Internal Geodynamics and Plate Tectonics
This domain examines the layered interior of the Earth, the geophysical evidence used to reconstruct it, and the way each plate boundary controls magmatism, metamorphism and deformation. PAU items usually give a map, a described cross-section or a rock association and ask you to identify the tectonic setting and justify it physically, not just to name it.
Interior structure and its evidence
- Crust: oceanic (about 7 km, basaltic, density near 2.9 g/cm³) and continental (30–70 km, granitic, near 2.7 g/cm³). The Mohorovičić discontinuity marks crust–mantle; the Gutenberg discontinuity (about 2900 km) separates mantle from core; the Lehmann discontinuity marks the inner core.
- The outer core is liquid, so S-waves cannot cross it. This creates the S-wave shadow beyond 103° from the epicentre, and the P-wave shadow between 103° and 143°.
- Isostasy: Airy's model balances relief with a low-density root; Pratt's model balances it with lateral density contrasts. Glacial rebound and the compacting, sinking Ebro Delta margin are consequences of vertical adjustment.
Boundary types and their signatures
- Divergent (Mid-Atlantic Ridge, East African Rift): normal faulting, shallow seismicity, tholeiitic basalt, high heat flow, ophiolite suites preserved at sutures.
- Ocean–ocean convergence: island arcs with calc-alkaline basalt and andesite. Ocean–continent convergence (Andes): andesite–dacite stratovolcanoes, a Benioff–Wadati zone and paired metamorphic belts, blueschist/eclogite on the oceanic side and greenschist/amphibolite inland. Continent–continent collision (Himalaya, Pyrenees): folding, thrusting, crustal thickening, essentially no volcanism.
- Transform boundaries (San Andreas): strike-slip, shallow earthquakes, no magma generation.
- Driving forces: slab pull dominates, followed by ridge push and basal drag from mantle convection.
Worked example
A region shows andesitic stratovolcanoes, earthquakes deepening from 30 km to 300 km along a plane dipping about 45°, and blueschist–eclogite assemblages. Reasoning: intermediate magma implies partial melting of a hydrated mantle wedge above a slab; the inclined seismic plane is the subducting lithosphere; high-P/low-T metamorphism requires a cold geotherm. Conclusion: ocean–continent subduction. A mid-ocean ridge is excluded because there is no tholeiitic suite and seismicity is not shallow-only; a hot spot is excluded because there is no age-progressive volcanic chain.
Traps that cost marks
Equating plates with continents (Iberia is part of the Eurasian plate); claiming collisions produce stratovolcanoes; forgetting that transform boundaries generate no magma; describing the whole core as solid; giving a boundary name without the mechanical justification the question rewards.