Physical Geography: Earth's Interior
What this domain tests
This domain examines how geographers infer Earth's internal structure without drilling: seismic-wave behaviour, density, gravity, magnetism, meteorite composition and the discontinuities between layers. Expect questions on layer boundaries, shadow zones and isostasy.
Core rules
- Crust: oceanic 5–10 km, basaltic and denser; continental 30–70 km, granitic. The Conrad discontinuity separates upper and lower crust.
- Mantle extends to 2,900 km; the Repetti discontinuity near 1,000 km separates upper and lower mantle; the asthenosphere is partly plastic, not molten.
- Core: liquid outer core, solid inner core. The Gutenberg discontinuity marks mantle–core; the Lehmann discontinuity marks outer–inner core.
- P-waves are longitudinal and pass through solids and liquids; S-waves are transverse and cannot pass through liquids. Both refract, creating a P shadow zone between about 103° and 142° and an S shadow beyond 103°.
- Density, pressure and temperature increase inward. Isostasy (Airy and Pratt) explains buoyant crustal blocks.
Worked example
A station 120° from an earthquake records P-waves but no S-waves. Reason: S-waves cannot cross the liquid outer core, so no direct S arrives beyond 103°. P-waves are refracted by the core, so the 103°–142° belt is a shadow; a 120° station may receive weak diffracted P. The inference is a liquid outer core - not a recording fault or a hollow Earth.
Traps
Mixing up Moho and Gutenberg; claiming S-waves cross the core; calling the asthenosphere magma; forgetting that refraction, not just blocking, creates the shadow zone.