Geology tricks that make deep time click

Hadean → Archean → Proterozoic → Phanerozoic. 'Hungry Alligators Prowl Ponds.'

Hadean4.6–4.0 Ga — hell, bombardment, no rocks

Archean4.0–2.5 Ga — first crust, first life

Proterozoic2.5 Ga–541 Ma — O₂ rise, snowball Earth

Phanerozoic541 Ma–present — visible complex life

Paleozoic (ancient life) → Mesozoic (middle, dinosaurs) → Cenozoic (recent, mammals). PAM (P=Paleozoic, A=and, M=Mesozoic then Cenozoic — mnemonic for Phanerozoic eras).

The Three Phanerozoic Eras — Paleozoic (541–252 Ma): ancient life — Cambrian Explosion, first fish, first land plants, first reptiles, first insects. Ends with Permian mass extinction (96% marine species gone — 'the Great Dying'). Mesozoic (252–66 Ma): middle life — dinosaurs (non-avian), first birds, first mammals (small), flowering plants. Ends with K-Pg extinction (Chicxulub asteroid). Cenozoic (66 Ma–present): recent life — mammals diversify, grasses evolve, primates, humans. We are in the Cenozoic (specifically Quaternary Period, Holocene Epoch, or Anthropocene). Quick memory: PAM — Paleozoic, and Mesozoic, and Cenozoic.

Paleozoic periods: Cambrian, Ordovician, Silurian, Devonian, Carboniferous, Permian. 'COSDCP (C=Cambrian, O=Ordovician, S=Silurian, D=Devonian, C=Carboniferous, P=Permian — the six Paleozoic periods).'

Cambrian541–485 Ma — explosion of animal life

Ordovician485–444 Ma — marine, ends with extinction

Silurian444–419 Ma — first land plants

Devonian419–359 Ma — first forests, amphibians

Carboniferous359–299 Ma — coal swamps, first reptiles

Permian299–252 Ma — Pangaea, Great Dying

Mesozoic: Triassic → Jurassic → Cretaceous. 'The Jurassic Classic.' TJC (T=Triassic, J=Jurassic, C=Cretaceous — the three Mesozoic periods).

Mesozoic Periods — Triassic (252–201 Ma): recovery after Permian extinction, first dinosaurs (~230 Ma), first mammals, first pterosaurs. Ends with Triassic-Jurassic extinction. Jurassic (201–145 Ma): dinosaur dominance — Brachiosaurus, Stegosaurus, Allosaurus. First birds (Archaeopteryx, 150 Ma). Pangaea begins breaking up, Atlantic opens. Cretaceous (145–66 Ma): flowering plants (angiosperms) diversify, T. rex, Triceratops, chalk deposits (Cretaceous = chalk in Latin). Ends with K-Pg (Cretaceous-Paleogene) extinction — Chicxulub asteroid impact, 66 Ma.

Big Five mass extinctions: End-Ordovician, Late Devonian, End-Permian (worst), End-Triassic, End-Cretaceous.

End-Ordovician444 Ma — glaciation, 85% species

Late Devonian375 Ma — ocean anoxia, 75%

End-Permian252 Ma — 96% marine, worst ever

End-Triassic201 Ma — 80%, CAMP volcanism

K-Pg66 Ma — Chicxulub, killed dinosaurs

Index fossil: widespread geographically + short-lived in time = precise age marker. Ammonites, trilobites, foraminifera.

Index Fossils — Good index fossil: (1) geographically widespread, (2) short temporal range, (3) easily recognized, (4) abundant. Examples: trilobites (Cambrian–Permian), ammonites (Devonian–Cretaceous), graptolites (Ordovician–Silurian), foraminifera (many zones in Cenozoic). Zone fossils: index fossils define biostratigraphic zones. Correlation: if same index fossil in two locations → same time period. Biostratigraphy: using fossils to date and correlate. Combine with radiometric dating: biostratigraphy gives relative; radiometric gives absolute.

Radiometric dating: parent isotope decays to daughter at known rate. Half-life = time to decay 50%. t = (t½/ln2) × ln(P+D/P).

