Plate tectonics
Domain 2 - Plate tectonics. The exam tests the theory as an integrated model: evidence, boundary types, driving forces and the Wilson cycle, often applied to Iberia and the western Mediterranean.
Evidence
- Continental drift (Wegener, 1912): matching coastlines, Glossopteris and Mesosaurus on both sides of the Atlantic, Permo-Carboniferous tillites, matching fold belts.
- Sea-floor spreading: magnetic stripes symmetric about ridges, alternating polarity, ocean floor nowhere older than about 200 Ma, age increasing away from ridges, thin sediment cover at ridges.
- Wadati–Benioff zones dipping 30–70° beneath island arcs; GPS rates of 1–10 cm per year (Nazca about 7 cm/yr, Mid-Atlantic about 2.5 cm/yr).
- Hot-spot chains such as Hawaii record plate motion over a fixed mantle plume, including the bend at about 47 Ma.
Boundaries and their signatures
- Divergent: normal faults, basaltic fissure volcanism, pillow lavas, ophiolite sequences, high heat flow, shallow seismicity.
- Convergent: ocean–ocean gives island arcs and back-arc basins; ocean–continent gives Andean-type andesitic arcs with accretionary prisms and mélanges; continent–continent gives folding and thrusting with crustal thickening and no modern volcanism (Himalaya).
- Transform: strike-slip, shallow earthquakes, no volcanism (San Andreas).
Driving forces
The dominant force is slab pull, aided by ridge push and resisted by basal drag and friction. Plates are the stiff lithosphere; convection supplies the energy but is not a simple conveyor belt.
Iberia and the Valencia region
Africa converges with Eurasia at roughly 4–5 mm per year. The Betic–Rif system, the Pyrenees and the extensional Valencia Trough record Mesozoic rifting and Cenozoic convergence; the Alboran domain shows extension behind the Gibraltar arc.
Worked example
Magnetic stripes 15 km wide on each side of a ridge correspond to one reversal of about 0.5 Ma. Half-spreading rate = 15 km ÷ 0.5 Ma = 30 km/Ma = 3 cm/yr, so full spreading is 6 cm/yr. If a matching stripe lies 90 km from the ridge, that crust is 90 ÷ 30 = 3 Ma old.
Traps
- Explaining plate motion only by mantle convection pushing plates, ignoring slab pull.
- Confusing a transform boundary with a passive continental margin.
- Assuming subduction always produces andesites, even in continent–continent collision.
- Reversing the logic of magnetic stripes: they are symmetric about the ridge, not about the continent.