Seafloor spreading

The process by which new oceanic crust forms at mid-ocean ridges and spreads outward on both sides, carrying plates apart and recording reversals of Earth's magnetic field.

Mid-ocean ridge: An undersea mountain chain where upwelling magma creates new oceanic crust along a divergent plate boundary. The site where spreading actually happens; every ridge is a spreading center.
Harry Hess: Princeton geologist who proposed in 1962 that seafloor spreads from ridges and sinks at trenches. Author of the seafloor spreading hypothesis, published as 'History of Ocean Basins'.
Juan de Fuca Ridge: A spreading center off the Pacific Northwest, mapped in detail to test magnetic stripe symmetry. The survey where Raff and Mason's magnetic stripes confirmed symmetric spreading.
Plate tectonics: The theory that Earth's rigid outer shell is broken into moving plates interacting at their boundaries. Spreading is the engine that made the full theory of plate tectonics cohere by 1968.
Earth expansion: The rejected hypothesis that Earth's radius grows, once proposed as an alternative to spreading. The rival explanation of ridges that spreading defeated without needing a growing planet.
Magnetic stripes: Symmetric bands of alternately magnetized ocean floor formed as cooling basalt locks in Earth's field polarity. The spreading record itself: stripe width measures how fast crust moves away from the ridge.
Continental drift: Alfred Wegener's 1912 proposal that continents move across Earth's surface, long rejected for lacking a mechanism. Spreading supplied the missing mechanism that made drift physically plausible.
Mid-Atlantic Ridge: The slow-spreading ridge running down the Atlantic, splitting Iceland and separating American and African plates. The ridge in Hess's original argument and the site of Project FAMOUS dives.
Seafloor age: Ocean crust ages linearly with distance from ridges; the oldest is roughly 200 million years old. The age-distance pattern predicted by spreading and confirmed by deep-sea drilling.
Geosyncline theory: The pre-plate theory that mountain belts form by sediment accumulating in sinking troughs. The dominant fixed-Earth framework that spreading made obsolete.

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