Topic 73: Earth Interior & Geological Time Scale

Study Material

Paper Paper II

Unit 3

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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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

Key facts

Earth's Crust — Two Types — Outermost solid layer; less than 1% of Earth's volume
Three Major Seismic Discontinuities — Mohorovičić (Moho) at ~35 km depth — separates crust from mantle; discovered 1909
The Mantle — Earth's Largest Layer — Extends from 35 km to 2,900 km depth; 84% of Earth's volume
Earth's Core — Outer and Inner - Outer core (2,900–5,100 km): liquid iron-nickel, 3,700–4,300°C;
Geological Time Scale — Hierarchy — Divides Earth's 4.6-billion-year history into Eons → Eras → Periods → Epochs

Key Points at a Glance

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
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
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
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
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
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
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
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
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
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
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
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

Introduction and Syllabus Scope

Earth interior and the geological time scale matter in RPSC Mains because the topic combines high-recall facts with conceptual links to plate tectonics, rocks, earthquakes, and long-term Earth history.

Why This Topic Matters

Topic 73 is among the most consistently tested geography topics in RPSC Mains. Over the 6-year PYQ record (2013–2023), it has yielded 29 marks at an average of 5.8 marks per year — the highest among the five World Geography topics (73–77) in Paper II. Questions have been asked in 2016, 2018, 2021, and 2023, making it one of the safest bets for 2026. According to the RPSC Mains 2018 syllabus, the written examination consists of 4 descriptive/analytical papers.

Two Interconnected Themes

The topic covers:

(a) Structural layers of Earth's interior — composition, thickness, temperature, and the discontinuities between them
(b) Geological time scale — Earth's 4.6-billion-year history divided into eons, eras, periods, and epochs, with key biological and physical events marking each division

Exam Strategy

For 5-mark answers, precise numbers — depths, temperatures, time ranges in million years — are essential. For 10-mark answers, connect the interior structure to plate tectonics, or trace the evolution of life through the geological time scale.

RPSC has asked both "fact retrieval" questions (Mesozoic era time range, Structure of Sial, Carboniferous characteristics) and conceptual questions (plate tectonics, rock types). The strongest answers use labelled diagrams for the interior, clean chronology for the time scale, and one Rajasthan or India example only where it supports the world-geography point.

Scope Note

This topic is world-scope (global physical geography). Do not confuse with Topic 78 (Physiography of India) or Topic 83 (Physiography of Rajasthan). The geological time scale is universal; specific reference to Indian geology (Gondwana coalfields, Deccan Traps) is valid supporting evidence but not the focus.

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PREDICTED Predicted RAS Questions

Based on PYQ trends and 2026 syllabus analysis

1 5M What is the Mohorovičić discontinuity? State its significance. 5 marks · 50 words

Model Answer

The Mohorovičić discontinuity (Moho), discovered by Croatian geologist Andrija Mohorovičić in 1909, is the boundary between Earth's crust and mantle at approximately 35 km depth under continents (10 km under oceans). Seismic P-wave velocity abruptly increases from ~6 km/s to ~8 km/s here, indicating a density and compositional change from lighter Sial/Sima crust to denser olivine-pyroxene mantle.

~50 words • 5 marks