Plate Tectonics: Evidence, Boundaries and Driving Forces
The theory
Plate tectonics explains the lithosphere as a mosaic of rigid plates moving over the weaker asthenosphere. Evidence: matching continental margins and fossils (Wegener's Pangaea), palaeomagnetic stripes recording sea-floor spreading, ocean-floor age increasing away from ridges, earthquake and volcano belts, and GPS-measured motion (centimetres per year).
Boundary types
- Divergent: plates separate; ridges produce new basaltic crust; shallow earthquakes; rift valleys on continents.
- Convergent: oceanic–continental gives subduction and volcanic arcs; oceanic–oceanic gives island arcs; continental–continental gives collision mountains and folding. Deep earthquakes occur along subducting slabs.
- Transform: plates slide past; shallow, strike-slip earthquakes; no volcanism.
Driving forces
Slab pull (dense subducting lithosphere sinking) dominates; ridge push and mantle convection contribute. Remember that convection is the broad engine, but slab pull is the strongest single force.
Traps
- Confusing lithosphere (crust plus rigid upper mantle) with crust.
- Placing deep earthquakes at ridges - deep seismicity marks subduction zones.
- Assuming continents drift through oceanic crust; plates move together and continents ride passively.
- Mixing up magnetic stripes: they are symmetric about the ridge and record field reversals.
Worked example
Two sites on the same ridge flank are 300 km apart, and ocean crust at the nearer site is 2 million years old while the farther is 5 million years. Spreading rate = distance ÷ time = 300 km ÷ 3 Ma = 100 km per million years, or about 10 cm per year. Rule: map the boundary type first, then predict earthquakes, volcanoes and topography.