-shaped pedestal. Name the landform, then explain the process step by step.
Explain two distinct weathering processes that operate in hot, arid environments, and how each breaks down rock.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.
Name one hot-desert landform shaped by wind erosion, and explain how the wind forms it.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.
How this is tested - the [10] essay: Paper 1 Option C ends with a 10-mark markband essay, usually weighing water against wind (or asking how important one process is) in shaping desert landforms.
Top band needs
- Accurate terms.
- Two or more landforms, each tied to a process.
- Named deserts as evidence.
- A weighing of the two processes (including over time).
- A clear conclusion.
Named deserts to use as evidence
- Sahara / Sahel — vast ergs (sand seas) of wind-built dunes, but also fossil wadis cut by wetter past climates (pluvials).
- Atacama (Chile) — one of the driest places on Earth; salt playas and salt weathering dominate.
- Death Valley (USA) — classic alluvial fans and the Badwater salt playa from flash floods.
Desert + feature: Name a desert + a feature, not just 'a hot desert'.
"Water, rather than wind, is the main force shaping hot desert landforms." To what extent do you agree?
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.
Markband marks: (1) Tie each landform to its process (wadi <- flash flood; dune <- aeolian deposition). (2) Anchor to a named desert (Sahara, Atacama, Death Valley). (3) Weigh water and wind, then end on an explicit judgement that answers the question.