Radiometric Dating — When mineral crystallizes: incorporates parent isotope but NOT daughter. Decay clock starts. Measure parent:daughter ratio → calculate age. Key systems: U-Pb (zircon, most precise, 4.6 Ga Earth age), K-Ar (volcanic rock, micas), Rb-Sr (metamorphic), C-14 (organic, < 50,000 yr). Half-life: U-238 = 4.47 Ga; K-40 = 1.25 Ga; C-14 = 5,730 yr. Isochron method: plots multiple samples, gives age without knowing initial composition. Cross-check: multiple systems on same sample → confidence. Dendrochronology calibrates C-14 back ~14,000 years.

Precambrian = 88% of Earth history. Great Oxidation Event (2.4 Ga), Snowball Earth (~720 Ma), first multicellular life (~1.2 Ga).

Precambrian Earth History — Hadean hell: magma ocean, moon-forming impact (~4.5 Ga). Archean: first stable crust (greenstone belts), first life (stromatolites in Pilbara, ~3.5 Ga, maybe earlier). Banded Iron Formations (BIFs): 3.8–1.8 Ga — iron precipitated as O₂ increased, now iron ore deposits. Great Oxidation Event (2.4 Ga): cyanobacteria photosynthesis raised atmospheric O₂ — killed most anaerobic life. Snowball Earth (Sturtian ~720 Ma, Marinoan ~635 Ma): global glaciation possibly to equator. Ediacaran fauna (635–541 Ma): first large complex multicellular life — soft-bodied, before Cambrian explosion.

Cenozoic (66 Ma–present): mammals diversify, India-Asia collision (Himalayas), ice ages, Homo sapiens.

The Cenozoic Era — Paleogene (66–23 Ma): mammals diversify into all niches vacated by dinosaurs. Paleocene-Eocene Thermal Maximum (PETM (Paleocene-Eocene Thermal Maximum), 56 Ma): rapid warming, carbon release — analog for current climate change. Eocene: warm, forests to poles, early whales, horses. Oligocene: grasslands expand, cooling begins. Neogene (23–2.6 Ma): Miocene — grasses spread globally, grazing mammals evolve. Pliocene: ice ages begin, Isthmus of Panama closes (3 Ma) → global ocean circulation change. Quaternary (2.6 Ma–present): Pleistocene ice ages, Homo sapiens (~300 ka), Holocene (11,700 ya–present) — our current interglacial.

Superposition (lower = older), Original Horizontality, Lateral Continuity, Cross-cutting Relationships — relative dating rules.

SuperpositionLower layers = older

HorizontalityOriginal deposition was horizontal

Lateral continuityLayers extend until interrupted

Cross-cuttingCuts are younger than what they cut

Cambrian Explosion (~541 Ma): most animal phyla appear in fossil record within ~20 million years. Burgess Shale.

The Cambrian Explosion — Before 541 Ma: mostly microbial life + Ediacaran soft-body forms. After: arthropods, molluscs, echinoderms, chordates, annelids — most modern animal body plans. Burgess Shale (British Columbia, 508 Ma): exceptional soft-body preservation — Anomalocaris (apex predator), Opabinia, Hallucigenia, Pikaia (early chordate). Chengjiang (China): even older Cambrian fossils. Causes debated: rising O₂, ozone protection, breaking up Gondwana, predator arms race, ecological innovation (eyes — 'light switch hypothesis'). Not truly instantaneous: 20 My is 'fast' in 541 My perspective.

Earth: 4.568 billion years old. Determined by U-Pb dating of meteorites (Canyon Diablo troilite). Moon confirms it.

Age of Earth — Clair Patterson (1956): U-Pb dating of Canyon Diablo iron meteorite → 4.550 Ga (±0.07). Earth and meteorites formed at same time from solar nebula. Oldest Earth rock: Acasta Gneiss (Canada), 4.031 Ga. Oldest mineral: Jack Hills zircon (Australia), 4.404 Ga (survived from original crust). Moon rocks (Apollo): ~4.4–4.5 Ga — consistent with Earth's age. Lord Kelvin (1862): calculated 20–400 Ma from cooling — wrong because he didn't know about radioactive heating. Lead contamination: Patterson's work also revealed massive industrial lead pollution — public health breakthrough.