Earth Structure
Earth Structure
The Earth is layered by density and composition. The crust is thin (about 5–70 km), solid and rocky; the mantle beneath it is largely solid silicate rock but slowly deforms and flows, carrying heat by convection; the outer core is liquid iron-nickel; the inner core is solid iron-nickel under immense pressure. Candidates must distinguish the compositional layers (crust, mantle, core) from the mechanical layers (lithosphere, asthenosphere, mesosphere, outer core, inner core). The lithosphere is the rigid crust plus the uppermost mantle; it is broken into plates that ride on the weaker, partially molten asthenosphere.
Know the evidence for Earth's internal structure: seismic waves, especially the way P-waves speed up and bend at boundaries and S-waves cannot pass through liquids (proving a liquid outer core); density and temperature increasing with depth; meteorite composition; and volcanic material brought from depth.
Traps in questions: confusing the mantle with the asthenosphere; saying the mantle is molten (it is mostly solid); claiming the crust is thickest under oceans (it is thinnest there, thickest under young mountain belts); muddling the liquid outer core with the solid inner core; and forgetting that plate movement is driven by convection plus slab pull and ridge push, not by tides or Earth's rotation.
Rule of thumb: work outward from the centre - inner core (solid), outer core (liquid), mantle (solid but flowing), crust (solid, brittle). For any "which layer is liquid?" question, answer the outer core.
Worked example: A question states that S-waves from an earthquake are not recorded on the far side of the Earth. Reasoning: S-waves are transverse and cannot travel through liquid, so they are blocked by the liquid outer core; P-waves are refracted but still recorded, confirming a solid inner core within the liquid outer core.