Solid Earth: Plate Tectonics, Rocks, Minerals and Processes
This section covers solid Earth processes. The exam tests whether you can link plate motions to rock associations, mineral stability, deformation and magma generation, and whether you can read geological evidence in sequence.
Plate boundaries and driving forces
- Divergent boundaries create new ocean crust and rift basins; convergent boundaries produce subduction, arcs, or continental collision; transform boundaries slide past one another.
- Slab pull is the dominant plate-driving force, with ridge push and mantle convection contributing.
- Subduction zones generate magma by flux melting of the mantle wedge; continental collisions thicken crust and produce metamorphism and folding.
Rocks and minerals
- Igneous rocks crystallise from melt; sedimentary rocks form by weathering, transport, deposition and lithification; metamorphic rocks recrystallise in the solid state.
- Silicate minerals dominate crust and mantle. Cleavage reflects atomic planes, not colour; polymorphs such as quartz and coesite record different pressure conditions.
- Bowen's reaction series explains why mafic minerals crystallise earlier and why magma composition changes during differentiation.
Worked example
Magnetic stripes on ocean crust are symmetric about a mid-ocean ridge. A stripe 150 km from the ridge has crust dated at 3.0 Ma. The half-spreading rate is 150 km divided by 3.0 Myr, or 50 km/Myr, which is 5 cm/yr. The full spreading rate is twice that, 10 cm/yr. Reasoning: distance from the ridge records crust created on one plate, while the full rate includes both plates.
Traps
- Using distance from the ridge as the full spreading rate.
- Assuming every convergent boundary has a volcanic arc; continent-continent collision generally does not.
- Identifying minerals by colour alone.
- Treating metamorphic grade as equivalent to depth without considering pressure and temperature paths.