Question 4 of 10: Question 3, Part 2: Cross-Section of the Earth

Question 3, Part 2: Cross-Section of the Earth (10 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Crust. The thin, rigid, outermost layer: 5–10 km thick under oceans (basaltic, density ≈3.0 g/cm³), 30–70 km under continents (granitic/felsic upper crust over a more mafic lower crust, average density ≈2.7 g/cm³). Chemically distinct from the mantle (SiO₂-Al-rich), and separated from it by the Mohorovičić (Moho) discontinuity.

Mantle. Extends from the Moho to ~2890 km depth (~84% of Earth's volume); composed of ultramafic silicate rock (peridotite: olivine + pyroxene), density increasing from ~3.3 g/cm³ near the top to ~5.5 g/cm³ near the core-mantle boundary as pressure compresses the same broad composition through several mineral phase changes (e.g. olivine → spinel → perovskite structures). Solid rock overall, but it flows plastically (convects) over geologic time; the upper, cooler, rigid part forms the base of the lithosphere, and a weaker, partially ductile asthenosphere underlies it.

Outer core. From ~2890 km to ~5150 km depth (thickness ~2260 km); LIQUID iron-nickel alloy (with minor light elements S, O, Si), density ~9.9–12.2 g/cm³. Its vigorous convective/ convective-plus-rotational (geodynamo) motion generates Earth's magnetic field.

Inner core. From ~5150 km to the centre at 6371 km (radius ~1220 km); SOLID iron-nickel alloy, density ~13.0 g/cm³ (the highest in the Earth) – solid despite being the hottest zone, because the immense pressure at the centre raises the melting point above the actual temperature there.