How are faults classified, recognised and related to the stress that formed them?
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.
A focused WJEC and Eduqas A-Level Geology G2 answer on the classification of normal, reverse, thrust and strike-slip faults by hanging-wall and footwall movement and stress regime, the terms used to describe fault planes (dip, throw, heave, slickensides, fault breccia), and how faults are recognised in the field and interpreted on geological maps.
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What this dot point is asking
Faults are the brittle response to stress, and WJEC wants you to classify them by movement and stress, describe the fault plane with the correct terms, and recognise faults in the field and on maps. This pairs with folds to complete the structural toolkit and underpins both map interpretation in T2 and earthquake hazards in T1.
The answer
The geometry of a fault
The four fault types
Faults are classified by how the hanging wall moves and by the stress that drives them:
Recognising faults
On a map, a fault appears as a line along which beds are offset, repeated or cut out; the juxtaposition of different ages across the line indicates the type and throw. In the field, the fault plane carries diagnostic evidence:
- Fault breccia (angular broken rock) and gouge (finely ground rock) in the fault zone.
- Slickensides: polished, scratched surfaces recording the direction of slip.
- Abrupt juxtaposition of contrasting rock types, and often springs or valleys following the weakened line.
Examples in context
The East African Rift is a system of normal faults forming horsts and grabens under tension as the crust extends. The Moine Thrust in north-west Scotland is a classic thrust that carried older rocks tens of kilometres over younger ones during the Caledonian orogeny, a landmark in the history of structural geology. The San Andreas Fault in California is a transform strike-slip fault where the Pacific and North American plates grind past each other, producing major earthquakes.
Try this
Q1. State which block moves down in a normal fault and the stress that causes it. [2 marks]
- Cue. The hanging wall moves down; tensional (extensional) stress causes it.
Q2. Distinguish throw from heave. [2 marks]
- Cue. Throw is the vertical component of displacement; heave is the horizontal component.
Q3. Name two features that would allow you to recognise a fault plane in the field. [2 marks]
- Cue. Fault breccia or gouge in the fault zone, and slickensides (polished, scratched surfaces) recording slip direction.
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 marksDescribe the relative movement and the stress regime associated with normal, reverse and strike-slip faults.Show worked answer →
Treat each fault by the movement of the hanging wall and the stress that drives it, because both are required.
A normal fault has an inclined fault plane; the hanging wall moves down relative to the footwall. It forms under tensional stress, which stretches and extends the crust, so normal faults are typical of rifts and constructive margins.
A reverse fault also has an inclined plane, but the hanging wall moves up relative to the footwall. It forms under compressional stress, which shortens the crust, so reverse and low-angle thrust faults are typical of convergent margins.
A strike-slip (tear) fault has a steep, often vertical plane; the blocks slide horizontally past each other with little vertical movement. It forms under shear stress, so strike-slip faults are typical of transform margins such as the San Andreas.
Markers reward hanging wall down under tension for normal, hanging wall up under compression for reverse, and horizontal sliding under shear for strike-slip.
WJEC Eduqas 20214 marksExplain how the displacement of beds and the presence of a fault breccia allow a fault to be recognised on a geological map and in the field.Show worked answer →
Give the map evidence and the field evidence, because the question names both.
On a map a fault is shown by the sudden displacement of beds along a line: the outcrop of a marker bed is offset, cut out or repeated where the fault crosses it, and the pattern of younger-against-older rocks across the line indicates the type and throw.
In the field a fault is recognised by physical evidence at the fault plane: a zone of broken rock (fault breccia) or finely ground rock (gouge), polished and scratched surfaces (slickensides) that record the direction of slip, and the abrupt juxtaposition of different rock types. Springs and topographic features may also follow the fault line.
Markers reward offset, repetition or cutting-out of beds across a line on the map, and fault breccia or gouge plus slickensides marking the plane in the field.
Related dot points
- 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.
- The geometry of folds (limbs, axial plane, hinge, fold axis, interlimb angle), the classification of folds (anticline, syncline, symmetrical, asymmetrical, overturned, recumbent, isoclinal) and the use of fold style to interpret the direction and intensity of compression.
A focused WJEC and Eduqas A-Level Geology G2 answer on fold geometry (limbs, hinge, axial plane, fold axis, interlimb angle), the classification of anticlines and synclines and of symmetrical, asymmetrical, overturned, recumbent and isoclinal folds, and how fold style is read to deduce the direction and intensity of compression.
- The types of unconformity (angular, disconformity, nonconformity) and their significance, the structures of mountain belts (nappes, thrust stacks), and the use of cross-cutting relationships and superposition to reconstruct the sequence of tectonic events.
A focused WJEC and Eduqas A-Level Geology G2 answer on the three types of unconformity and what each records, the structures of orogenic belts such as nappes and thrust stacks, and how cross-cutting relationships, superposition and unconformities are combined to reconstruct the sequence of folding, faulting, intrusion, uplift and erosion in a region.
- The interpretation of geological maps: reading dip and strike from outcrop patterns, the rule of Vs for outcrops crossing valleys, recognising horizontal, dipping, folded and faulted strata and unconformities, and using the pattern to deduce the underlying structure.
A focused WJEC and Eduqas A-Level Geology T2 answer on interpreting geological maps: reading dip and strike from outcrop patterns, applying the rule of Vs where outcrops cross valleys, and recognising horizontal, dipping, folded, faulted and unconformable strata to deduce the underlying structure.
- The causes of earthquakes (elastic rebound, focus and epicentre), seismic waves (P, S and surface waves), the measurement of size (magnitude and intensity), the primary and secondary hazards (ground shaking, liquefaction, landslides, tsunami), and the prediction and mitigation of seismic risk.
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Sources & how we know this
- WJEC Eduqas A-level Geology specification — WJEC Eduqas (2017)