Internal Geodynamics: Earth's Structure, Plate Tectonics and Magmatic–Metamorphic Processes
Domain 1 tests Earth's internal layering, the mechanics of plate tectonics, and the magmatic and metamorphic processes that built the Canarian archipelago. The PAU rewards candidates who move in one chain: from seismic evidence to a layer model, from a plate setting to a magma type, from a mineral assemblage to a metamorphic grade.
Layering: evidence before labels
Two overlapping classifications are examined. By composition: crust (oceanic ~7 km basaltic; continental 30–70 km granitic), mantle (peridotite), core (Fe–Ni). By rheology: lithosphere, asthenosphere, mesosphere, outer and inner core. Key seismic markers: P-wave velocity jumps to about 8 km/s at the Moho; it drops abruptly at the Gutenberg discontinuity (~2900 km); S-waves vanish beyond ~103° because the outer core lacks shear strength. Supporting evidence comes from meteorite composition, mantle xenoliths in alkaline lavas, heat flow and geomagnetism.
Plate tectonics: settings and forces
Slab pull dominates the driving forces, with ridge push and mantle convection secondary. Divergent boundaries (Mid-Atlantic Ridge) generate basaltic crust and normal faults; convergent boundaries produce Benioff–Wadati seismicity and calc-alkaline arcs; transform boundaries produce strike-slip faults. The Canaries are intraplate volcanic islands on anomalously thick oceanic lithosphere of the African plate, linked to a mantle plume, not to any subduction zone.
From mantle to magma
Melting requires decompression, volatile addition (flux melting) or heat input - never simply 'rising pressure'. Small-degree partial melting of peridotite yields alkali basalt/basanite; fractional crystallization and assimilation drive the alkaline trend basalt → trachyte → phonolite. Metamorphism is contact (aureole, hornfels) or regional (foliation, index minerals); facies - greenschist, amphibolite, granulite, blueschist, eclogite - are defined by mineral assemblage, not by depth alone.
Worked example
A basanite from La Palma carries peridotite xenoliths. Explain the ascent. Reasoning: peridotite is mantle rock, so the source is sub-crustal; xenoliths are dense and would settle in a slowly rising body, so ascent must have been rapid along fractures; high Na₂O, low SiO₂ and normative nepheline indicate a small melt fraction at high pressure, not slab-derived calc-alkaline magma. Conclusion: rapid intraplate alkaline magmatism from a garnet-lherzolite source.
Traps that cost marks
Claiming the core is liquid - the inner core is solid Fe–Ni. Describing the Canaries as a volcanic arc. Confusing lithosphere with crust. Writing 'magma forms because pressure increases'. Naming a metamorphic facies without giving a diagnostic mineral assemblage.
- Lithosphere = crust + rigid upper mantle; asthenosphere is the ductile low-velocity layer.
- Slab pull is the dominant plate-driving force.
- Canarian volcanism is intraplate alkaline (basanite–trachyte–phonolite), plume-related.
- S-wave shadow and the Gutenberg discontinuity prove a liquid outer core.
- Metamorphic facies are recognised by mineral assemblage, not by depth alone.