Rock layers and stress
Most layered rocks begin as flat strata. The oldest layer is at the bottom (law of superposition). Deep inside the crust, forces called stress push on rock.
- Compression: squeezing from the sides.
- Tension: pulling apart.
- Shear: pushing in opposite directions along a line.
Rock can respond in two ways. If it is hot, slow-pushed or deep, it behaves like soft clay and bends (ductile). If it is cold, shallow or pushed fast, it behaves like a dry biscuit and breaks (brittle).
Folds: anticline and syncline
A fold is a bend in rock layers. Folds are made by compression, often at plate boundaries, and many fold mountains are made this way.
- Anticline: an arch. The oldest rocks are in the middle (core). The limbs slope down away from the top.
- Syncline: a trough. The youngest rocks are in the middle. The limbs slope in towards the centre.
- Axial plane: an imaginary surface that cuts the fold in half. In an open fold the planes are upright. In an overturned fold, one limb is tipped over.
- Limbs are the two sides of a fold; the hinge is where the bend is sharpest.
Folds can be as small as a hand sample or as big as a mountain range. Tip: the land surface does not always match the fold. A valley can sit on an anticline after erosion.
Faults: normal, reverse and strike-slip
A fault is a crack in rock where one block has slid past the other. The fault plane may be tilted. The block above the plane is the hanging wall; the block below is the footwall.
- Normal fault (tension): hanging wall moves down. The crust is stretched. Two normal faults facing each other make a down-dropped valley, a graben (rift valley). The block left high between them is a horst.
- Reverse fault (compression): hanging wall moves up. The crust is shortened. A reverse fault with a gentle tilt is a thrust fault.
- Strike-slip fault (shear): blocks slide sideways. Roads, rivers and fences are cut and shifted.
Faults can be inactive (old) or active. Sudden slip on an active fault releases energy as an earthquake. Long cracks with no slip, just a split, are called joints. Joints help weathering and quarrying.
Structural trends and how we read structures
In a region, folds and faults often line up in the same direction. This direction is called the structural trend or grain of the land. It comes from the direction of the old squeezing or stretching. For example, a belt of folded mountains runs roughly at right angles to the push that made it.
Geologists read structures using:
- Strike and dip on a geological map (direction of the layer and angle of tilt).
- Age order: in an anticline the oldest rocks are in the centre; in a syncline they are at the edges.
- Offset: if a layer is cut and shifted, a fault is present, and the shifted amount is the slip.
- Unconformity: a surface where younger layers lie on tilted or eroded older ones. It records a time gap with folding and erosion.
Try it: fold a towel, snap a biscuit
- Lay 4 coloured cloths or towels in a stack. Push the two ends slowly together. Predict first: where will the arch form? Name the anticline and syncline.
- Snap a dry biscuit by pulling and by pushing. Slow and gentle bends clay; fast breaks the biscuit. That is ductile and brittle.
- In the 3D above, press each force button and say aloud whether the top block went up, down or sideways.
Key formulas and definitions
- Compression → folds or reverse/thrust faults
- Tension → normal faults, graben (rift valley), horst
- Shear → strike-slip faults
- Anticline: oldest rock in the core. Syncline: youngest rock in the core
- Hanging wall = block above the fault plane; footwall = block below
Worked examples
1. In a normal fault, which block moves down and what force made it?
The hanging wall moves down relative to the footwall. The force is tension (pulling apart).
2. A road cuts across a hill. The rock layers form an arch. The oldest layer is in the middle. What is the fold?
It is an anticline: an arch with the oldest rock at the core.
3. A straight river is cut in two and the halves are shifted sideways by 20 m. Name the fault and the force.
It is a strike-slip fault, made by shear (sideways pushing). The slip is 20 m.
Common mistakes
- Mixing up anticline and syncline. Anticline = arch (A shape), oldest in the core. Syncline = trough (sink), youngest in the core.
- Saying a normal fault means “ordinary”. It is normal because the hanging wall moves down, the way gravity pulls, and it comes from tension.
- Mixing up hanging wall and footwall. Hanging wall is above the tilted fault plane; footwall is below.
- Believing all folds make hills. Erosion can turn an anticline into a valley and a syncline into a ridge.