External Geodynamics: Weathering, Erosion, Transport, Sedimentation and Sedimentary Environments
Domain 2 examines the exogenous cycle: how rock is broken down, moved and deposited, and how the resulting sediments record the environment. Island settings make this examinable through basalt weathering, barranco flash-flood transport, aeolian dunes and volcaniclastic marine deposits.
Weathering: disintegration before transport
Physical weathering: freeze–thaw, salt crystallisation (critical on coastal lava), thermal expansion, unloading and biological wedging. Chemical weathering: hydrolysis (feldspar → clay), oxidation (Fe²⁺ → Fe³⁺, the reddish 'caliche' and almagre tones), carbonation and dissolution. Basalt weathers fast to smectite and palagonite; quartz-rich rocks resist. Weathering produces regolith, which soil formation then profiles.
Erosion, transport and deposition
Agents: running water, wind, waves and currents, ice (absent in the Canaries today but present in summit periglacial zones), and gravity (rockfalls, debris flows). The Hjulström curve relates grain size to entrainment, erosion and deposition velocity. Transport modes: traction (bedload), saltation, suspension, and solution. Rounding and sorting increase with transport distance and time; maturity also requires compositional stability (quartz survives, olivine does not).
Sedimentary environments
Continental (fluvial, aeolian, lacustrine, glacial), transitional (deltaic, beach, tidal flat, dune field such as Maspalomas), marine (neritic reef, pelagic, turbidite fan). Each environment has diagnostic structures: cross-bedding in dunes, graded bedding in turbidites, ripple marks, mud cracks, bioturbation. Sedimentary rocks: clastic (conglomerate, sandstone, lutite), chemical and biochemical (limestone, evaporite, chert), plus volcaniclastic (tuff, agglomerate, hyaloclastite).
Worked example
A sequence shows: angular volcanic clasts in a coarse matrix, fining upward into cross-bedded sands, then horizontally laminated silts with plant remains. Reasoning: angular clasts plus poor sorting = short transport, near source, likely flash-flood debris flow in a barranco; the fining-upward and cross-bedding record waning fluvial traction; the laminated silts with plant debris indicate a low-energy floodplain or pond. The whole set is a fining-upward alluvial sequence, not a marine one - no marine fossils or wave structures.
Traps that cost marks
Confusing weathering (in situ) with erosion (removal). Saying 'rounding proves long transport' without noting that soft grains round quickly. Describing graded bedding as cross-bedding. Forgetting volcaniclastic rocks in island settings.
- Angular, poorly sorted sediment = short transport, high energy.
- Maturity = well sorted, well rounded, quartz-rich.
- Graded bedding → turbidites; cross-bedding → dunes or channels.
- Hydrolysis of feldspar produces clay; oxidation of iron produces red colour.
- Barrancos deliver coarse volcaniclastics in flash floods to alluvial fans and the sea.