Plate Tectonics, Earthquakes & Volcanoes
Key terms and concepts about plate motion, boundary types, and seismic waves.
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Questions Covered in This Set
10 cards to master
Who proposed continental drift, and what evidence did he use?
Alfred Wegener (1912) — matching coastlines, matching fossils (Glossopteris, Mesosaurus), matching mountain belts, and glacial scratches in now-tropical areas. He could not explain the driving force.
What seafloor discovery confirmed plate motion in the 1950s–60s?
Symmetric magnetic stripes recording magnetic field reversals on both sides of mid-ocean ridges — proof of seafloor spreading.
Lithosphere vs. asthenosphere
Lithosphere: rigid outer shell (crust + brittle top mantle), ~100 km thick, broken into ~15 plates. Asthenosphere: hot, weak, slowly flowing solid upper mantle the plates slide on.
What forces drive plate motion?
Mantle convection, ridge push, and especially slab pull (cold dense plate sinking drags the rest of the plate). Plates move 2–10 cm/year.
What forms at a divergent boundary?
Mid-ocean ridges, rift valleys, new seafloor, shallow earthquakes, and gentle basaltic volcanoes — e.g., Mid-Atlantic Ridge, East African Rift, Iceland.
What happens at an ocean–continent convergent boundary?
Dense oceanic crust subducts under continental crust, creating a deep trench, explosive inland volcanoes, and a coastal mountain range (Nazca plate → Andes, Peru–Chile Trench).
What forms at an ocean–ocean convergent boundary?
The older, colder, denser plate subducts, forming a trench and a curved volcanic island arc — Japan, the Aleutians, the Marianas.
What forms at a continent–continent convergent boundary?
Neither plate sinks, so crust crumples and thickens: huge mountains and large earthquakes but almost no volcanism (India + Asia → Himalayas, rising ~5 mm/year).
Why does oceanic crust always subduct?
Density decides who sinks: oceanic basalt is ~3.0 g/cm³ versus continental granite ~2.7 g/cm³, so oceanic crust is denser and goes down.
Focus vs. epicenter; P-waves vs. S-waves
Focus (hypocenter) is the underground rupture point; epicenter is the surface point directly above. P-waves are fastest, compressional, travel through solids and liquids; S-waves are slower shear waves that travel only through solids.
