Earth's 4.6 billion years divided into four major time chunks — the biggest divisions of geologic time
Hadean (4.6–4.0 Ga): hell-like, heavy bombardment, no preserved rocks (oldest crust ~4.0 Ga, Jack Hills zircons ~4.4 Ga). Archean (4.0–2.5 Ga): first stable crust, first life (stromatolites ~3.5 Ga), anoxic atmosphere. Proterozoic (2.5 Ga–541 Ma): Great Oxidation Event (2.4 Ga), Snowball Earth episodes, first multicellular life (~1.2 Ga). Phanerozoic (541 Ma–present): visible life — Paleozoic, Mesozoic, Cenozoic. 88% of Earth's history is Precambrian. The Phanerozoic is just the most recent 12%.
The three eras of visible life — each separated by a mass extinction
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.
Six periods spanning 289 million years — from the Cambrian Explosion to the Great Dying
Cambrian (541–485 Ma): explosion of complex animal life — most animal phyla appear. Ordovician (485–444 Ma): marine invertebrates diversify; ends with glaciation + mass extinction. Silurian (444–419 Ma): first vascular land plants, first jawed fish. Devonian (419–359 Ma): 'Age of Fishes' — first forests, first amphibians. Carboniferous (359–299 Ma): vast coal swamps (source of most coal), first reptiles. Permian (299–252 Ma): Pangaea assembled; ends with biggest mass extinction in history (~96% marine, ~70% terrestrial species).
Cambrian
541–485 Ma — explosion of animal life
Ordovician
485–444 Ma — marine, ends with extinction
Silurian
444–419 Ma — first land plants
Devonian
419–359 Ma — first forests, amphibians
Carboniferous
359–299 Ma — coal swamps, first reptiles
Permian
299–252 Ma — Pangaea, Great Dying
Mesozoic Periods
Mesozoic: Triassic → Jurassic → Cretaceous. 'The Jurassic Classic.' TJC.
Mesozoic Periods
The Age of Reptiles — three periods spanning 186 million years
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.
Mass Extinctions
Big Five mass extinctions: End-Ordovician, Late Devonian, End-Permian (worst), End-Triassic, End-Cretaceous.
The Big Five Mass Extinctions
Five catastrophic events that reset the history of life on Earth
1. End-Ordovician (444 Ma): glaciation, 85% species lost. 2. Late Devonian (375–360 Ma): ~75% species, ocean anoxia. 3. End-Permian / 'The Great Dying' (252 Ma): ~96% marine, ~70% terrestrial species — worst ever. Cause: Siberian Traps volcanism (volcanic winter + ocean acidification). 4. End-Triassic (201 Ma): ~80% species — Central Atlantic Magmatic Province volcanism. 5. K-Pg (66 Ma): ~76% species — Chicxulub asteroid + Deccan Traps. Current: 6th mass extinction underway (Holocene, anthropogenic) — rates 100–1,000× background.
End-Ordovician
444 Ma — glaciation, 85% species
Late Devonian
375 Ma — ocean anoxia, 75%
End-Permian
252 Ma — 96% marine, worst ever
End-Triassic
201 Ma — 80%, CAMP volcanism
K-Pg
66 Ma — Chicxulub, killed dinosaurs
Index Fossils
Index fossil: widespread geographically + short-lived in time = precise age marker. Ammonites, trilobites, foraminifera.
Index Fossils
Fossils that tell you the age of a rock layer — the geologist's time stamps
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
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
The most powerful tool for establishing absolute ages — using radioactive decay as a clock
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 Earth
Precambrian = 88% of Earth history. Great Oxidation Event (2.4 Ga), Snowball Earth (~720 Ma), first multicellular life (~1.2 Ga).
Precambrian Earth History
The long hidden history of Earth — before animals, before complex life, before much of what we recognize
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.
The Age of Mammals — the last 66 million years that led to us
Paleogene (66–23 Ma): mammals diversify into all niches vacated by dinosaurs. Paleocene-Eocene Thermal Maximum (PETM, 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.
Stratigraphic Principles
Superposition (lower = older), Original Horizontality, Lateral Continuity, Cross-cutting Relationships — relative dating rules.
Steno's Laws
The four foundational principles that allow geologists to read rock sequences like a book
Nicolas Steno (1669): formalized the first geological principles. Superposition: in undisturbed sequence, lower layers older than upper. Original horizontality: sediments deposit horizontally (tilted layers = later deformation). Lateral continuity: layers extend laterally until thinning or interrupted. Cross-cutting relationships: feature cutting through rock is younger than the rock it cuts (fault, dike, intrusion). Inclusion: fragments of one rock in another → the fragments are older. Unconformity: surface of erosion = time gap. These four rules (plus unconformities and cross-cutting) are the foundation of relative dating.
Superposition
Lower layers = older
Horizontality
Original deposition was horizontal
Lateral continuity
Layers extend until interrupted
Cross-cutting
Cuts are younger than what they cut
Cambrian Explosion
Cambrian Explosion (~541 Ma): most animal phyla appear in fossil record within ~20 million years. Burgess Shale.
The Cambrian Explosion
The most dramatic event in the history of animal life — complex animals appear almost overnight (geologically)
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.
Age of Earth
Earth: 4.568 billion years old. Determined by U-Pb dating of meteorites (Canyon Diablo troilite). Moon confirms it.
Age of Earth
How we know Earth is 4.568 billion years old — and why it took until 1956 to prove it
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.