External geodynamics: weathering, erosion, transport and sedimentation
External geodynamics covers the breakdown of rock at the surface, the removal and transport of the debris, and its deposition and lithification. The PAU asks you to separate weathering from erosion, to infer the transport agent from texture, and to interpret sedimentary structures and facies.
Weathering
- Physical: freeze–thaw shattering, salt crystallisation, thermoclasty and exfoliation, root wedging. Clasts form without mineralogical change.
- Chemical: dissolution (limestone in carbonated water), hydrolysis (feldspar to clay), oxidation (Fe²⁺ to Fe³⁺, giving red soils) and carbonation. Hot humid climates favour chemical work.
- Soil is the final product of weathering plus organic matter.
Erosion, transport and deposition
- Gravity, water, wind, ice and waves remove material; mass movement (creep, slides, flows) is a distinct step.
- Rivers carry bed load (traction, saltation), suspended load and dissolved load. The Hjulström curve shows that clay needs higher velocities to be eroded than sand, yet stays in suspension once disturbed.
- Rounding increases with distance and abrasion; sorting reflects how consistent the agent is. Glacial till is angular and unsorted, dune sand is well rounded and well sorted, fluvial gravel is rounded and moderately sorted.
- Structures: graded bedding (turbidites, fining upward), cross-bedding (dune migration, palaeocurrents), ripples, mud cracks (emergence) and bioturbation.
- Diagenesis - compaction, cementation and recrystallisation - turns sediment into rock.
Worked example
A sandstone body fines upward, shows parallel then ripple lamination and is capped by mudstone with marine fossils. Reasoning: the fining-upward sequence and current structures record waning flow, so this is a turbidite deposited by a density current in deep water, not a beach or a dune field; the mudstone marks a return to quiet pelagic settling.
Traps that cost marks
- Calling weathering erosion; erosion implies transport.
- Reading well-rounded grains as long transport without checking sorting.
- Confusing cross-bedding with graded bedding.
- Assuming karst is only underground: lapiaz, dolines and simas are expressions of the same limestone dissolution.