The theory that the ocean floor is created at the spreading (accretionary) plate margins within the ocean basins. Igneous rocks rise along conduits from the mantle, giving rise to volcanic activity in a narrow band along the mid-ocean ridges. As these cool, the basaltic lavas and dykes form the upper part of the oceanic crust, and the underlying magma chamber solidifies to form layer 3 of the oceanic crust. The newly formed oceanic crust spreads perpendicularly away from the ridge, probably in response to mantle convective motions (see plate tectonics). As the basalts originally cooled, they became magnetized by the ambient geomagnetic field. As this field reverses polarity, oceanic crust formed at different times is characterized by oceanic magnetic anomalies that are parallel to the ridge at which they originally formed (Vine and Matthews, 1963). These anomalies allow the dating of the oceanic crust and the determination of its past relative motion. The creation of new ocean floor was implicit in the previous concept of continental drift, but is mainly characterized by the narrowness of the zone within which the new ocean floor is formed. It is now a fundamental concept within the plate-tectonic theory.