Seafloor spreading, magnetic striping, and hydrothermal circulation
Slow: Mid-Atlantic Ridge ~2.5 cm/yr. Fast: East Pacific Rise ~15 cm/yr. Full rate = distance between conjugate plates / time. Faster ridges are wider, shallower, smoother.
Geomagnetic reversals (every ~200 kyr on average) are recorded symmetrically as new basalt cools through the Curie point (~580°C for magnetite). Key evidence for seafloor spreading (Vine-Matthews 1963).
Seawater percolates down fractures, heats to 350–400°C, and re-emerges as black smokers. Precipitates sulfide chimneys; supports unique chemosynthetic ecosystems.
Slow ridges have a prominent axial rift valley (1–2 km deep, 15–20 km wide). Fast ridges have an axial high — no valley. Controlled by magma supply and rheology.