Notes · GeologyUK · A-Levels

Rock deformation

This chapter examines how rocks respond to stress and record it as structures. It covers stress and strain and the brittle-versus-ductile behaviour of rock, the geometry and classification of folds, the classification of faults and their relationship to the principal stresses through Anderson's theory, and the joints and reactivated structures that complete the deformational record.

4 sections~15 min reading time3 competenciesLevel Standard 3 · Advanced 1

T·0555 / 13

Exam profile

AO1 · Describe the types of fold, fault and joint and the stress regimes that produce themAO2 · Classify structures and calculate throw and displacement, relating faults to principal stressesAO3 · Interpret deformed sequences to deduce the stresses and deformation history

Operators:describeexplainclassifycalculatededuceinterpret

basic level

AS-Level expects you to describe the main types of fold and fault and to recognise them in cross-section.

higher level

The full A-Level requires you to relate faults to principal stresses, calculate throw and heave, and interpret a deformation history.

Depth

Text

Contents · 4 sections

Rock deformation

Stress, strain and rock behaviour#

StandardWJEC/Eduqas A level Geology (Component 2) — Rock deformation

Elastic, plastic and brittle behaviour

Fig. 1A stress-strain graph: rock deforms elastically (recoverable) at first, then either flows plastically (ductile) or, if brittle, fractures at the fracture point.

Stress is the force acting per unit area on a rock, and strain is the resulting change in shape or volume. Stress can be compressive (squeezing, at convergent settings), tensional (stretching, at divergent settings) or shear (side-by-side, at transform settings), and it is the type and orientation of stress that determines the structures produced. When stress is equal in all directions it is called confining (or lithostatic) pressure, which simply squeezes a rock without distorting it; it is differential (directed) stress, unequal in different directions, that deforms rock into folds and faults.

Rocks respond to increasing stress in stages. At first the strain is elastic — the rock deforms but springs back to its original shape when the stress is removed, storing energy as it does so (the energy released in earthquakes). Beyond the elastic limit the behaviour diverges: a brittle rock fractures suddenly, releasing the stored energy as an earthquake and producing faults and joints, whereas a ductile rock deforms plastically, flowing into permanent folds without breaking. Which path a rock takes is central to whether a region faults or folds.

The controls on brittle versus ductile behaviour are temperature, confining pressure, strain rate and rock type. High temperature and high confining pressure — the conditions deep in the crust — favour ductile (plastic) behaviour and folding; low temperature and low pressure near the surface favour brittle behaviour and faulting. A slow strain rate allows a rock to flow ductilely, whereas a sudden, fast stress makes even a normally ductile rock snap. Rock type matters too: a well-bedded mudstone folds readily, while a massive, brittle sandstone or a cold granite tends to fracture.

This explains a pattern seen throughout structural geology: the same rocks can fold at depth and fault near the surface, and a single mountain belt often shows deep ductile folds passing up into brittle faults and thrusts. Reading a structure therefore tells us not only the direction of the stress but the conditions — the depth, temperature and rate — under which the rock deformed. The distinction between elastic, brittle and ductile behaviour underlies both the folds of the next section and the faults and earthquakes that follow.

Worked example

Predicting fold or fault

Two identical mudstone beds are stressed: one is deep in the crust at high temperature and pressure, the other is near the surface and cold. Predict how each deforms and explain why.

The deep bed
High temperature and high confining pressure favour ductile behaviour, so the deep bed flows plastically and folds without breaking.
The shallow bed
Low temperature and low confining pressure favour brittle behaviour, so the shallow bed fractures and faults.
Conclude
The same rock folds at depth and faults near the surface because the conditions, not the rock, decide the behaviour.

Result: The deep, hot, confined bed folds (ductile); the shallow, cold bed faults (brittle) — behaviour is set by the conditions.

Typical mistakes

Confusing confining (equal, all-round) pressure, which does not distort a rock, with differential (directed) stress, which does.
Assuming a rock either only folds or only faults; the same rock can fold at depth (ductile) and fault near the surface (brittle).

