Joints are fractures with no movement, formed by cooling, drying or pressure release; an unconformity is a buried erosion surface separating older rocks below from younger rocks above, recording a gap in time during which deposition stopped and erosion occurred; unconformities and joints are interpreted from cross-sections to reconstruct geological history.

What this dot point is asking

Eduqas wants you to know that joints are fractures with no movement (formed by cooling, drying or pressure release), and that an unconformity is a buried erosion surface separating older rocks below from younger rocks above, recording a gap in time. You need to describe the sequence of events an unconformity records (deposition, uplift, erosion, renewed deposition) and to read both structures from cross-sections to reconstruct geological history, a central Component 2 skill.

The answer

Joints: fractures without movement

A joint is a crack or fracture in a rock along which no movement has taken place (this is the key difference from a fault). Joints form by:

• Cooling and contraction: as a lava flow or intrusion cools, it shrinks and cracks, often into regular polygonal columns (the basalt columns of the Giant's Causeway).
• Drying out: wet sediment (mud) shrinks as it dries, opening mud cracks (desiccation cracks).
• Pressure release (unloading): when overlying rock is eroded away, the rock beneath expands slightly and cracks parallel to the surface.

Joints are important in the field because they control how rock breaks, where water flows, and how a quarry or cliff weathers.

Unconformities: gaps in the rock record

An unconformity is a buried surface of erosion that separates older rocks below from younger rocks above, with a gap in time between them. During that gap, deposition stopped and erosion removed some rock, so part of the geological history is simply missing at that point.

The classic angular unconformity records this sequence:

1. Deposition of the lower beds as horizontal layers.
2. Folding or tilting and uplift of those beds by Earth movements (so they are no longer horizontal).
3. Erosion of the uplifted, tilted rocks, creating a flat erosion surface and the time gap.
4. Renewed deposition of younger sediment horizontally on top of the erosion surface.
5. Lithification of the younger beds, leaving the unconformity buried between the two sets.

You can recognise an angular unconformity on a cross-section because the beds below the surface are tilted or folded at a different angle from the flat-lying beds above.

Reading them to reconstruct history

Joints and unconformities are read alongside folds and faults to work out the order of events:

• An unconformity always means: the rocks below formed, were uplifted and eroded, and then the rocks above were laid down. It marks a major break.
• Joints tell you about the conditions after a rock formed (cooling, drying, unloading).

Together with the principles of superposition (younger on top) and cross-cutting (a structure is younger than what it cuts), these let you build the geological history of an area from its cross-section.

Examples in context

Example 1. Hutton's unconformity at Siccar Point. James Hutton's famous Scottish locality shows steeply tilted greywacke truncated and overlain by gently dipping red sandstone, the discovery that revealed the immensity of geological time.

Example 2. Columnar basalt. The hexagonal columns of the Giant's Causeway are cooling joints, opened by contraction as a thick basalt lava flow cooled, with no movement along them.

Try this

Q1. State the key difference between a joint and a fault. [1 mark]

• Cue. A joint has no movement along it; a fault has displacement (the beds are offset).

Q2. List two processes that can form joints in rocks. [2 marks]

• Cue. Any two of: cooling and contraction, drying out (desiccation), pressure release (unloading after erosion).

Q3. State what an unconformity tells you about the geological history of an area. [2 marks]

• Cue. There was a gap in time during which the lower rocks were uplifted and eroded before the upper rocks were deposited, so part of the record is missing.