Key Points at a Glance

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

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

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

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

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)

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

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

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)

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

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

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

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