Earth as a system: structure, composition and dynamics
Questions in this domain test whether you can treat the Earth as one system of coupled reservoirs - geosphere, atmosphere, hydrosphere and biosphere - rather than four unrelated topics. In the PAU you are typically asked to justify the internal structure from indirect evidence, to separate chemical from mechanical layering, and to explain a feedback that links the deep Earth to the surface.
Chemical and mechanical layering
- Chemical layers: crust (felsic to mafic silicates), mantle (peridotite, Mg–Fe silicates) and core (Fe–Ni alloy).
- Mechanical layers: rigid lithosphere (crust plus uppermost mantle, about 100 km), ductile asthenosphere with a seismic low-velocity zone, mesosphere, liquid outer core and solid inner core.
- Discontinuities: Mohorovičić (crust/mantle), Gutenberg at about 2900 km (mantle/outer core) and Lehmann (outer/inner core).
Indirect evidence for the interior
- P waves cross solids and fluids; S waves cross solids only. S waves disappear beyond 103° from the epicentre and P waves are refracted into a shadow between 103° and 142°, which proves a liquid outer core.
- Mean density of the Earth is 5.5 g/cm³ against about 2.7 g/cm³ for surface rocks, so the interior must be denser.
- Iron meteorites serve as compositional analogues, and the geomagnetic field requires a convecting, electrically conducting fluid.
- Isostasy: Airy compensation uses crustal roots, Pratt compensation uses lateral density changes; post-glacial rebound proves that the asthenosphere flows.
Feedbacks
A classic example is the silicate-weathering thermostat: rising temperature speeds chemical weathering, which consumes CO₂, weakens the greenhouse effect and cools the planet.
Worked example
A student writes that the mantle must be molten because lava comes from it. Reasoning: S waves traverse the whole mantle, so the bulk mantle behaves as a solid; melting is restricted to a few percent in the low-velocity zone, at ridges and above subducting slabs, and lava is a segregated melt rather than the bulk rock. The correct conclusion is a solid mantle with local partial melting.
Traps that cost marks
- Equating the lithosphere with the crust; the lithosphere includes the uppermost mantle.
- Confusing Mohorovičić and Gutenberg, or placing the mantle/core boundary at 6371 km instead of about 2900 km.
- Claiming the inner core is liquid; it is solid because pressure raises the melting point faster than temperature rises.
- Ignoring that the S-wave shadow begins at 103°, not at 180°.