Seafloor spreading, magnetic striping, and plate divergence
Mid-ocean ridges are divergent plate boundaries where magma wells up from the mantle, creating new oceanic crust. As the seafloor spreads symmetrically away from the ridge axis, it records Earth's magnetic field direction at the time of solidification — preserving a tape-recorder of geomagnetic reversals. The Mid-Atlantic Ridge spreads at ~2.5 cm/yr; the East Pacific Rise at up to 15 cm/yr. Harry Hess's 1962 "History of Ocean Basins" and the Vine-Matthews-Morley hypothesis (1963) linking magnetic anomalies to seafloor spreading became the cornerstone evidence for plate tectonics.