Unit 3: Earth's Systems and the Rock Cycle
Reading the Earth as a set of interacting systems
Unit 3 asks you to treat Earth as four connected spheres - atmosphere, hydrosphere (including the cryosphere), geosphere and biosphere - and to trace matter and energy between them. Candidates must understand plate tectonic processes (divergence, convergence, transform boundaries), the rock cycle, and how weathering, erosion, transport and deposition link the geosphere to the hydrosphere and atmosphere. You should be able to interpret stratigraphic sequences using superposition, cross-cutting relationships and index fossils, and to explain how the fossil record and isotopic dating constrain Earth's history.
Traps that appear in questions
- Confusing weathering with erosion. Weathering breaks rock down in place; erosion moves the material. A question about "breakdown without transport" wants weathering.
- Mixing up the three rock types. Igneous rocks crystallise from melt; sedimentary rocks form from deposited and lithified material; metamorphic rocks recrystallise under heat and pressure without melting. A rock that has melted and re-solidified is igneous, not metamorphic.
- Misreading relative dating. A cross-cutting intrusion is younger than the layers it cuts, but an inclusion within a rock is older than its host.
- Ignoring scale. Human timescales versus geological timescales cause many errors in reasoning about rates.
Worked example: a rule of thumb for relative dating
A sequence shows horizontal sedimentary layers A, B and C (A at the bottom), cut by a vertical igneous dyke D, with an unconformity above C overlain by layer E. Apply the principles in order: A is oldest (superposition), then B, then C; D cuts all three, so D is younger than C; E sits above the erosion surface, so E is younger than D. Order: A, B, C, D, E. State the principle you used for each step - markers reward the reasoning, not just the order.