G5: Rock Deformation & Stress
How rocks respond to stress: brittle deformation (fractures and faults) vs ductile deformation (folds), and the types of faults.
Rock Deformation & Stress
How rocks respond to stress: brittle deformation (fractures and faults) vs ductile deformation (folds), and the types of faults.
Key Fact: Stress is the force applied to rock; strain is the resulting deformation
Key Fact: Brittle deformation: rocks fracture under stress, producing joints, faults and fractures (dominant at shallow depths, low temperature, fast strain rate)
Key Fact: Ductile deformation: rocks bend or flow under stress, producing folds and cleavage (dominant at depth, high temperature, slow strain rate)
Key Fact: Normal faults form under tension: the hanging wall moves down relative to the footwall
Key Fact: Reverse (thrust) faults form under compression: the hanging wall moves up relative to the footwall
Key Fact: Strike-slip faults form under shear stress: blocks move horizontally past each other
📋 Key Vocabulary and Concepts
For Rock Deformation & Stress, you must know:
- Stress: The force per unit area applied to a rock body (compression, tension or shear)
- Strain: The deformation (change in shape or volume) of a rock in response to stress
- Hanging wall: The block of rock above a fault plane; moves relative to the footwall below
- Footwall: The block of rock below a fault plane
- Joint: A fracture in rock along which no movement has occurred (unlike a fault)
❓ Practice Questions
Q: What determines whether a rock deforms in a brittle or ductile manner?
Q: How does a normal fault differ from a reverse fault?
Q: What is the difference between a joint and a fault?
Q: What type of stress produces a strike-slip fault?
Q: Why do rocks fold rather than fracture deep in the crust?
✅ Answers
- Temperature, pressure (depth), strain rate and rock type. High temperature and pressure (great depth) favour ductile deformation (folding). Low temperature and pressure (shallow depth) and rapid stress favour brittle deformation (faulting).
- A normal fault forms under tension, the hanging wall moves down relative to the footwall. A reverse fault forms under compression, the hanging wall moves up relative to the footwall.
- A joint is a fracture with no movement along it. A fault is a fracture where the rock blocks have moved relative to each other.
- Shear stress, forces acting parallel to the fault plane cause the blocks to slide horizontally past each other (e.g. San Andreas Fault).
- At great depth, high temperatures and pressures make rocks more ductile (plastic). They can bend and flow slowly under stress rather than breaking, producing folds instead of faults.
🎯 Exam Tips
- Brittle vs ductile: depth and temperature are the key controls
- Normal fault = tension (hanging wall down); Reverse fault = compression (hanging wall up)
- Joints have NO movement; faults HAVE movement
- Stress is the cause; strain is the effect
📝 Exam Technique
GCSE Geology Exam Tips — Rock Deformation & Stress:
1. For Rock Deformation & Stress 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 Rock Deformation & Stress to fieldwork observations and map data
5. For evaluation, weigh competing geological theories with evidence
1. For Rock Deformation & Stress 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 Rock Deformation & Stress to fieldwork observations and map data
5. For evaluation, weigh competing geological theories with evidence
⚠️ Common Errors
✗ All rocks fracture under stress ✓ Rocks near the surface fracture (brittle), but at depth under high temperature and pressure they fold (ductile)
✗ Normal faults form under compression ✓ Normal faults form under TENSION, the hanging wall moves DOWN. Reverse faults form under compression, the hanging wall moves UP
✗ Joints and faults are the same thing ✓ Joints are fractures with NO movement; faults are fractures WITH movement along the plane
✍️ Model Answer
Full-Mark Response
Explain how rocks respond to stress and describe the different types of fault. [6 marks]
When stress is applied to rocks, they respond through strain (deformation). The type of deformation depends on conditions: at shallow depths with low temperature and pressure, and under rapid stress, rocks deform brittly, they fracture, producing joints (fractures with no movement) and faults (fractures with movement). At greater depths with high temperature and pressure, and under slow stress, rocks deform ductilely, they bend and fold rather than fracture. Three main fault types correspond to three stress types. Under tension, normal faults form where the hanging wall (block above the fault plane) moves down relative to the footwall (block below), extending the crust (e.g. in rift valleys). Under compression, reverse faults form where the hanging wall moves up relative to the footwall, shortening the crust; low-angle reverse faults are called thrust faults. Under shear stress, strike-slip faults form where blocks slide horizontally past each other with no vertical movement (e.g. San Andreas Fault). The type of fault therefore indicates the type of stress that acted on the rock, making faults valuable for reconstructing past tectonic conditions.
📊 AO Deep Dive
Assessment Objective Analysis
AO1 (Knowledge & Understanding): Demonstrate knowledge and understanding of rock deformation & stress, including key geological concepts, observational data and theoretical models relevant to OCR J357.
AO2 (Application of Knowledge): Apply knowledge and understanding of rock deformation & stress to both familiar and unfamiliar geological contexts, using field evidence and theoretical principles to explain phenomena.
AO3 (Analysis & Evaluation): Analyse geological data related to rock deformation & stress, evaluate evidence from observations and investigations, and construct reasoned arguments using scientific methodology.
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🎬 Video Resources
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