73. Earth Interior and Geological Time Scale — Full Notes

CORE Key Points at a Glance

1. 1

   Earth's Crust — Two Types

   • Outermost solid layer; less than 1% of Earth's volume
   • Continental crust: 30–70 km thick (avg 35 km), composed of Sial (Silica + Aluminium), density 2.7 g/cm³
   • Oceanic crust: 5–10 km thick, composed of Sima (Silica + Magnesium), density 3.0 g/cm³
2. 2

   Three Major Seismic Discontinuities

   • Mohorovičić (Moho) at ~35 km depth — separates crust from mantle; discovered 1909
   • Gutenberg discontinuity at 2,900 km — separates mantle from outer core; discovered 1914
   • Lehmann discontinuity at 5,100 km — separates outer core from inner core; discovered 1936
3. 3

   The Mantle — Earth's Largest Layer

   • Extends from 35 km to 2,900 km depth; 84% of Earth's volume
   • Composed mainly of olivine and pyroxene (Sima composition); temperature 1,000–3,700°C
   • Asthenosphere (100–350 km): partially molten zone that enables plate tectonic movement
4. 4

   Earth's Core — Outer and Inner

   • Outer core (2,900–5,100 km): liquid iron-nickel, 3,700–4,300°C; circulation generates Earth's magnetic field via geodynamo
   • Inner core (5,100–6,371 km): solid iron-nickel despite ~5,500°C due to extreme pressure; density ~13 g/cm³
   • S-waves absent in outer core — proof it is liquid
5. 5

   Geological Time Scale — Hierarchy

   • Divides Earth's 4.6-billion-year history into Eons → Eras → Periods → Epochs
   • Four eons: Hadean (4,600–4,000 Ma), Archean (4,000–2,500 Ma), Proterozoic (2,500–541 Ma), Phanerozoic (541 Ma–present)
   • First three eons together = Precambrian (88% of Earth's history)
6. 6

   Palaeozoic Era (541–252 Ma) — Six Periods

   • Six periods: Cambrian, Ordovician, Silurian, Devonian, Carboniferous, Permian
   • Cambrian Explosion (~541 Ma): rapid diversification; most major animal phyla appear
   • Carboniferous (359–299 Ma): coal-forming swamp forests; first reptiles; atmospheric O₂ = 35% — PYQ 2023
   • Era ended with Permian-Triassic extinction (252 Ma) — 96% of all marine species extinct
7. 7

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

   • Three periods: Triassic (252–201 Ma), Jurassic (201–145 Ma), Cretaceous (145–66 Ma)
   • First dinosaurs (~230 Ma); first birds — Archaeopteryx (150 Ma); flowering plants (~130 Ma)
   • K-Pg mass extinction (~66 Ma): Chicxulub asteroid impact; 75% of species extinct; dinosaurs vanish
8. 8

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

   • Three periods: Paleogene (66–23 Ma), Neogene (23–2.58 Ma), Quaternary (2.58 Ma–present)
   • India collides with Asia (~50 Ma) → Himalayan orogeny begins
   • Pleistocene (2.58–0.012 Ma): ~20 glaciation cycles; sea levels 120 m lower than today
   • Homo sapiens evolved ~300,000 years ago; modern civilisation in the Holocene (12,000 BP–present)
9. 9

   Seismic Wave Analysis — Reading Earth's Interior

   • P-waves (Primary/Compressional): travel through all media (solid, liquid, gas)
   • S-waves (Secondary/Shear): travel only through solids; absent in outer core → proves it is liquid
   • Shadow zone (103°–143° from earthquake epicentre) for P-waves: confirmed liquid outer core
10. 10

    Isostasy — Floating Crust

    • Concept of gravitational equilibrium where crust "floats" on denser mantle
    • Airy model: mountain ranges have deep "roots" compensating for height
    • Pratt model: lower-density rocks exist beneath mountains
    • Explains post-glacial rebound — mountains rise after glacial ice melts
11. 11

    Plate Tectonics — Unified Theory

    • Unified continental drift (Wegener, 1912) and sea-floor spreading (Hess, 1960)
    • Earth's lithosphere divided into 7 major plates: Pacific, North American, Eurasian, African, Antarctic, Indo-Australian, South American
    • Convection currents in mantle (driven by primordial heat + radioactive decay) drive plate movement
12. 12

    Rock Cycle — Three Rock Types

    • Igneous rocks: crystallised from magma/lava — granite (intrusive), basalt (extrusive)
    • Sedimentary rocks: deposited and compacted layers — sandstone, limestone, coal, petroleum
    • Metamorphic rocks: altered by heat/pressure — marble (from limestone), quartzite (from sandstone), slate (from shale)
    • All fossil fuels (coal, petroleum) form only in sedimentary sequences