Earth's Interior — Layers, Composition, and Discontinuities

2.1 The Crust

The crust is the outermost solid layer of the Earth, representing less than 1% of Earth's volume. It is the zone where all human activity, mineral extraction, and surface geological processes occur.

Continental Crust

• Thickness: 30–70 km (average 35 km; up to 70 km under the Himalayas and Andes)
• Composition: Primarily Sial — Silica (SiO₂) and Aluminium (Al₂O₃) dominant minerals; granites and gneisses are characteristic rocks
• Density: ~2.7 g/cm³ (lighter, hence floats on denser mantle)
• Age: Ancient rocks up to 4.0 billion years old (Acasta Gneiss, Canada — oldest known crustal rocks)

Oceanic Crust

• Thickness: 5–10 km (much thinner than continental crust)
• Composition: Primarily Sima — Silica (SiO₂) and Magnesium (MgO) dominant; basalts are characteristic rocks
• Density: ~3.0 g/cm³ (denser — subducts under continental crust at convergent plate boundaries)
• Age: Young — no oceanic crust older than ~200 million years (continuously recycled by plate tectonics)

Key Minerals in the Crust: Feldspar (51.3%), Quartz (12.5%), Pyroxene (11%), Mica (5%), Olivine (3%), others.

2.2 Discontinuities — The Boundaries Between Layers

Three major seismic discontinuities divide Earth into its primary layers, identified through the behaviour of seismic waves:

Discontinuity	Depth	Separates	Discovery
Mohorovičić (Moho)	~35 km (continental), ~10 km (oceanic)	Crust from Mantle	Andrija Mohorovičić, 1909 (Yugoslavia)
Gutenberg (Wiechert-Gutenberg)	2,900 km	Mantle from Outer Core	Beno Gutenberg, 1914
Lehmann	5,100 km	Outer Core from Inner Core	Inge Lehmann, 1936
Conrad	~15–20 km	Upper and Lower Crust	V. Conrad, 1925

How Discontinuities are Detected

Seismic waves (P-waves and S-waves) change velocity and direction (refraction) or reflect at boundaries between materials of different density/elasticity. Sudden velocity changes indicate a discontinuity. The complete absence of S-waves in the outer core zone proved its liquid state.

2.3 The Mantle

The mantle extends from the Moho (~35 km) to the Gutenberg discontinuity (2,900 km), comprising about 84% of Earth's volume and 68% of Earth's mass.

Upper Mantle (35–670 km)

• Composition: Olivine [(Mg,Fe)₂SiO₄] and Pyroxene — rocks called peridotite
• Asthenosphere (100–350 km): A partially molten zone where rock is plastic/ductile. Temperature exceeds the melting point of rock, but pressure keeps it largely solid. This zone "lubricates" plate movement. Seismic wave velocities slow markedly here (Low Velocity Zone — LVZ)
• Lithosphere: The rigid crust + uppermost solid mantle above the asthenosphere (~0–100 km) forms the tectonic plates

Lower Mantle (670–2,900 km)

• Minerals transform to denser phases under pressure — Perovskite (MgSiO₃) and Post-perovskite dominate
• Temperature: 3,000–3,700°C; fully solid despite high temperature due to extreme pressure
• Convection currents in the mantle (driven by residual primordial heat + radioactive decay of U, Th, K) drive plate tectonics

Heat Sources in Earth's Interior

1. Primordial heat — residual from Earth's formation (~4.5 billion years ago)
2. Radiogenic heat — decay of radioactive isotopes: Uranium-238, Thorium-232, Potassium-40
3. Ratio: approximately 50:50 primordial vs radiogenic

2.4 The Core

Outer Core (2,900–5,100 km)

• Liquid iron-nickel (Fe-Ni alloy) with minor amounts of lighter elements (sulphur, oxygen, silicon)
• Temperature: 3,700–4,300°C
• S-waves do not pass through it (confirming liquid state)
• The geodynamo: Differential rotation of the liquid outer core around the solid inner core generates Earth's magnetic field — essential for protecting life from solar wind and cosmic radiation
• The magnetic poles are ~11° offset from the geographic poles; they "wander" and have reversed hundreds of times in Earth's history (paleomagnetism evidence for plate tectonics)

Inner Core (5,100–6,371 km)

• Solid iron-nickel — remains solid despite temperature of ~5,500°C due to extreme pressure (3.6 million atmospheres)
• Density: ~13 g/cm³
• Radius: ~1,271 km
• The inner core rotates slightly faster than the rest of Earth (differential rotation — observed through seismic tomography)

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