External Geodynamics and Geological Processes
This domain covers the surficial system that transfers mass from relief to basins: weathering, erosion, transport, sedimentation, soil formation and the hazards that arise when these processes act fast. PAU questions typically ask you to link a climate, a lithology and a landform, or to explain why a deposit is sorted, rounded or chemically mature.
Weathering
- Physical: freeze–thaw (needs water in joints and repeated crossing of 0 °C), salt crystallisation in porous stone, thermal fatigue, unloading/expansion, root wedging.
- Chemical: hydrolysis of feldspar to clay plus dissolved ions; carbonation of limestone producing calcium bicarbonate; oxidation of Fe²⁺ to Fe³⁺ giving red-brown soils; dissolution of halite and gypsum.
- Controls: climate (water and temperature), mineral resistance (Bowen's series read in reverse - quartz survives best, olivine worst), joint density and exposed surface area.
Erosion, transport and sedimentation
- Agents: water, wind, ice, gravity. Transport modes are bedload (traction, saltation), suspended load and dissolved load; the Hjulström curve shows that clay and gravel need higher velocities to be entrained than sand.
- Rounding and sorting increase with transport distance; quartz-rich, matrix-poor sandstones indicate mature, long-transported sediment.
- Depositional environments are read from grain size trends, sedimentary structures (cross-bedding, ripples, graded bedding) and fossil content. Diagenesis (compaction, cementation) converts sediment to rock.
Soils
A profile has O, A, B, C and R horizons. Key exam variables are texture (sand/silt/clay proportions), pH, organic matter, cation exchange capacity (highest in clay and humus) and drainage. In Catalonia, rendzina-type soils sit on limestone, terra rossa on karstified carbonate, and acidic brown soils on siliceous Montseny and Gavarres bedrock.
Worked example
A granite cliff shows unweathered rounded corestones embedded in a sandy, clay-bearing residue. Reasoning: water penetrates joint sets and attacks feldspar by hydrolysis, producing clay and dissolved Na⁺, K⁺ and silica; quartz is inert and remains as grus-sized sand; alteration proceeds inward from joints, leaving intact cores. Prediction: the residual material is clay-rich, quartz-dominated and chemically immature in its clay fraction but mature in its resistant minerals.
Traps that cost marks
Saying erosion and weathering are the same thing; describing limestone as chemically inert; assuming rounding proves long transport without checking composition; confusing soil texture (particle size) with structure (aggregation).