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ssss1 The formation of oceanic crust along the ridge axis generates layer 2 pillow basalts and dikes and layer three gabbros of the oceanic crust (blue) and layer 4 mantle peridotites (gray). Sediment deposition atop these rocks produces layer 1 (yellow) of the crust. Sea floor spreading (black arrows) carries these laterally away from the ridge axis in both directions.

constructive.

underplating

linear magnetic anomalies

ssss1 Model depicts the production of alternating normal (colored) and reversed (white) magnetic bands in oceanic crust by progressive sea floor spreading and alternating normal and reversed periods of geomagnetic polarity (a–c). The age of such bands should increase away from the ridge axis.

Source: Courtesy of USGS.

Sea floor spreading was convincingly demonstrated in the middle to late 1960s by paleomagnetic studies and radiometric dating which showed that the age of ocean floors systematically increases in both directions away from the ridge axis, as predicted by sea floor spreading (ssss1).