Active revision

Explain why the deep core of a mountain belt shows folds while its higher levels show faults, referring to temperature and confining pressure.

Active recall

Recall the key points — then reveal.

Sources: WJEC/Eduqas A level Geology specification (WJEC / Eduqas) · British Geological Survey — geological structures (British Geological Survey)

Folds#

StandardWJEC/Eduqas A level Geology (Component 2) — Rock deformation

Anticline and syncline

Fig. 2An anticline (up-arch, oldest beds in the core) beside a syncline (trough, youngest beds in the core), formed by horizontal compression.

Folds are bends in originally near-horizontal rock layers, produced by ductile deformation, most often under compression. The parts of a fold have precise names that the examination expects: the limbs are the sloping sides, the hinge is the line of greatest curvature, the axial plane is the surface that divides the fold symmetrically through the hinges, and the fold axis is the line along which the hinge runs. Describing a fold means using this vocabulary accurately, because the geometry records the direction and intensity of the stress.

The two basic folds are the anticline and the syncline. An anticline is an up-arch in which the beds dip away from the crest on both sides and, crucially, the oldest beds are found in the core (centre); a syncline is a down-fold or trough in which the beds dip inward toward the centre and the youngest beds are in the core. This relationship between the shape and the age of the beds in the core is important, because erosion can flatten a fold so that its shape is no longer obvious, and then the pattern of ages across the outcrop is what reveals whether it was an anticline or a syncline.

Folds are classified by their symmetry and the tilt of their axial plane, which records how intense and how one-sided the compression was. A symmetrical fold has a vertical axial plane and limbs of equal dip, indicating balanced compression; an asymmetrical fold has an inclined axial plane and limbs of unequal dip; an overturned fold has an axial plane so steeply inclined that one limb has been rotated past vertical; and a recumbent fold has a near-horizontal axial plane, indicating very intense, one-directional compression as in the core of a mountain belt. A monocline is a simple one-sided step in otherwise horizontal beds.

Folds matter beyond description. Their tightness and style let a geologist gauge the amount of crustal shortening a region has suffered, and hence the intensity of an orogeny; anticlines form important traps for oil and gas, met in the resources chapter, because buoyant hydrocarbons rise into the crest beneath a sealing cap rock; and the way beds dip on either side of a fold controls the outcrop patterns seen on geological maps. Interpreting a fold in cross-section — naming it, reading the ages in its core and gauging the compression — is a recurring examination task.

Worked example

Reading a fold from bed ages

A cross-section shows folded beds where the oldest rocks are exposed along the central axis and successively younger beds appear on both sides. Name the fold, describe how the beds dip, and state the stress that formed it.

Use the age pattern
Oldest beds in the core, younger beds outward on both sides, is the defining feature of an anticline.
Describe the dips
In an anticline the limbs dip away from the central axis on both sides (an up-arch).
State the stress
Anticlines form by ductile folding under horizontal compression, as at a convergent margin or in an orogeny.

Result: The structure is an anticline (oldest beds in the core, limbs dipping outward), formed by horizontal compression.

Typical mistakes

Identifying an anticline or syncline from its present shape alone; after erosion, the age pattern (oldest in the core = anticline) is the reliable test.
Confusing the axial plane (which bisects the fold) with the limbs (the sloping sides).

Active revision

In an eroded, flat-lying outcrop the beds get older toward a central line and younger away from it. State whether the structure is an anticline or a syncline and justify your answer.

Active recall

Recall the key points — then reveal.

Sources: WJEC/Eduqas A level Geology specification (WJEC / Eduqas) · British Geological Survey — folds (British Geological Survey)

Faults and the principal stresses#

AdvancedWJEC/Eduqas A level Geology (Component 2) — Rock deformation

A normal fault

Fig. 3A normal fault: the hanging wall has dropped relative to the footwall under tension; the offset of the marker bed defines the throw (vertical) and heave (horizontal).

