Interior of the Earth

Introduction

The Earth has a radius of about 6,371 km. Humans have never drilled deeper than about 12 km (the Kola Superdeep Borehole in Russia), so our knowledge of the Earth's interior comes entirely from indirect evidence — primarily the study of seismic waves produced by earthquakes.

Sources of Information About the Earth's Interior

• Direct sources (limited):
• Mining and drilling operations (surface only)
• Rock samples from volcanoes (magma from depth)
• Meteorites (analogues for Earth's composition)

• Indirect sources (primary):
• Seismic waves: vibrations generated by earthquakes that travel through the Earth
• Gravitational and magnetic field measurements
• Volcanic eruptions
• Temperature and pressure studies

Seismic Waves — The Key to Understanding the Interior

Earthquakes generate two main types of seismic waves that travel through the Earth's interior:

• 1. P-waves (Primary waves / Compressional waves):
• Travel through solids, liquids, and gases
• Fastest seismic waves (~6-13 km/s in the crust/mantle)
• Move by compressing and expanding material in the direction of travel
• Can pass through the Earth's core

• 2. S-waves (Secondary waves / Shear waves):
• Travel only through solids — cannot pass through liquids or gases
• Slower than P-waves (~3.5-7.5 km/s)
• Move by shaking material perpendicular to the direction of travel
• Cannot pass through the outer core — this is how we know the outer core is liquid

• 3. Surface waves (L-waves / Love and Rayleigh waves):
• Travel along Earth's surface, not through the interior
• Most destructive during earthquakes
• Slowest of all seismic waves

Shadow Zones

When seismic waves pass through Earth's interior, certain areas on the opposite side of the globe do NOT receive seismic waves from an earthquake — these are called shadow zones.

• P-wave shadow zone: a ring-shaped zone between 103 degrees and 143 degrees from the earthquake epicentre — P-waves are refracted (bent) by the core.
• S-wave shadow zone: the entire region beyond 103 degrees on the opposite side — because S-waves cannot pass through the liquid outer core at all.

The existence of shadow zones proves that the Earth's interior is not uniform and has a liquid outer core.

Internal Structure of the Earth

The Earth's interior is divided into three main layers based on composition:

1. Crust

• Outermost, thinnest layer
• Continental crust: 30-50 km thick; made of lighter granitic rocks (rich in Si and Al — called SIAL)
• Oceanic crust: 5-10 km thick; made of denser basaltic rocks (rich in Si and Mg — called SIMA)
• The boundary between the crust and mantle is the Mohorovicic Discontinuity (Moho), where seismic wave speeds increase sharply.

2. Mantle

• Extends from the Moho to a depth of 2,900 km
• Made mainly of silicate rocks rich in iron and magnesium (ultrabasic rocks like peridotite)
• Upper mantle includes the rigid lithosphere (crust + upper mantle) and the semi-molten asthenosphere (where isostatic adjustments occur)
• The boundary between the mantle and core is the Gutenberg Discontinuity.

3. Core

• Radius of about 3,500 km; made mainly of iron (Fe) and nickel (Ni) — called NIFE
• Outer Core: liquid; generates Earth's magnetic field through convection of molten iron
• Inner Core: solid despite extreme heat, due to immense pressure
• The boundary within the core (outer-inner) is the Lehmann Discontinuity.

Important Discontinuities (Boundaries)

| Discontinuity | Location | Significance |
|---|---|---|
| Conrad Discontinuity | Within continental crust | Separates upper (granitic) and lower (basaltic) crust |
| Mohorovicic (Moho) | Crust-Mantle boundary (~35 km continental) | Sharp increase in seismic wave velocity |
| Gutenberg Discontinuity | Mantle-Core boundary (~2,900 km) | S-waves stop; P-wave velocity drops |
| Lehmann Discontinuity | Outer core-Inner core boundary (~5,100 km) | P-wave velocity increases again |

Temperature, Pressure, and Density

• Temperature increases with depth at about 1 degree C per 32 m in the upper crust (geothermal gradient), but the rate slows deeper.
• Pressure increases steadily with depth.
• Density increases from about 2.7 g/cm³ (crust) to about 13 g/cm³ (inner core).
• The inner core is solid because extreme pressure prevents melting despite temperatures of 5,000-6,000 degrees C.

• Students often confuse P-waves and S-waves: remember S-waves = Shear = can only go through Solids. P-waves = Primary = can pass through everything.
• The shadow zone is not a "dead zone" for all waves — only specific types in specific areas.
• SIAL (Silicon + Aluminium) = continental crust; SIMA (Silicon + Magnesium) = oceanic crust; NIFE (Nickel + Iron) = core. These abbreviations are commonly tested.
• The Moho is NOT the boundary between mantle and core — that is the Gutenberg Discontinuity.

Our knowledge of Earth's interior comes mainly from seismic wave analysis. The Earth has three main layers — crust, mantle, and core — separated by important discontinuities (Moho and Gutenberg). P-waves and S-waves behave differently in solid vs. liquid materials, revealing the liquid outer core and solid inner core. Temperature, pressure, and density all increase with depth, and the inner core is solid due to immense pressure.