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sea-floor spreading
An important phenomenon arising from plate tectonics. Along mid-ocean ridges material emerges from the Earth's mantle to form new oceanic crust. This material, primarily basalt, spreads out to either side of the ridges at a rate of the order of 10–50 mm/year. Newly laid down basalt is able to "fossilize" the prevailing geomagnetism: the main evidence for sea-floor spreading comes from the symmetric pattern of alternately magnetized strips of basalt on either side of the ridge.
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