Processes and Landforms in Tropical Environments (Cambridge AS & A‑Level – Topic 7)
1. Tropical Climates (Syllabus 7.1)
1.1 Geographical Position and General Characteristics
- Located between 23.5° N and 23.5° S (the tropics).
- Mean annual temperature > 20 °C; small annual temperature range (≈ 5 °C) but relatively large diurnal range in some regions.
- High annual rainfall (≥ 1 500 mm) with a pronounced seasonality in many areas.
1.2 Köppen–Geiger Sub‑types
| Code | Climate type | Rainfall pattern | Typical region |
|---|
| Af | Equatorial (humid‑tropical) | Rainfall distributed evenly throughout the year; no dry month. | Amazon Basin, Congo Basin |
| Am | Monsoon‑influenced humid‑tropical | Very wet summer, short “dry‑short” month. | Western Ghats (India), parts of SE Asia |
| Aw / As | Seasonally humid‑tropical (savanna) | Distinct wet season (summer) and dry season (winter); dry season ≥ 3 months. | Sahel, northern Brazil, northern Australia |
1.3 Climate Drivers
- Inter‑tropical Convergence Zone (ITCZ) – migrates north‑south with the Sun’s zenith point; brings intense convection and heavy rain when overhead.
- Monsoon systems – continental response to the seasonal shift of the ITCZ; produce a wet summer and a dry winter over large land masses.
- El Niño‑Southern Oscillation (ENSO) – inter‑annual variation in Pacific sea‑surface temperatures; can cause droughts (e.g., Indonesia) or floods (e.g., East Africa).
1.4 Required Graphical Knowledge (AO1)
- Annual temperature diagram – flat curve, small range.
- Annual precipitation diagram – distinguishes Af/Am (no dry month) from Aw/As (pronounced dry season).
- ITCZ migration diagram – shows north‑south movement over the year and associated rainfall belts.
2. Weathering in Tropical Regions (Syllabus 7.2 – Processes)
2.1 Physical (Mechanical) Weathering
- Thermal expansion & contraction – large diurnal temperature swings cause stress in rock surfaces, especially on exposed cliffs.
- Hydraulic action – intense rain‑fall and rapid surface runoff exploit joints, widen fissures and can undercut granite cliffs, leading to rock‑fall.
- Freeze‑thaw – limited to high‑altitude tropical zones (e.g., Andes, East African highlands).
- Biological mechanical weathering – root wedging, burrowing animals, and lichen “pennate” growth physically pry rocks apart.
2.2 Chemical Weathering
- Hydrolysis – feldspar + H₂O → clay minerals (kaolinite) + soluble ions; dominant in warm, moist conditions.
- Oxidation – Fe²⁺ → Fe³⁺ + O₂ → Fe‑oxides (laterite, red‑brown soils).
- Dissolution (carbonic‑acid weathering) – CO₂ + H₂O ↔ H₂CO₃; H₂CO₃ + CaCO₃ → Ca²⁺ + 2 HCO₃⁻ (key for limestone).
- Solution of silicates – minor in granites but can occur along fractures where acidic water is present.
2.3 Biological Weathering
- Root wedging – expanding roots exert pressure on joints.
- Organic acids – lichens, mosses and microbial colonies release citric, oxalic and other acids that accelerate hydrolysis and dissolution.
- Bioturbation – earthworms, insects and small mammals expose fresh rock surfaces and increase aeration.
2.4 Linking Weathering to Climate (Cause‑and‑Effect Chains – AO3)
- High temperature → faster chemical reaction rates (Arrhenius equation).
- Abundant rainfall → continuous supply of water for hydrolysis, oxidation and carbonic‑acid dissolution.
- Dense vegetation → production of organic acids + root wedging → enhanced chemical & mechanical breakdown.
- Rapid breakdown → formation of deep, nutrient‑rich lateritic soils (oxisols) → supports rainforest vegetation → maintains high humidity → reinforces the weathering cycle.
3. Granite Landforms in Tropical Areas (Syllabus 7.2 – Granite)
3.1 Typical Granite Landforms
- Bornhardts – massive, dome‑shaped inselbergs with steep sides (e.g., Uluru, Australia).
- Kopjes – isolated, often rounded granite outcrops scattered across savanna plains (e.g., Serengeti, Tanzania).
- Tors – clusters of exposed boulders formed by differential weathering along joint sets.
- Domes – rounded hills where outer layers have been stripped, exposing a resistant core.
- Pavements – flat, exposed surfaces where weathering leaves a residual network of clasts and fissures.
- Mesas – flat‑topped hills with steep sides; a granite caprock protects underlying softer strata.
3.2 Formation Processes (Step‑by‑step)
- Jointing during cooling creates planes of weakness.
- Physical weathering (thermal stress, hydraulic action) widens joints.
- Chemical weathering – hydrolysis of feldspar weakens the matrix, especially along joints; oxidation produces lateritic coatings.
- Biological activity – root wedging and organic acids further enlarge fractures.
- Differential erosion removes the less resistant material, leaving the more resistant granite core as tors, bornhardts, etc.
