Internal structure and dynamics of the Earth
Domain 1 - Internal structure and dynamics of the Earth. PAU questions here ask you to justify a layered model from seismic evidence and then explain the heat engine that moves plates. Marks come from separating compositional from mechanical layering and quoting real discontinuities with depths.
Seismic evidence for layering
- P waves (compressional) cross solids, liquids and gases; S waves (shear) only cross solids, so a liquid layer blocks them.
- Refraction and reflection at discontinuities define the interfaces: Moho (crust–mantle, 7–10 km oceanic, 30–70 km continental), Gutenberg (2,900 km, mantle–outer core), Lehmann (5,150 km, outer–inner core).
- The S-wave shadow begins at 103° from the epicentre; a P-wave shadow runs from 103° to 143°. Only a liquid outer core explains both.
- Velocity rises with density and rigidity, but a low-velocity zone at 100–250 km marks the asthenosphere (a few percent partial melt).
Compositional versus mechanical layers
- Compositional: crust (granitic continental, basaltic oceanic), mantle (peridotite, olivine, then bridgmanite below 660 km), core (Fe–Ni with light elements).
- Mechanical: lithosphere (crust plus uppermost mantle, elastic, roughly 100 km), asthenosphere, mesosphere, outer core, inner core. The lithosphere is not the crust - this is the most frequent error.
- Isostasy: the Airy model (thicker crust floats deeper, giving mountain roots) and the Pratt model (equal depth, different density). Post-glacial rebound in Fennoscandia proves that material beneath the lithosphere flows.
Heat and dynamics
- Heat sources are accretion and gravitational differentiation plus radioactive decay of 40K, 238U, 235U and 232Th.
- The continental geotherm averages 25–30 °C per km; heat flow peaks at mid-ocean ridges and is lowest in old cratons.
- Solid-state mantle convection (creep of centimetres per year) together with slab pull drives plates; the geodynamo is convection of liquid iron in the outer core, organised by rotation.
Worked example
A station records P at 00:00 and S at 06:00, so S–P is 6 min. Near 40° the S–P interval grows about 8 s per degree, so 360 s ÷ 8 s/° ≈ 45° of epicentral distance, roughly 5,000 km (travel-time tables give about 4,400 km at 40°). One station gives distance only: three stations are needed to triangulate the epicentre, and focal depth requires arrival curvature or a station close to the epicentre.
Traps that cost marks
- Saying the mantle is liquid because it convects - it is solid and flows by creep.
- Confusing lithosphere with crust, or the Moho with the lithosphere–asthenosphere boundary.
- Claiming S waves cross the outer core.
- Forgetting that the inner core stays solid near 5,000 °C because pressure raises the melting point.