EVSC10001 · The Global Environment
Weathering, Soils & Biogeography
This chapter covers how rock breaks down in place by physical (mechanical, size only) versus chemical (mineral-altering) weathering, how those products build a layered soil profile by pedogenesis, and how climate then sorts vegetation into the Köppen–Geiger biomes. It matters because EVSC10001 Part A tests the weathering-to-soil-to-biome chain hard, and every short-answer demands a clearly labelled diagram — so knowing the O–A–E–B–C–R horizons and the climate rules is worth real marks.
What this chapter covers
- 01Weathering vs erosion (in-situ breakdown vs transport)
- 02Physical weathering: frost, salt, thermal, exfoliation
- 03Chemical weathering: hydrolysis, dissolution, oxidation, carbonation
- 04Controls on rate: mineral stability, surface area, climate
- 05Pedogenesis & the five soil-forming factors (cl,o,r,p,t)
- 06Soil horizons O–A–E–B–C–R: leaching vs illuviation
- 07Köppen–Geiger zones A–E and aridity thresholds
- 08Altitude lapse rate & fire regime (pyrophobic vs pyrophytic)
Sketch and label a soil profile
- +1Draw a vertical column with a downward depth arrow on one side; this is the profile framework where horizons stack most-organic/most-weathered at the top to fresh bedrock at the base.
- +2Label the horizons in order top-to-bottom: O (organic litter/humus), A (topsoil, organic + mineral mix), E (pale leached/eluvial layer), B (subsoil zone of accumulation), C (weathered parent material), R (unweathered bedrock).
- +1Annotate the E horizon as the site of leaching (eluviation) — clay, Fe and soluble ions are washed OUT, leaving it pale and depleted.
- +1Annotate the B horizon as the site of illuviation — the same clay/Fe is deposited IN lower down, so the B is enriched and often redder or clay-rich; one line noting soil forms in situ by pedogenesis, not by deposition.
Key terms
- Weathering
- The in-situ breakdown and decomposition of rock by contact with the atmosphere, water and organisms, with no transport of the material (transport is erosion).
- Hydrolysis
- The most important chemical-weathering reaction: silicate minerals such as feldspar react with weak carbonic acid to form clay minerals (e.g. kaolinite) plus silicic acid and dissolved cations.
- Regolith / saprolite
- The broken, partly chemically-decomposed mantle of rock material left in place by weathering; saprolite is deeply weathered bedrock that still preserves the original rock fabric.
- Pedogenesis
- The set of soil-forming processes — additions, transformations, translocations and losses — that build a soil in place over time from parent material; soil is made, not deposited.
- Illuviation
- The deposition of clay, iron oxides and soluble material into a lower horizon (typically the B horizon), having been leached (eluviated) out of the overlying E horizon.
- Köppen–Geiger classification
- An empirical climate scheme that sorts the globe into five main zones — A tropical, B arid, C warm-temperate, D snow/continental, E polar — using temperature and a precipitation (dryness) threshold to predict the broad biome.
Weathering, Soils & Biogeography FAQ
What is the difference between weathering and erosion?
Weathering is the breakdown of rock where it sits (in situ) — loosening and decomposing it in place. Erosion is the removal and transport of that loosened material by water, wind, ice or gravity. A boulder cracked by frost is weathering; the fragments washing downslope is erosion.
How do physical and chemical weathering differ?
Physical (mechanical) weathering changes only the SIZE of the rock — it breaks it into smaller pieces with no change in composition. Chemical weathering changes the MINERALS themselves, turning primary minerals into new clays plus dissolved ions. They reinforce each other: physical break-up creates the surface area that lets chemical reactions proceed faster.
What are the soil horizons in order?
Top-to-bottom they are O (organic litter), A (topsoil), E (leached/eluvial, pale), B (subsoil, zone of accumulation/illuviation), C (weathered parent material) and R (unweathered bedrock). The E sits between A and B and is the pale, leached one — a common point students lose marks on.
Why is chemical weathering fastest in the tropics?
Chemical weathering rate rises with BOTH temperature and moisture, so warm, wet equatorial climates drive the deepest weathering — producing thick regolith, abundant kaolinite clay and, at the extreme, residual iron/aluminium oxides (laterite). Cold or dry zones are dominated instead by slow physical weathering and thin regolith.
Why does a single mountain hold several biomes?
Temperature falls with altitude at an environmental lapse rate of about 5.5–6.5 °C per 1000 m, so climbing a slope stacks cooler climate zones one above another — the same gradient you would cross travelling toward the pole, compressed into a few kilometres. Soil and the fire regime then fine-tune which vegetation occupies each band.
Exam move
Treat this chapter as one cause-and-effect chain — climate drives weathering, weathering builds the soil, soil plus climate set the biome — because Part A rewards linking the three rather than listing facts. Drill the two distinctions the markers test first (weathering vs erosion; physical = size only vs chemical = new minerals), then memorise the O–A–E–B–C–R order cold, keeping E as the pale leached layer between A and B. Since every Part A short-answer is scored on a labelled diagram, practise drawing three by hand under a 10-minute clock: the vertical soil profile (horizons + leaching/illuviation arrows), the pole-to-equator regolith-depth/weathering-type profile, and a Köppen climograph — and always add a depth or latitude axis so the sketch reads at a glance.
