Historical Geology, Stratigraphy, Fossils and Geological Time
Domain 3 covers how geologists order events and date them, and how fossils and strata record Earth history. In the PAU it typically appears as a stratigraphic column or cross-section to interpret, plus dating method questions - including the age of the Canarian islands.
Relative dating: the principles
The principles of superposition, original horizontality, lateral continuity, cross-cutting relationships (a fault or intrusion is younger than what it cuts), inclusions (a xenolith is older than its host) and faunal succession let you build a relative chronology. Unconformities mark gaps: angular (tectonism then erosion), disconformity (erosion of horizontal beds), nonconformity (sediment on igneous or metamorphic basement).
Absolute dating
Radioactive decay is exponential; the half-life is fixed. Useful systems: U–Pb in zircon (oldest rocks, >4 Ga), K–Ar and Ar–Ar (volcanic rocks, 10⁵–10⁹ yr, ideal for Canarian lavas), Rb–Sr (metamorphic and plutonic), ¹⁴C (organic carbon only, 0–50 ka, useless on basalt). Younger-over-older relationships plus magnetic polarity chrons calibrate the time scale.
Fossils and the geological time scale
Fossils are index species when short-lived and widespread (ammonites, trilobites, graptolites, foraminifera, calcareous nannoplankton). Biozones allow correlation; fossils also indicate palaeoenvironment (reef corals = shallow warm sea) and palaeoclimate. Time scale: Hadean, Archaean, Proterozoic, Phanerozoic (Palaeozoic, Mesozoic, Cenozoic). Major events: Great Oxidation Event, Cambrian explosion, Permo-Triassic extinction, K–Pg impact, Quaternary glaciations.
Worked example
A cross-section shows horizontal limestones containing ammonites, cut by a basalt dyke, with a granite beneath a nonconformity and an angular unconformity above folded shales. Order the events. Reasoning: folded shales are oldest (deformation before erosion); the granite intruded the shales (cross-cutting) and was then exposed (nonconformity); the limestones were deposited horizontally on top; the dyke cuts the limestones, so it is younger; the angular unconformity records the deformation phase, not a depositional event. The dyke can then be dated by K–Ar, not ¹⁴C.
Traps that cost marks
Dating basalt with ¹⁴C. Reversing superposition. Treating an unconformity as a rock unit. Saying fossils give 'absolute' ages when they give relative correlation. Forgetting that K–Ar dates the cooling of lava, not the sediment around it.
- Canarian shields are Miocene (Fuerteventura, Gran Canaria, ~20–14 Ma); El Hierro and La Palma are the youngest (<2 Ma).
- ¹⁴C only for organic carbon younger than ~50 ka.
- Cross-cutting = younger; inclusion = older.
- Index fossils: short range, wide distribution.
- Magnetic polarity chrons provide global correlation of volcanic sequences.