Geological Time Scale — Earth's 4.6-Billion-Year History

3.1 Understanding the Time Scale

The geological time scale (GTS) is an internationally standardised system of chronological dating. It divides Earth's history into named intervals (eons, eras, periods, epochs) based on three types of evidence:

• Stratigraphic record — sequences of rock strata
• Palaeontological record — fossil assemblages (index fossils mark boundaries)
• Radiometric dating — decay rates of radioactive isotopes (mainly U-Pb, K-Ar, Rb-Sr)

Hierarchy: Eon > Era > Period > Epoch > Age

Abbreviations used:

• Ma = Million years ago (mega-annum)
• Ga = Billion years ago (giga-annum)
• BP = Before Present

3.2 The Four Eons

Eon	Duration	% of Earth's History	Key Events
Hadean	4,600–4,000 Ma	~13%	Earth formed; moon-forming impact; oceans form; earliest crust; no life
Archean	4,000–2,500 Ma	~33%	First life (prokaryotes, ~3.7 Ga); first continents (cratons); first banded iron formations
Proterozoic	2,500–541 Ma	~43%	Great Oxidation Event (~2,400 Ma); first eukaryotes (~1,800 Ma); first multicellular life; Snowball Earth episodes
Phanerozoic	541 Ma–present	~12%	Explosion of complex life; all major geological events in familiar record

Note: The first three eons (Hadean + Archean + Proterozoic) = Precambrian (88% of Earth's history). Precambrian life was microbial, leaving few fossils.

3.3 The Palaeozoic Era (541–252 Ma) — "Ancient Life"

Duration: ~289 million years. Divided into 6 periods.

1. Cambrian Period (541–485 Ma)

• The Cambrian Explosion (~541 Ma): rapid diversification producing most major animal phyla (arthropods, molluscs, chordates, echinoderms) within a few million years
• Burgess Shale (Canada) — world's finest Cambrian fossil site
• Atmospheric CO₂: ~20× present levels; O₂: ~60% of present; no terrestrial life
• Gondwana supercontinent forming in southern hemisphere (India part of Gondwana)

2. Ordovician Period (485–444 Ma)

• Diversification of marine invertebrates; first vertebrates (jawless fish)
• Mass extinction (~444 Ma) — 2nd largest in Earth's history; caused by Gondwana glaciation → sea level fall + cooling

3. Silurian Period (444–419 Ma)

• First plants colonise land; first vascular plants; first jawed fish; first insects (primitive)
• Coral reefs proliferate

4. Devonian Period (419–359 Ma) — "Age of Fishes"

• Diversification of fishes (sharks, bony fish); first forests (trees); first amphibians move to land
• Devonian Mass Extinction (~372 Ma) — marine die-off

5. Carboniferous Period (359–299 Ma) — PYQ 2023 (5 marks)

Key characteristics (asked directly in 2023):

• Climate: Warm, humid, equatorial rainforest conditions; high oxygen (~35% — enabling giant insects)
• Vegetation: Dense coal-forming swamp forests of giant tree ferns (Lepidodendron, Sigillaria), horsetails (Calamites), and seed ferns (Glossopteris in Gondwana)
• Coal formation: Dead plant material accumulating in oxygen-poor swamps → peat → compressed over millions of years → coal. World's largest coal seams (including India's Gondwana coalfields) are Carboniferous/Permian origin
• Animal life: First reptiles evolved from amphibians; giant insects (dragonfly Meganeura wingspan 70 cm); first amniote eggs
• Geology: Pangaea begins assembling; Appalachian/Hercynian mountain-building episodes

6. Permian Period (299–252 Ma)

• Assembly of Pangaea complete (~270 Ma)
• Reptiles diversify; first therapsids (mammal ancestors)
• Permian-Triassic (P-T) Mass Extinction (252 Ma) — the "Great Dying": 96% of all marine species and 70% of terrestrial vertebrate species extinct — largest mass extinction in Earth's history; caused by Siberian Traps volcanism releasing CO₂ and SO₂

3.4 The Mesozoic Era (252–66 Ma) — "Age of Reptiles" — PYQ 2021

Duration: ~186 million years. Divided into 3 periods.

1. Triassic Period (252–201 Ma)

• Recovery from P-T extinction; Pangaea exists as single landmass
• First dinosaurs (Late Triassic, ~230 Ma); first mammals (tiny shrews); first pterosaurs (flying reptiles)
• India still part of Gondwana supercontinent

2. Jurassic Period (201–145 Ma)

• Pangaea begins breaking apart — Laurasia (north) and Gondwana (south) separated by Tethys Sea
• Dinosaurs dominate: Brachiosaurus, Stegosaurus, Allosaurus (famous from fossil record)
• First birds evolve from theropod dinosaurs (Archaeopteryx — 150 Ma, transitional fossil)
• India begins separating from Gondwana

3. Cretaceous Period (145–66 Ma) — largest period of Mesozoic

• Angiosperms (flowering plants) appear and diversify (~130 Ma)
• Sea levels among highest in Earth's history; warm climate; no polar ice caps
• Tethys Sea narrows as India drifts northward toward Asia at ~15 cm/year
• Deccan Traps volcanism (India) begins ~67 Ma — massive basalt flows covering ~1.5 million km²
• K-Pg (Cretaceous-Paleogene) Mass Extinction (66 Ma): Chicxulub asteroid impact (Yucatán, Mexico) — 10 km diameter asteroid; 75% of all species extinct including all non-avian dinosaurs. Evidence: global iridium layer in sediment at K-Pg boundary; shocked quartz; impact crater (180 km diameter)

Key Mesozoic Facts for RPSC

• Mesozoic Era: 252–66 Ma (asked directly in 2021: "What is the time range of the Mesozoic era?")
• Correct answer: 252 million years ago to 66 million years ago
• "Age of Reptiles" or "Age of Dinosaurs"
• Three periods: Triassic, Jurassic, Cretaceous

3.5 The Cenozoic Era (66 Ma–Present) — "Age of Mammals"

Duration: ~66 million years. Three periods.

1. Paleogene (66–23 Ma)

• Rapid mammal diversification after dinosaur extinction
• India collides with Asia (~50 Ma) → Himalayan orogeny begins; Tethys Sea closes; Mediterranean Sea isolated
• First horses, whales, primates appear

2. Neogene (23–2.58 Ma)

• Grasslands spread globally; horses diversify; hominid ancestors appear (~7 Ma)
• Himalayas reach current heights; Monsoon system strengthens (~8 Ma)
• Gondwana coalfields in India already formed; Himalayan rivers established

3. Quaternary (2.58 Ma–Present)

• Pleistocene Epoch (2.58–0.012 Ma): ~20 glaciation cycles; massive ice sheets covered North America, Europe, Scandinavia to 3 km thickness; sea levels 120 m lower than today; megafauna (mammoths, woolly rhinos, sabre-tooth tigers)
• Holocene Epoch (12,000 BP–present): Current warm interglacial; rise of human civilisation; Harappan, Mesopotamian, Egyptian civilisations; India's rivers reach current patterns

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