Internal geodynamics: plate tectonics, magmatism and metamorphism
Internal geodynamics explains why the lithosphere is broken into plates, how they move and what happens at their edges. The PAU links each plate boundary to the magmatism and metamorphism it produces, and asks you to read a tectonic setting backwards from the rocks and from a pressure–temperature path.
Plate boundaries and driving forces
- Divergent: mid-ocean ridges with normal faults, shallow seismicity and tholeiitic MORB; continental rifts add alkaline volcanism.
- Convergent: ocean–ocean gives island arcs and back-arc basins; ocean–continent gives Andean-type margins with calc-alkaline andesites and granodiorites; continent–continent gives collision, thrusts, folding and no volcanism.
- Transform: strike-slip motion, shallow earthquakes, neither volcanism nor metamorphism.
- Driving forces: slab pull dominates, ridge push is secondary, and mantle convection with basal drag acts as either a passive or a resisting force.
Magmatism
Magma series run from tholeiitic (low potassium, ridges and ocean islands) through calc-alkaline (subduction, typical of the Betic and Pyrenean arcs) to alkaline (intraplate). In a magma chamber, fractional crystallisation follows Bowen's reaction series, so early removal of olivine and calcium plagioclase leaves a silica-rich residual melt.
Metamorphism
Metamorphism is solid-state recrystallisation driven by temperature, pressure and fluids. Regional orogenic metamorphism produces the slate–schist–gneiss sequence with index minerals chlorite, biotite, garnet, staurolite, kyanite and sillimanite. Contact metamorphism creates hornfels aureoles and marble. Blueschist and eclogite appear where the geothermal gradient is low, as in subduction zones, whereas granulite records high temperature.
Worked example
A belt contains blueschist passing into eclogite and, twenty kilometres inland, greenschist. Reasoning: blueschist requires high pressure with low temperature, only achievable with a cold descending slab; the seaward belt therefore marks the subduction zone, and the pair of belts with contrasting gradients is a paired metamorphic belt, as in Japan.
Traps that cost marks
- Treating granite as the direct product of mantle melting instead of crustal melting or fractionation.
- Confusing contact and regional metamorphism, or assigning marble and hornfels to regional burial.
- Saying plates float on magma; they move over ductile peridotite.
- Placing Iceland or the Canaries on a destructive boundary.