- Scale of expression – local (tors, kopjes) vs regional (inselberg fields, extensive bornhardts).
3.3 Additional Weathering Factors Specific to Granite
- Minor solution of quartz and feldspar in highly acidic water (important on steep cliffs).
- Enhanced hydraulic action during tropical thunderstorms that can undercut cliff bases, leading to rock‑falls and the creation of talus slopes.
4. Limestone Landforms in Tropical Areas (Syllabus 7.2 – Limestone)
4.1 Chemical Basis of Karst Development
Calcite dissolution is driven by carbonic acid formed when atmospheric CO₂ dissolves in rainwater:
CO₂ + H₂O ⇌ H₂CO₃
H₂CO₃ + CaCO₃ ⇌ Ca²⁺ + 2 HCO₃⁻
In tropical climates the high temperature increases CO₂ solubility in water and accelerates the reaction.
4.2 Karst Types (Sub‑aerial vs Marine)
- Sub‑aerial karst – develops on land; includes cone, tower and cockpit karst.
- Marine karst – forms at the sea‑shore where wave action enlarges solution cavities, producing sea‑caves, blowholes and platform karst.
4.3 Typical Sub‑aerial Karst Forms
- Cone karst – gently sloping, conical hills with shallow depressions (e.g., Guilin, China).
- Tower karst – steep, vertical towers rising from a flat plain (e.g., Ha Long Bay, Vietnam).
- Cockpit karst – deeply dissected, maze‑like terrain with steep‑sided basins (e.g., Madagascar’s “Tsingy”).
4.4 Associated Karst Features
- Sinkholes (dolines) – surface depressions caused by collapse of underground voids.
- Solution valleys – narrow, linear valleys formed by longitudinal dissolution along joints.
- Disappearing streams & springs – surface water enters the subterranean system (disappearing) or re‑emerges (spring).
- Caves & caverns – large underground voids created by progressive dissolution.
- Limestone pavements – flat surfaces with clints (blocks) separated by grikes (fissures).
- Speleothems – stalactites, stalagmites and columns formed from secondary precipitation of calcite.
4.5 Formation Timescales
- Rapid dissolution in warm, humid climates can produce noticeable karst features within 10⁴–10⁵ years.
- Large cave systems may require several hundred thousand years to develop fully.
5. Vegetation, Soils & Ecosystems (Syllabus 7.3)
5.1 Tropical Rainforest Vegetation
- Multi‑layered canopy: emergent trees (30–45 m), main canopy (20–30 m), understory, shrub layer, forest floor.
- Dominant families: Dipterocarpaceae (SE Asia), Fabaceae (Amazon), Moraceae (tropical Africa).
- High species richness – > 300 species ha⁻¹ in some plots; high endemism.
5.2 Tropical Savanna Vegetation
- Grassland matrix with scattered fire‑adapted trees (Acacia, Baobab, Combretum).
- Seasonal leaf‑fall linked to the dry season; grasses are C₄ (high water‑use efficiency).
5.3 Soil Orders and Profiles
| Soil order | Typical profile (A‑E horizons) | Key properties | Typical vegetation |
|---|
| Oxisol |
O – A – Bt – C – R |
Very deep, highly weathered; low CEC, high Fe‑Al oxides; acidic (pH ≈ 4–5). |
Lowland tropical rainforest. |
| Latosol |
O – A – Bt – C – R |
Well‑developed B‑horizon, moderate to high Fe‑Al oxides; pH ≈ 5–6. |
Seasonally humid‑tropical savanna. |
| Tropical red/brown earth |
O – A – B – C – R |
Less weathered; higher base saturation; pH ≈ 6–7; often limed for agriculture. |
Cultivated areas, secondary forest. |
5.4 Soil Fertility Indicators (AO2)
- Organic carbon (C) – proxy for nutrient supply; low in Oxisols, higher in Latosols.
- Electrical conductivity (EC) – indicates soluble salts; generally low in tropical soils.
- pH – controls nutrient availability; acidic soils limit phosphorus and calcium.
5.5 Links Between Vegetation, Soils & Landforms
- Granite‑derived lateritic soils support nutrient‑poor rainforests; thin soils on limestone favour calciphile species and karst‑dependent flora.
- High biodiversity indices (species richness, Shannon‑Wiener) are positively correlated with structural complexity of the canopy and with soil depth.
- Vegetation stabilises soils on steep karst and granite slopes, reducing erosion and limiting the expansion of sinkholes.
6. Changes, Pressures & Management (Syllabus 7.4)
6.1 Main Human Pressures
- Population growth & settlement expansion – encroachment into forested or savanna areas.
- Land‑use change – large‑scale monocultures (oil palm, soy), cattle ranching, shifting cultivation.
- Resource extraction – logging (legal & illegal), mining (bauxite, gold), hydro‑electric dams.
- Fire – natural (lightning) and anthropogenic (clearing, poaching); alters savanna‑forest boundaries.
- Climate‑change impacts – altered rainfall patterns, increased frequency of extreme events, shifts in vegetation zones.
