G15: Weathering & Soils
Physical, chemical and biological weathering processes, factors affecting weathering rates, and soil formation.
Weathering & Soils
Physical, chemical and biological weathering processes, factors affecting weathering rates, and soil formation.
Key Fact: Weathering is the in-situ breakdown of rock at or near Earth's surface by physical, chemical or biological processes
Key Fact: Physical (mechanical) weathering: freeze-thaw, exfoliation (onion-skin weathering), thermal fracture, salt weathering
Key Fact: Chemical weathering: oxidation (rusting), hydrolysis (feldspar to clay), carbonation (limestone solution by weak acid), hydration
Key Fact: Biological weathering: root growth in cracks, burrowing organisms, lichen acids dissolving rock
Key Fact: Factors affecting weathering: climate (temperature and rainfall), rock type, surface area, time, vegetation
Key Fact: Soil forms from weathered rock (regolith) mixed with organic material; it develops horizons (O, A, B, C) over time
📋 Key Vocabulary and Concepts
For Weathering & Soils, you must know:
- Freeze-thaw weathering: Physical weathering where water enters cracks, freezes and expands (~9%), wedging rock apart; common in cold climates
- Exfoliation: Physical weathering where rock layers peel off in sheets due to expansion from pressure release or thermal cycling
- Hydrolysis: Chemical weathering where water reacts with feldspar minerals, converting them to clay minerals and dissolved silica
- Carbonation: Chemical weathering where carbonic acid (from CO2 dissolved in rainwater) dissolves carbonate rocks like limestone
- Regolith: The layer of loose, weathered rock and mineral fragments covering bedrock; the parent material from which soil develops
❓ Practice Questions
Q: Describe how freeze-thaw weathering works.
Q: How does hydrolysis weather feldspar?
Q: What is the difference between weathering and erosion?
Q: What factors control the rate of weathering?
Q: Describe the layers (horizons) in a typical soil profile.
✅ Answers
- Water enters cracks in rock. When temperature drops below freezing, the water expands by ~9% as it turns to ice, exerting pressure on the crack walls. Repeated freeze-thaw cycles gradually widen the crack until fragments of rock break off.
- Water reacts with feldspar minerals in granite, breaking their chemical bonds and converting them into clay minerals (kaolinite) plus dissolved silica and potassium/sodium ions. This weakens the rock and is a major process in granite weathering.
- Weathering is the in-situ breakdown of rock where it sits (no transport involved). Erosion is the removal and transport of weathered material by agents such as water, wind or ice.
- Climate (temperature and moisture: warm and wet accelerates chemical weathering; cold promotes freeze-thaw), rock type and mineralogy (some minerals weather faster than others), surface area (more joints = faster weathering), time, and vegetation cover.
- O horizon: organic layer (leaf litter). A horizon: topsoil with organic matter and minerals. B horizon: subsoil with accumulated leached minerals (clay, iron). C horizon: partially weathered parent rock (regolith).
🎯 Exam Tips
- Three weathering types: physical, chemical, biological, know examples of each
- Weathering = in place; Erosion = involves transport
- Climate is the main control: warm+wet = chemical; cold = physical
- Soil horizons: O, A, B, C from top to bottom
📝 Exam Technique
GCSE Geology Exam Tips — Weathering & Soils:
1. For Weathering & Soils questions, use correct geological terminology and classify features precisely
2. Reference specific geological time periods and processes
3. When interpreting data, consider the quality and limitations of the evidence
4. Apply your understanding of Weathering & Soils to fieldwork observations and map data
5. For evaluation, weigh competing geological theories with evidence
1. For Weathering & Soils questions, use correct geological terminology and classify features precisely
2. Reference specific geological time periods and processes
3. When interpreting data, consider the quality and limitations of the evidence
4. Apply your understanding of Weathering & Soils to fieldwork observations and map data
5. For evaluation, weigh competing geological theories with evidence
⚠️ Common Errors
✗ Weathering and erosion are the same ✓ Weathering breaks rock in place (no movement); erosion removes and transports the broken material
✗ Freeze-thaw only happens in polar regions ✓ Freeze-thaw occurs wherever temperatures fluctuate above and below freezing, including temperate mountain regions
✗ Chemical weathering is fastest in cold, dry climates ✓ Chemical weathering is fastest in warm, wet climates because chemical reaction rates increase with temperature and water is essential for most chemical weathering processes
✍️ Model Answer
Full-Mark Response
Describe the three types of weathering and explain how climate affects weathering rates. [6 marks]
Physical (mechanical) weathering breaks rock without changing its chemical composition. Freeze-thaw weathering occurs when water enters cracks, freezes and expands by ~9%, forcing cracks apart. Exfoliation (onion-skin weathering) happens when rock layers peel away due to pressure release or daily temperature cycling. Salt weathering occurs when evaporating salt crystals grow in pores and split the rock. Chemical weathering alters rock minerals through reactions. Carbonation dissolves limestone by weak carbonic acid (CO2 in rainwater): CaCO3 + H2CO3 = Ca(HCO3)2 (soluble). Hydrolysis converts feldspar in granite to clay minerals and dissolved silica, weakening the rock. Oxidation rusts iron-bearing minerals (e.g. pyrite to iron oxide). Biological weathering involves organisms: plant roots grow into rock cracks, widening them; lichens secrete organic acids that dissolve minerals; burrowing animals break up rock. Climate is the dominant control on weathering rates. In warm, wet climates (tropical), chemical weathering is very rapid because reactions need both heat and moisture. In cold climates, physical weathering (freeze-thaw) dominates because water freezes repeatedly. In hot, dry climates (deserts), weathering is slow overall but physical processes like thermal fracture and salt weathering dominate. Climate therefore determines both the rate and the dominant type of weathering in a region.
📊 AO Deep Dive
Assessment Objective Analysis
AO1 (Knowledge & Understanding): Demonstrate knowledge and understanding of weathering & soils, including key geological concepts, observational data and theoretical models relevant to OCR J357.
AO2 (Application of Knowledge): Apply knowledge and understanding of weathering & soils to both familiar and unfamiliar geological contexts, using field evidence and theoretical principles to explain phenomena.
AO3 (Analysis & Evaluation): Analyse geological data related to weathering & soils, evaluate evidence from observations and investigations, and construct reasoned arguments using scientific methodology.
📝 Exam Questions by Topic
🎬 Video Resources
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