A fault is a fracture along which the rocks on either side have moved relative to one another, produced by brittle failure. Its geometry is described with a standard vocabulary: the fault plane is the surface of movement, and where it is inclined the block resting on top of it is the hanging wall and the block beneath it the footwall. The vertical component of the displacement of a marker bed is the throw and the horizontal component is the heave; the dip is the angle of the fault plane below horizontal. Measuring throw and heave, and identifying which block is the hanging wall, is the basis of classifying and quantifying a fault.

Faults are classified by the sense of movement. In a normal fault the hanging wall has moved down relative to the footwall, which lengthens (extends) the crust and is caused by tension; in a reverse fault the hanging wall has moved up, which shortens the crust and is caused by compression; a thrust fault is a low-angle reverse fault (dipping less than about 45 degrees) that carries older rocks over younger over large distances. In a strike-slip (tear or wrench) fault the movement is horizontal along the fault, caused by shear, with little vertical displacement.

Anderson's theory of faulting links each fault type to the orientation of the three principal stresses (the greatest, intermediate and least, usually written as the maximum, intermediate and minimum compressive stresses). Because the Earth's surface can support no shear stress, one principal stress is always roughly vertical, and this fixes the fault type. When the maximum stress is vertical, the crust extends and normal faults form (dipping steeply, about 60 degrees); when the maximum stress is horizontal and the minimum vertical, the crust shortens and reverse or thrust faults form (dipping gently, about 30 degrees); when both the maximum and minimum are horizontal and the intermediate is vertical, strike-slip faults form (near vertical). This theory lets a geologist read the stress state directly from the faults.

Faulting is also quantitative. The throw and heave of a fault are related to the dip of the fault plane by simple trigonometry, so one can be calculated from the other, and the total displacement can be found; matching offset marker beds across a fault gives the throw directly from a cross-section or map. Faults, once formed, are planes of weakness that can be reactivated by later stresses, sometimes with a different sense of movement, so an ancient normal fault may be reactivated as a reverse fault in a later compression. Recognising, classifying, quantifying and sequencing faults is among the most examined of structural skills.

\text{heave} = \frac{\text{throw}}{\tan(\text{dip})}

Throw, heave and dip

The vertical throw and horizontal heave of a fault are related by the dip of the fault plane: throw divided by heave equals the tangent of the dip, so the heave equals the throw divided by tan(dip).

Fault types and their stress regimes

Fig. 4Faults classified by the sense of movement and, following Anderson's theory, by which principal stress is vertical: tension (normal), compression (reverse/thrust) or shear (strike-slip).

Worked example

Classifying and quantifying a fault

In a cross-section, a fault dipping at 60 degrees has moved the hanging wall downward, giving a marker bed a vertical throw of 20 m. Classify the fault, name the stress, and calculate the heave.

Classify
The hanging wall has moved down relative to the footwall, so this is a normal fault, formed by tension (extension) with the maximum principal stress vertical.
State the relationship
The heave equals the throw divided by the tangent of the dip.
$\text{heave} = \frac{\text{throw}}{\tan(60^{\circ})}$
Calculate
tan 60 degrees = 1.732, so heave = 20 / 1.732 = 11.5 m.
$\text{heave} = \frac{20}{1.732} = 11.5\ \text{m}$

Result: A normal (tensional) fault with a throw of 20 m and a heave of 11.5 m.

Typical mistakes

Muddling normal and reverse faults; in a normal fault the hanging wall goes down (extension), in a reverse fault it goes up (compression).
Forgetting that the hanging wall is the block above an inclined fault plane and the footwall the block below it.

Active revision

A fault dips at 60 degrees and drops the hanging wall so that a marker bed has a throw of 30 m. Classify the fault, state the stress that caused it, and calculate the heave.

Active recall

Recall the key points — then reveal.

