Stress and strain: the three stress regimes (compression, tension and shear) and the strain (deformation) they produce; elastic, ductile and brittle behaviour; the factors that control deformation style (temperature, confining pressure, strain rate, rock type and pore fluid pressure); competent and incompetent rocks; and why rocks deform ductilely at depth but brittlely near the surface.

What this dot point is asking

Eduqas wants you to distinguish the three stress regimes (compression, tension and shear) from the strain (the deformation) they produce, to define elastic, ductile and brittle behaviour, to explain the controls on which one occurs (temperature, confining pressure, strain rate, rock type and pore fluid pressure), to contrast competent and incompetent rocks, and to explain why rocks tend to deform ductilely at depth but brittlely near the surface. This is the foundation of the whole module: folds, faults, joints and the structures you read on maps are all the strain that follows from these ideas.

The answer

Stress and strain

These two words are not interchangeable, and Eduqas tests the distinction.

There are three stress regimes:

• Compression squeezes the rock (forces directed towards each other), shortening and thickening the crust. It produces folds and reverse or thrust faults, at convergent (destructive) margins.
• Tension pulls the rock apart (forces directed away from each other), lengthening and thinning the crust. It produces normal faults, at divergent (constructive) margins and rifts.
• Shear acts as two parallel forces in opposite directions (a "tearing" couple). It produces strike-slip (tear) faults, at conservative margins.

Elastic, ductile and brittle behaviour

A rock can respond to stress in three ways, depending on conditions:

• Elastic. Strain is recoverable: the rock returns to its original shape when the stress is removed (like a spring). Strain is proportional to stress here, up to the elastic limit.
• Ductile. Past the elastic limit, the rock deforms permanently by flowing or bending without breaking, giving folds. The rock stays deformed when unloaded.
• Brittle. The rock deforms permanently by fracturing (breaking) along faults or joints, rather than flowing. Sudden brittle failure releases stored elastic energy as earthquakes.

Beyond the elastic limit, then, deformation becomes permanent and is either ductile (folding) or brittle (faulting).

Controls on deformation style

The same rock can fold in one setting and fracture in another. The controls are:

• Temperature. High temperature (at depth) weakens minerals and lets atoms creep and recrystallise, favouring ductile flow. Cold, near-surface rock is rigid and tends to fracture.
• Confining pressure. High all-round (lithostatic) pressure at depth suppresses the opening of fractures, holding the rock together, so it deforms ductilely. Low confining pressure near the surface lets fractures open freely, so behaviour is brittle.
• Strain rate. A slow strain rate gives the rock time to creep and flow (ductile); a fast strain rate (an earthquake, an impact) gives brittle fracture. Rate alone can flip the behaviour.
• Rock type. Some rocks are inherently weaker or more able to flow (shale, salt, marble) than others (quartzite, well-cemented sandstone).
• Pore fluid pressure. High-pressure fluids in the pore spaces push the grains apart and reduce the friction that holds a fault closed, making brittle failure easier (this is why fluids can trigger faulting). They can also weaken the rock and promote dissolution.

The pattern to remember: high temperature, high confining pressure and low strain rate all favour ductile behaviour, while their opposites favour brittle failure.

Competent and incompetent rocks

Within a layered sequence, rocks differ in strength:

• A competent rock is strong and rigid (for example quartzite, dolomite, well-cemented sandstone, igneous rock). It resists flow and tends to fracture or to fold into open, blocky folds.
• An incompetent rock is weak and easily deformed (for example shale, mudstone, salt). It flows readily, accommodating strain by tight, contorted folding.

In a folded sequence of alternating beds, competent beds keep an even thickness and control the fold shape, while incompetent beds thicken in the hinges and thin on the limbs as they flow to fill space.

Why depth controls the style

Because temperature and confining pressure both rise with depth, there is a broad change down through the crust: shallow rocks are cold and at low confining pressure, so they deform brittlely (faults, joints, earthquakes), whereas deep rocks are hot and at high confining pressure, so they deform ductilely (folds, flow, ductile shear zones). The change-over is the brittle-ductile transition, typically around 10 to 15 km in the continental crust. This is why most earthquakes are shallow and why deeply buried rocks are folded rather than simply broken.

Examples in context

Example 1. Shallow faulting versus deep folding in a mountain belt. In the same compressional belt, the cold upper crust fails by brittle thrust faulting and earthquakes, while the hot lower crust flows ductilely into tight folds and shear zones, a vertical change driven entirely by rising temperature and confining pressure with depth.

Example 2. Salt as an incompetent layer. Rock salt is so weak that it flows under its own load, rising as salt diapirs that pierce and fold the overlying strata, the clearest demonstration of an incompetent rock deforming ductilely while the competent beds around it fracture.

Try this

Q1. State the three stress regimes and the structure each typically produces. [3 marks]

• Cue. Compression (folds, reverse or thrust faults), tension (normal faults), shear (strike-slip or tear faults).

Q2. Explain why a rock is more likely to deform ductilely at depth than near the surface. [2 marks]

• Cue. Temperature and confining pressure both rise with depth; high temperature weakens minerals and lets them flow, and high confining pressure suppresses fracturing, so deep rocks flow (ductile) while cold, low-pressure surface rocks fracture (brittle).

Q3. Distinguish a competent rock from an incompetent rock, with one example of each. [2 marks]

• Cue. A competent rock is strong and rigid (for example quartzite) and controls the fold shape; an incompetent rock is weak and flows easily (for example shale or salt), thickening in fold hinges.