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Cameroon GCE A-Level Geology study materials
Cameroon GCE A-Level Geology study resources

Cameroon GCE A-Level Geology: Historical Geology

Cameroon GCE A-Level Geology · Work through your exam, one topic at a time.

All guide sections

Historical Geology

What this domain tests

Historical Geology tests relative and absolute dating, the geological time scale, major events in Earth history and the evidence used to reconstruct them. Cameroon examples - the Archean Ntem Complex and the Pan-African belt - are often used to test whether you can place local rocks in global time.

Core rules and concepts

  • Relative dating principles: superposition, original horizontality, lateral continuity, cross-cutting relationships, inclusions, faunal succession. Unconformities: angular (tilted beds overlain by flat-lying), disconformity (erosion surface between parallel beds), nonconformity (sediment on igneous/metamorphic basement).
  • Absolute dating uses radioactive decay: N = N0e^(-λt), t1/2 = ln2/λ. Useful systems: U-Pb zircon (oldest, high closure temperature), Rb-Sr and Sm-Nd (whole-rock isochrons), K-Ar and Ar-Ar (volcanic rocks, but reset by heating), C-14 (organic material younger than about 50 ka).
  • Time scale: Hadean, Archean, Proterozoic, Phanerozoic; Phanerozoic = Paleozoic, Mesozoic, Cenozoic; Periods from Cambrian to Quaternary.
  • Key events: Great Oxidation Event ~2.4 Ga; Snowball Earth in the Neoproterozoic; Cambrian explosion 541 Ma; end-Permian extinction 252 Ma; end-Cretaceous extinction 66 Ma; Pan-African orogeny ~600 Ma; Cameroon Volcanic Line from ~65 Ma.

Worked example

A wood sample contains 12.5% of its original 14C. Each half-life (5,730 years) halves the amount: 100% to 50% to 25% to 12.5%, which is three half-lives. Age = 3 × 5,730 = 17,190 years. The same sample cannot be dated by K-Ar because wood contains no potassium feldspar or mica, and 14C is useless for a 200 Ma sandstone because all original 14C has decayed.

Traps that cost marks

Using 14C on igneous or very old rocks; treating K-Ar ages as unreset after burial metamorphism; confusing unconformity types; mixing relative and absolute ages in one statement; placing the Pan-African orogeny in the Mesozoic; forgetting that the geological time scale is defined by GSSPs, not by arbitrary dates.