How do rocks respond to stress, and what controls brittle versus ductile behaviour?
The concepts of stress and strain, the difference between compressional, tensional and shear stress, elastic, brittle and ductile behaviour, and the factors (temperature, confining pressure, strain rate, rock type and fluids) that control how a rock deforms.
A focused WJEC and Eduqas A-Level Geology G2 answer on stress and strain, the three stress regimes, elastic, brittle and ductile behaviour, and the controls (temperature, confining pressure, strain rate, rock competence and fluid pressure) that decide whether a rock fractures or flows when deformed.
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What this dot point is asking
G2 is the mechanics of deformation, and this dot point sets up the vocabulary for the whole topic. WJEC wants secure definitions of stress and strain, the three stress regimes, the three styles of behaviour (elastic, brittle, ductile), and the conditions that decide which style a given rock shows. Get this right and folds, faults and structural map work follow logically.
The answer
Stress and strain
There are three stress regimes, each tied to a plate setting:
- Compressional stress squeezes and shortens (convergent margins); it tends to fold rocks and form reverse and thrust faults.
- Tensional stress stretches and pulls apart (divergent margins); it tends to form normal faults and rifts.
- Shear stress slides one block past another (transform margins); it tends to form strike-slip faults and shear zones.
Elastic, brittle and ductile behaviour
A rock can respond in three ways as stress increases:
What controls the behaviour
Whether a rock fractures or flows depends on five factors:
- Temperature: hotter (deeper) rocks are weaker and more ductile.
- Confining pressure: high pressure at depth suppresses fracturing and favours flow.
- Strain rate: slow deformation favours ductile flow; fast deformation favours brittle failure.
- Rock type (competence): competent rocks (sandstone, limestone) fracture; incompetent rocks (shale, salt) flow.
- Fluids: high pore-fluid pressure lowers the effective stress and promotes brittle failure.
Examples in context
Salt tectonics in the North Sea and the Gulf of Mexico shows incompetent salt flowing ductilely and rising as diapirs, while the surrounding sandstones and shales fault and fold, a direct illustration of competence. The elastic rebound model of earthquakes (after the 1906 San Francisco event) shows elastic strain building across a locked fault, then released as brittle slip. Welsh slate records ductile flattening at low grade, with platy minerals rotated into a new cleavage under directed stress.
Try this
Q1. Define stress and strain. [2 marks]
- Cue. Stress is the force per unit area applied to a rock; strain is the change in shape or volume that results.
Q2. State the structure produced by tensional stress and by compressional stress. [2 marks]
- Cue. Tensional stress gives normal faults (and rifts); compressional stress gives folds and reverse or thrust faults.
Q3. Explain why a rock is more likely to flow ductilely at depth than near the surface. [3 marks]
- Cue. At depth the higher temperature weakens the rock and the high confining pressure suppresses fracturing, so it flows rather than breaks.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC Eduqas 20195 marksExplain the factors that determine whether a rock deforms in a brittle or a ductile manner.Show worked answer →
Set out the controls and say which way each pushes the rock, because the marks track named factors.
Temperature: higher temperature, deeper in the crust, makes rocks weaker and more ductile, so they flow rather than fracture. Cold rocks near the surface tend to be brittle.
Confining pressure: high confining pressure at depth suppresses fracturing and promotes ductile flow; low pressure near the surface allows brittle failure.
Strain rate: slow deformation gives ductile behaviour because the rock has time to flow; rapid deformation (such as an earthquake or an impact) gives brittle failure.
Rock type (competence): competent rocks such as sandstone and limestone tend to fracture; incompetent rocks such as shale and salt tend to flow.
Fluids: high pore-fluid pressure reduces the effective stress holding a rock together and promotes brittle failure along fractures.
So deep, hot, slow, high-pressure conditions on a weak rock give ductile folding; shallow, cold, fast conditions on a strong rock give brittle faulting.
Markers reward temperature, confining pressure, strain rate, rock competence and fluids, each correctly linked to brittle or ductile behaviour.
WJEC Eduqas 20214 marksDistinguish between compressional, tensional and shear stress, giving the structure each tends to produce.Show worked answer →
Define each stress by the direction of the forces and link it to a structure, because both are asked.
Compressional stress acts to squeeze and shorten a rock, with forces directed towards each other. It tends to produce folds and reverse (thrust) faults, and is typical of convergent margins.
Tensional stress acts to stretch and pull a rock apart, with forces directed away from each other. It tends to produce normal faults and rifting, typical of divergent margins.
Shear stress acts to slide one part of a rock past another in opposite, parallel directions. It tends to produce strike-slip (tear) faults and shear zones, typical of transform margins.
Markers reward squeeze-shorten giving folds and reverse faults, stretch giving normal faults, and slide-past giving strike-slip faults.
Related dot points
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- The classification of faults (normal, reverse, thrust, strike-slip) by the relative movement of the hanging wall and footwall and by the stress regime, the terminology of fault planes (dip, throw, heave, slickensides), and the recognition of faults in the field and on maps.
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- Contact, regional and dynamic metamorphism, the controls of temperature, pressure and fluids, the increase of grade and the use of index minerals and metamorphic facies, and the textures (slate, schist, gneiss, hornfels, marble, quartzite) they produce.
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Sources & how we know this
- WJEC Eduqas A-level Geology specification — WJEC Eduqas (2017)