📖 PART 1: Interior of the Earth
The interior of the Earth cannot be observed directly. Information about its
structure comes from: seismic waves (behaviour of earthquake P and S waves), volcanic
eruptions, mining data, and meteorite study.
Three Main Layers
| Layer |
Depth |
Composition |
State |
Temperature |
| Crust |
0–70 km (oceanic: ~5–10 km; continental: 30–70 km) |
Sial (Silicon + Aluminium) — oceanic crust; Sima (Silicon + Magnesium/Iron) on ocean floor |
Solid |
200–900°C |
| Mantle |
70–2,900 km |
Peridotite (olivine, pyroxene — rich in iron, magnesium, silicon) |
Upper mantle solid; Asthenosphere (partial melt/semi-plastic); lower mantle solid |
900–3,700°C |
| Core |
2,900–6,371 km (centre) |
Nickel + Iron (Ni-Fe → Nife) |
Outer core: LIQUID; Inner core: SOLID (under extreme pressure) |
4,000–6,000°C+ |
The Crust — Sial and Sima
- Sial (Continental Crust): Silicon + Aluminium. Lighter, less dense (~2.7 g/cm³). Forms
the continents. Granite-type rocks. Floats on the denser Sima below.
- Sima (Oceanic Crust): Silicon + Magnesium (and iron). Denser (~3.0 g/cm³). Forms the
ocean floor. Basalt-type rocks.
- Mohorovičić Discontinuity (Moho): The boundary between the crust and the mantle — where
seismic wave speed increases sharply. Named after Croatian seismologist Andrija Mohorovičić.
The Mantle
- Largest layer by volume (~84% of Earth's total volume). Extends from Moho (~35 km) to ~2,900 km depth.
- Asthenosphere: A weak, semi-plastic zone in the upper mantle (~100–350 km depth). Rock
here is close to melting point — can flow slowly. The tectonic plates "float" and move on the
asthenosphere.
- Below the asthenosphere, the mantle is more solid (lower mantle or mesosphere).
- Gutenberg Discontinuity: Boundary between mantle and outer core (~2,900 km depth) —
marked by sudden change in seismic wave behaviour (S-waves stop here, since outer core is liquid).
The Core
- Outer Core: Liquid iron-nickel; ~2,900–5,150 km depth. Movement of liquid iron
generates Earth's magnetic field (geodynamo effect).
- Inner Core: Solid iron-nickel; ~5,150–6,371 km depth. Despite extreme temperature,
enormous pressure keeps it solid.
- Lehmann Discontinuity: Boundary between outer and inner core (~5,150 km depth),
discovered by Danish seismologist Inge Lehmann (1936).
📝 Quick Revision Facts
| Fact |
Value / Description |
| Thinnest layer of Earth |
Crust (5–70 km thick) |
| Thickest layer of Earth |
Mantle (2,830 km thick) |
| Core composition |
Nickel + Iron (Nife) |
| Outer core state |
Liquid (generates Earth's magnetic field) |
| Inner core state |
Solid (despite high temperature — due to pressure) |
| Moho discontinuity |
Boundary between crust and mantle |
| Gutenberg discontinuity |
Boundary between mantle and outer core (~2,900 km) |
| Lehmann discontinuity |
Boundary between outer and inner core (~5,150 km) |
| Asthenosphere |
Semi-plastic upper mantle; tectonic plates move on it |
📌 Chapter Summary
- Earth has 3 layers: Crust (Sial + Sima), Mantle (solid upper + semi-plastic Asthenosphere + solid
lower), Core (liquid outer + solid inner).
- Crust: Sial = continental (Al, Si, lighter), Sima = oceanic (Mg, Fe, denser). Moho = crust-mantle
boundary.
- Mantle: peridotite; largest layer (~84% volume); Asthenosphere allows plate movement. Gutenberg
discontinuity = mantle-core boundary.
- Core = Nife (Nickel + Iron). Outer core = liquid → generates Earth's magnetic field. Inner core =
solid. Lehmann discontinuity separates them.