Earth science - geology, strata, fossils, and Earth's history
What the exam tests
Rock types, strata, fossils, geological time and Earth processes. Expect interpretation of rock sequences and simple plate-tectonic explanations.
Core rules
- Igneous rock forms from cooled magma or lava; sedimentary rock forms from compacted deposits; metamorphic rock forms under heat and pressure.
- The law of superposition: in undisturbed sedimentary strata, lower layers are older.
- Cross-cutting faults or igneous intrusions are younger than the rocks they cut.
- An unconformity is a gap in the record caused by erosion or non-deposition.
- Fossils are mainly found in sedimentary rock. Index fossils are widespread and short-lived, so they correlate layers.
- Earth's history is divided into eras and periods; plate movement causes earthquakes, volcanoes and mountain building.
Worked example
A cliff shows sandstone at the bottom, shale above, then limestone, with a vertical igneous dyke cutting all three. The sandstone is oldest, then shale, then limestone. The dyke is youngest because it cuts across all the layers. If the limestone contains a trilobite index fossil, the limestone can be correlated with other rocks of the same age.
Traps
- Superposition does not apply if strata have been overturned by folding.
- Fossils are rare in igneous rock because heat destroys remains.
- A fault is younger than the beds it displaces, but older than any undisturbed bed deposited on top.