Rocks deform when stressed: compression produces folds (anticlines arch upwards, synclines sag downwards) and reverse faults, while tension produces normal faults; the type and orientation of folds and faults are evidence of the direction of past Earth movements and are shown on geological maps and cross-sections.

What this dot point is asking

Eduqas wants you to explain how rocks deform when stressed, and to use folds and faults as evidence of past Earth movements. Compression (squeezing) produces folds (anticlines and synclines) and reverse faults; tension (pulling apart) produces normal faults. You need to name the structures, describe the movement, link each to the stress that made it, and recognise them on geological maps and cross-sections, which is a core Component 2 skill.

The answer

How rocks respond to stress

Stress is the force per unit area applied to a rock; strain is the resulting change in shape or volume. Rocks deform in two main ways:

• Ductile (plastic) deformation bends and folds the rock without breaking it. This tends to happen slowly, deep down, where high temperature and confining pressure let the rock flow.
• Brittle deformation breaks the rock, producing fractures (joints and faults). This tends to happen nearer the surface, where the rock is cooler and more rigid.

The two main stresses are compression (forces acting towards each other, squeezing) and tension (forces acting away from each other, pulling apart).

Folds: the product of compression

A fold is a bend in originally flat (horizontal) beds, produced by compression. The two basic folds are:

• Anticline: beds arched upwards into an arch. The oldest beds are in the centre (core) of the fold.
• Syncline: beds sagging downwards into a trough. The youngest beds are in the centre.

Each fold has limbs (the sloping sides) and an axial plane (the imaginary plane dividing the fold symmetrically). The orientation of the fold (which way the limbs dip, how tightly it is folded) records the direction and strength of the compression.

Faults: fractures with movement

A fault is a fracture along which the rocks have moved relative to one another (unlike a joint, where there is no movement). The block above an inclined fault plane is the hanging wall; the block below is the footwall. The two faults you must know are:

• Normal fault: the hanging wall slips down relative to the footwall. This extends (lengthens) the crust, so it is produced by tension.
• Reverse fault: the hanging wall is pushed up relative to the footwall, so older rocks can ride up over younger ones. This shortens the crust, so it is produced by compression.

Folds and faults as evidence

Because each structure records a specific stress, mapping them reveals the history of Earth movements:

• Anticlines, synclines and reverse faults record compression (a colliding plate margin or mountain belt).
• Normal faults (and rift valleys) record tension (a stretching or rifting region, such as a constructive margin).

This is why Component 2 hands you a map or cross-section and asks you to infer the forces that shaped the area.

Examples in context

Example 1. The folded Welsh borderland. Tightly folded Silurian rocks in the Welsh borders record the compression of an ancient continental collision that built a mountain belt long since eroded.

Example 2. The East African Rift. A line of normal faults drops the rift floor down between the highlands, recording tension as the continent is being pulled apart, the opposite stress to a fold belt.

Try this

Q1. State which type of stress produces folds and which produces normal faults. [2 marks]

• Cue. Folds form under compression (squeezing); normal faults form under tension (pulling apart).

Q2. Describe the relative movement of the hanging wall in a reverse fault and what it does to the crust. [2 marks]

• Cue. The hanging wall is pushed up relative to the footwall, shortening (compressing) the crust.

Q3. State where the oldest beds are found in an anticline. [1 mark]

• Cue. In the centre (core) of the fold.