External geological processes: weathering, erosion, transport, sedimentation and landforms
Domain 5 - External geological processes. The exam expects a clear chain from weathering through erosion, transport and deposition to landforms, with the agent (water, ice, wind, gravity) correctly identified.
Weathering
- Physical: freeze–thaw, salt crystallisation, unloading/exfoliation, thermal fatigue, root wedging. Produces regolith and joint-blocky terrain.
- Chemical: hydrolysis of feldspars to clays, oxidation of iron, carbonation and solution of limestone, hydration. Fastest in warm, humid climates.
- Controls: climate, lithology, relief, vegetation and time. Weathering is in situ; nothing is removed.
Erosion, transport and deposition
- Agents: running water, ice, wind, waves and gravity. Erosion removes; transport moves; deposition ends the journey.
- The Hjulström curve: clay needs high velocity to be entrained because grains cohere, sand is easiest to move, gravel needs fast flow; settling velocity rises with grain size and density.
- Rounding and sorting increase with transport distance; glacial till is the classic poorly sorted, angular deposit; wind transport produces well-rounded, frosted, well-sorted sand.
- Sedimentary structures as palaeoenvironmental indicators: cross-bedding (dunes, channels), graded bedding (turbidites), ripple marks (flow direction), mudcracks (subaerial exposure), bioturbation.
Landforms
- Fluvial: V-shaped valleys, meanders, oxbow lakes, terraces, alluvial fans, deltas, peneplains.
- Karst: carbonic acid dissolution of limestone gives dolines, poljes, caves, stalactites and sinking streams.
- Glacial: cirques, arêtes, U-valleys, moraines, erratics, till plains.
- Coastal and aeolian: cliffs, wave-cut platforms, spits and bars, dunes, yardangs, barchans.
Worked example
A river leaves a mountain front and spreads gravel, then sand, then mud across a plain. The gravel is rounded but poorly sorted (short, high-energy transport), the sand is better sorted with cross-bedding, and the mud is deposited from suspension. The vertical sequence coarsens downward if the river later cuts into the fan; the grain-size trend plus sorting lets you infer transport distance and energy.
Traps
- Using weathering and erosion as synonyms.
- Claiming ice is a good sorting agent - meltwater sorts, the ice itself does not.
- Reading current direction from symmetric ripples instead of asymmetric ones.
- Forgetting that karst requires jointed, soluble rock and a water table.