Geology: Earth's Structure and Plate Tectonics
Earth's interior
Seismology is the main probe. P waves pass through solids and liquids; S waves pass only through solids. The S-wave shadow zone between 103° and 142° from the epicentre proves the outer core is liquid, while the inner core - bounded by the Lehmann discontinuity - is solid because pressure overrides temperature. Learn the discontinuities: Mohorovičić (crust–mantle), Gutenberg (mantle–core, about 2900 km), Lehmann (about 5150 km). Continental crust is granitic and 30–70 km thick; oceanic crust is basaltic, 5–10 km, and denser. The mantle is solid but plastic - it flows on geological time scales and is not a magma ocean. Isostasy keeps crustal blocks in buoyant balance.
Plate tectonics
Plates move over the asthenosphere. Divergent boundaries create crust (Mid-Atlantic Ridge; Hess's magnetic striping), convergent boundaries destroy it (ocean–ocean → island arc; ocean–continent → Andean arc; continent–continent → Himalaya), transform boundaries slide. Subduction produces the Wadati–Benioff zone of deep earthquakes. India drifted north at roughly 15 cm/yr in the Cretaceous, slowing to about 5 cm/yr; the Deccan Traps erupted around 66 Ma over the Réunion hotspot, and continued convergence explains why the Koyna region (Seismic Zone IV) records reservoir-triggered earthquakes.
Worked example
If S waves are missing between 103° and 142° but P waves are recorded, what follows? P waves refract through the core; S waves cannot cross a liquid, so the gap marks a liquid outer core. Reasoning: wave behaviour reveals the state of matter, not just its density.
Traps
Do not call the mantle molten. Do not confuse the Moho with the Gutenberg discontinuity. A plate boundary is not the same as a fault - most faults are intra-plate. The Deccan Traps are flood basalts of a hotspot, not an ocean ridge.