Sources: WJEC/Eduqas A level Geology specification (WJEC / Eduqas) · British Geological Survey — faults (British Geological Survey)

Joints and the reactivation of structures#

StandardWJEC/Eduqas A level Geology (Component 2) — Rock deformation

Joints, faults and their sequence

Fig. 5Joints (no displacement) contrasted with faults (displacement); cross-cutting relationships allow deformation events to be placed in order.

Joints are fractures along which there has been no displacement, distinguishing them from faults; they are brittle features that open a rock along planes of weakness. They form in several ways: by the release of pressure as overlying rock is eroded (unloading joints, which run parallel to the surface), by cooling and contraction (as in the spectacular columnar joints of a basalt lava flow, where hexagonal columns form as the lava shrinks), and by the same regional stresses that produce folds and faults. Joints usually occur in sets of parallel fractures, and two or more sets divide a rock into blocks.

Joints have a large practical importance out of proportion to their simple geometry. They control how water and fluids move through otherwise impermeable rock, so they govern the permeability of aquifers and hydrocarbon reservoirs, met in the resources chapter; they are the planes along which weathering penetrates and along which rock is quarried; and, in engineering geology, the orientation and spacing of joints control the stability of a rock slope, a tunnel or a dam foundation. A rock cut across its joints stands firm, but the same rock cut parallel to a joint set may slide.

Because a fault, once formed, is a permanent plane of weakness, later stresses tend to reactivate old structures rather than break fresh rock. A fault formed under one stress regime may move again, sometimes in a different sense: an ancient normal fault produced by extension may later be reactivated as a reverse fault when the region is compressed, and deep basement faults can be reactivated repeatedly over hundreds of millions of years. This inheritance means that the structures seen today may be the cumulative product of several episodes of deformation superimposed on the same weaknesses.

Reading the structural record therefore means sequencing the events. Cross-cutting relationships apply to structures just as to rocks — a fault that offsets a fold must be younger than the fold, and a joint set cut by a fault predates it — so a deformed sequence can be unravelled into an order of events: deposition, then folding, then faulting, then reactivation, and so on. This structural sequencing, combined with the dating principles of the time-and-change chapter, is how a geologist reconstructs the full tectonic history recorded in an outcrop or on a map.

Worked example

Sequencing brittle structures

An outcrop shows a folded sandstone cut by a normal fault, which is itself cut by a set of open joints. Place the three structures in order and justify the sequence.

Apply cross-cutting
A structure that cuts across another must be younger than the one it cuts.
Order the events
The fold is cut by the fault, so folding came first; the fault is cut by the joints, so faulting came next; the joints are youngest.
State the history
The sequence is: deposition of the sandstone, then folding (compression), then normal faulting (extension), then jointing.

Result: Order: folding, then faulting, then jointing — read from the cross-cutting relationships.

Typical mistakes

Calling any fracture a fault; a joint shows no displacement, whereas a fault does.
Assuming a fault moved only once; old faults are often reactivated, sometimes in a different sense, by later stresses.

Active revision

A basalt shows regular hexagonal columns and is cut by a fault. Explain the origin of the columns and state which formed first, the columns or the fault.

Active recall

Recall the key points — then reveal.

Sources: WJEC/Eduqas A level Geology specification (WJEC / Eduqas) · British Geological Survey — joints and fractures (British Geological Survey)

References & sources

Sources

WJEC / Eduqas

WJEC/Eduqas A level Geology specification

British Geological Survey

British Geological Survey — geological structures

Rock deformation

4 sections~15 min reading time3 competenciesLevel Standard 3 · Advanced 1

Rock deformation

Key points

Predicting fold or fault

Key points

Reading a fold from bed ages

Key points

Classifying and quantifying a fault

Key points

Sequencing brittle structures

Sources

Rock deformation

Key points

Predicting fold or fault

Key points

Reading a fold from bed ages

Key points

Classifying and quantifying a fault

Key points

Sequencing brittle structures

Sources