Harry Hess and Robert Dietz proposed a new hypothesis, sea floor spreading, during the 1960s. Sea floor spreading differed from the theory of continental drift in that the motions of the continents were driven by motion of the sea floor. Unlike continental drift, the sea floor spreading hypothesis proposed an explanation for sea floor motion, namely convection of the mantle. Plate tectonics is a refinement of the sea floor spreading hypothesis. These are zones of divergence , where adjoining plates are moving apart.
Ocean crust sinks into the mantle at deep-sea trenches.
These are zones of convergence , where adjoining plates are colliding, and ocean crust is being subducted and destroyed. Basalt contains magnetic mineral particles which orient themselves to the Earth's magnetic field while the basalt is still fluid. The Earth's magnetic field has reversed many times. A reversal leaves the position of the poles constant, but reverses their polarity. Note that the pattern repeats on each side of the ridge, suggesting that crust was formed symmetrically about the ridge axis.