Earth's Surface Processes, Rocks, and Geologic History
This domain tests the rock cycle, weathering and erosion, landform development, and the methods geologists use to reconstruct Earth's history. Questions often require you to interpret rock textures, relative dating diagrams, and half-life calculations.
Rocks and the Rock Cycle
Igneous rocks form from cooling magma or lava; slow cooling makes coarse crystals (intrusive), while fast cooling makes fine or glassy textures (extrusive). Sedimentary rocks form by compaction and cementation of clasts or by chemical precipitation; bedding, cross-bedding, and fossils preserve environments. Metamorphic rocks recrystallize under heat and pressure; foliation develops when pressure is directed. The rock cycle links these via melting, weathering, erosion, deposition, and metamorphism.
Surface Processes
Weathering breaks rock in place; erosion transports it. Streams sort sediment, form meanders and deltas, and shape valleys. Groundwater moves through porous and permeable rocks; the water table marks the saturated zone. Glaciers carve U-shaped valleys and deposit till and moraines. Mass wasting moves material downslope under gravity.
Geologic History and Dating
Relative dating uses superposition, original horizontality, cross-cutting relationships, inclusions, and unconformities. Absolute dating uses radioactive decay: after n half-lives, the parent fraction is (1/2)^n. Index fossils correlate rocks over wide areas but only within short time intervals.
Worked Example: Radiometric Age
A volcanic rock contains 12.5% of its original parent isotope. How old is it if the half-life is 200 million years? 12.5% = 1/8 = (1/2)^3, so three half-lives have passed. Age = 3 × 200 Ma = 600 Ma. Reasoning: each half-life halves the parent; the daughter fraction is 87.5%, assuming no daughter was present initially.
Traps That Cost Marks
- Cross-cutting features are younger than the rocks they cut; inclusions are older than the host.
- Unconformities represent missing time, not continuous deposition.
- Metamorphic dating can record cooling or resetting, not always original formation.
- Coarse texture means slow cooling, not necessarily intrusion at great depth.
- Stream erosion and deposition change with gradient and discharge.