6.2 Evaluating Management Strategies (AO3)
- Define clear objectives – conservation, sustainable use, livelihood improvement.
- Effectiveness – use measurable indicators (forest‑cover change, biodiversity indices, water quality, carbon sequestration).
- Sustainability – assess ecological (e.g., regeneration rates), economic (profitability, cost‑benefit) and social (community support, equity) dimensions.
- Trade‑offs – e.g., reduced timber extraction improves biodiversity but may lower local income; eco‑tourism generates revenue but can increase visitor pressure.
- Participatory approaches – involve local communities, respect indigenous land rights, incorporate traditional knowledge.
- Adaptive management – monitor outcomes, adjust actions in response to new data or changing conditions.
7. Comparative Table: Granite vs. Limestone Landforms
| Aspect | Granite | Limestone |
|---|
| Dominant weathering process |
Physical (thermal, hydraulic) + chemical (hydrolysis of feldspar, oxidation) |
Chemical dissolution by carbonic acid (sub‑aerial and marine) |
| Typical landforms |
Bornhardts, kopjes, tors, domes, pavements, mesas |
Cone, tower, cockpit karst; sinkholes, caves, pavements, speleothems |
| Surface texture |
Rough, blocky, often rounded; joint‑controlled fissures |
Smooth, fissured (clints & grikes); often polished by solution |
| Formation timescale |
Millions of years; slow removal of weathered material |
Millions of years but relatively rapid dissolution in humid climates (10⁴–10⁵ yr for visible features) |
| Drainage pattern |
Surface runoff; well‑developed dendritic networks |
Sub‑surface drainage; disappearing streams, underground rivers |
| Soil development |
Thin, sandy, often lateritic; low fertility, high Fe‑Al oxides |
Thin, calcareous; high pH, prone to erosion when vegetation removed |
8. Case Study: The Amazon Basin (Humid‑Tropical Region)
8.1 Physical Geography
- Area ≈ 5.5 million km²; lowland plateau (100–300 m a.s.l.).
- Climate: mean annual temperature 26 °C; annual rainfall 2 000–3 000 mm, little seasonal variation.
- Geology: Precambrian granitic‑gneissic shield inter‑bedded with sedimentary sandstones and limestone lenses.
8.2 Processes & Landforms
- Intense chemical weathering of granitic bedrock produces deep lateritic soils (oxisols) that support the rainforest.
- Localized limestone outcrops (e.g., Carajás) develop karst features – sinkholes, caves, and solution valleys.
- River incision creates extensive fluvial terraces and alluvial floodplains; tributary valleys expose granite tors in some upland sections.
8.3 Human Pressures
- Deforestation for cattle ranching and soy – ≈ 17 % forest loss since 1970.
- Mining (gold, bauxite) – tailings dams, heavy‑metal contamination of waterways.
- Infrastructure (highways, hydro‑electric dams) – habitat fragmentation, altered river regimes.
8.4 Management & Evaluation
- Protected‑area network – > 60 % of the basin under some protection; effectiveness varies with enforcement and funding.
- Payments for Ecosystem Services (PES) – REDD+ – aims to reward carbon sequestration; evaluation uses carbon stock change, biodiversity monitoring and community benefit distribution.
- Challenges – illegal logging, weak governance, competing economic interests; adaptive management and stronger community participation are essential.
9. Key Concepts for Assessment (Syllabus 7.5)
- Explain how tropical climate (high temperature, abundant rainfall, ITCZ/monsoon dynamics) accelerates physical, chemical and biological weathering.
- Identify and describe the main granite landforms (bornhardts, kopjes, tors, domes, pavements, mesas) and the processes that create them, including cause‑and‑effect chains.
- Describe the formation of limestone karst, distinguishing sub‑aerial (cone, tower, cockpit) and marine karst, and name associated features (sinkholes, caves, speleothems, pavements).
- Compare granite and limestone landforms in terms of weathering mechanisms, surface texture, drainage patterns and soil development.
- Discuss the interaction between vegetation, soils and landforms in tropical rainforests and savannas, using soil‑profile diagrams and biodiversity indicators.
- Analyse the main human pressures on tropical environments and evaluate at least two management strategies using criteria of effectiveness, sustainability and socio‑economic trade‑offs.
- Use accurate geographical terminology and support answers with labelled diagrams (e.g., cross‑section of a granite tor, karst cave system, tropical climate diagram, weathering‑cycle flow chart).
10. Suggested Diagrams for Revision
- Cross‑section of a tropical granite tor showing jointing, weathering fronts, root wedging and talus slope.
- Karst landscape illustrating cone, tower and cockpit karst with sinkholes, underground streams, caves and speleothems.
- ITCZ migration diagram with accompanying annual precipitation graphs for Af, Am and Aw/As climates.
- Flow‑chart of the tropical weathering cycle linking climate, vegetation, physical and chemical weathering.
- Soil profile diagrams for Oxisol, Latosol and tropical red/brown earth, annotated with key properties (pH, CEC, colour).