Structural Geology and Tectonics
What this domain tests
Structural Geology and Tectonics tests your ability to describe and interpret deformation: stress and strain, faults and folds, strike and dip, and plate-boundary processes. Regional questions link structures to Cameroon's tectonic framework - the Cameroon Volcanic Line, the Benue Trough and the Central African Shear Zone.
Core rules and concepts
- Stress is a tensor with three principal axes σ1 ≥ σ2 ≥ σ3. Normal faults need σ1 vertical; reverse faults need σ1 horizontal and σ3 vertical; strike-slip faults need σ1 and σ3 horizontal, σ2 vertical.
- Strike is the compass direction of the line of intersection with horizontal; dip is the angle of steepest descent, measured perpendicular to strike. Record dip direction (e.g., 090°/40°) or use the right-hand rule.
- Apparent dip is always less than true dip: tan(apparent) = tan(true) × sin(angle between the traverse and the true dip direction).
- Folds: anticlines close upwards, synclines close downwards; limbs, hinge line, axial plane, plunge. Younging indicators (cross-bedding, graded bedding, sole marks) distinguish anticline from syncline in inverted or recumbent folds.
- Faults: hanging wall and footwall; normal = extension, reverse/thrust = shortening, strike-slip = shear. Throw is vertical separation, heave is horizontal.
- Plate boundaries: divergent (ridges, normal faults), convergent (subduction, thrusts, fold belts), transform (strike-slip). Hotspots produce chains such as the Cameroon Volcanic Line.
Worked example
A bed dips 40° towards 090°. You measure along a traverse trending 040°. The angle between the traverse and the dip direction is 50°. tan(apparent) = tan 40° × sin 50° = 0.839 × 0.766 = 0.643, so the apparent dip is 32.7°, about 33°. This is less than 40°, as it must be. If the traverse were parallel to strike, the apparent dip would be 0°.
Traps that cost marks
Reporting dip direction as strike (they differ by 90°); confusing normal and reverse faults; calling any upward-closing fold an anticline without checking younging; mixing throw and heave; forgetting that apparent dip can never exceed true dip; treating the Cameroon Volcanic Line as a plate boundary - it is intraplate.