Faults and Fault Displacement
When Rocks Break Instead of Bend
In the previous lesson you saw that compressional stress can slowly bend buried rocks into folds. But rocks do not always bend — sometimes they fracture. A fault is a planar fracture along which measurable displacement has occurred. The type of fault that forms depends on the direction and nature of the stress applied to the rock.
Geologists classify faults by comparing the movement of two key blocks: the hanging wall and the footwall. Imagine standing on the fault plane itself — the block above you is the hanging wall, the block beneath your feet is the footwall. Whether the hanging wall moves up or down relative to the footwall tells you immediately which type of fault you are looking at.
There are four main fault types on the specification: normal, reverse, thrust, and strike-slip. Each is the product of a different type of tectonic stress — tensional, compressional, or shear.
Normal Faults — Tensional Stress
A normal fault forms when tensional stress pulls the crust apart. The hanging wall slides down relative to the footwall. Normal faults are the signature structure of divergent plate boundaries, where two plates move apart. The East African Rift Valley is a classic modern example: as the African plate is pulled apart by tensional stress, a central block drops down between parallel normal faults to form a rift valley. Fault planes in normal faults are typically steep, dipping at 60° or more.
On a geological map, you can recognise a normal fault by the displacement of rock units. A distinctive feature is that younger rocks are exposed on the downthrown side of the fault and older rocks on the upthrown side, because the fault cuts part of the succession out. A sequence that is repeated points to a reverse or thrust fault, not a normal one. In cross-section, the hanging wall drops, widening the exposure of younger surface rocks in the centre of a downfaulted block.
Reverse and Thrust Faults — Compressional Stress
A reverse fault forms when compressional stress drives the hanging wall upward relative to the footwall. This is the opposite of a normal fault. Reverse faults have steep fault planes, typically dipping at 45° or more.
When the fault plane is shallow — dipping at 45° or less — the structure is specifically called a thrust fault. Thrust faults are spectacular: compressional stress can push entire sheets of older rock many kilometres over younger rocks. The Moine Thrust in north-west Scotland is a famous example, where ancient Precambrian rocks were thrust horizontally over younger Cambrian strata during the Caledonian orogeny. The diagnostic field observation is finding older rocks sitting directly on top of younger ones — apparently the “wrong way round” by the law of superposition.
This mix-up is easy to make. The key is the word “normal” — it refers to what happens normally under gravity: the hanging wall slips down. A reverse fault is the reverse of that — the hanging wall moves up. In a diagram, find the fault plane first and see which way it slopes. The block sitting on top of the sloping plane is the hanging wall. If that block has moved up, the fault is reverse. If it has moved down, the fault is normal. Practise sketching both types with arrows showing the hanging-wall movement direction.
Strike-Slip Faults — Shear Stress
A strike-slip fault forms when shear stress causes the two sides of a fault to slide horizontally past each other. There is little or no vertical movement. Strike-slip faults are associated with conservative plate boundaries, where two plates grind sideways past each other. The San Andreas Fault in California is the world’s most studied strike-slip fault: the Pacific Plate moves north-west past the North American Plate at roughly 5 cm per year.
On a geological map, strike-slip faults are identified by the sideways offset of rock units, dykes, or other linear features that cross the fault. If you face the fault and the block on the other side has moved to the right, it is called a dextral (right-lateral) fault. If it has moved to the left, it is sinistral (left-lateral). Identifying the offset of a distinctive marker — such as a dyke displaced by a fault — is a common exam question on the practical-skills component.
Normal fault → tensional stress, hanging wall down (divergent boundaries). Reverse/thrust fault → compressional stress, hanging wall up (convergent boundaries). Strike-slip fault → shear stress, horizontal movement (conservative boundaries).
When a fault cuts across a series of rock units on a map, the units appear displaced — they start on one side of the fault line and continue at a different position on the other. For a normal or reverse fault, the displacement is vertical: units on the downthrown side are lower. For a strike-slip fault, units are shifted sideways. The amount of shift is called the fault displacement. You can calculate horizontal displacement by measuring the offset of a marker horizon on the map and applying the map scale. Always include units (metres or kilometres) in your answer.
Faults and Earthquakes
Faults are geologically active as long as the stress driving them continues. Movement is rarely smooth — blocks lock together until stress overcomes friction, then slip rapidly, releasing energy as seismic waves. This is the mechanism of most earthquakes. The zone of crushed and ground-up rock along the fault plane is called fault breccia. Deep faults that cut right through the lithosphere are particularly significant because they control the location of volcanic activity and earthquake zones, both of which you will meet in the plate boundaries lesson.