The ocean floor varies considerably in depth. An oceanic ridge system can be traced around the world. Sea-floor spreading proposed that rising magma was forming new oceanic crust along the oceanic ridges, the ocean floor was moving away from the ridges, and that old crust was destroyed at oceanic trenches. Polarity refers to the orientation of the earth's magnetic field. The magnetic field has switched polarity at irregular intervals during earth history.
Compass needles point toward the magnetic north pole during periods of normal polarity.
During intervals of reverse polarity compass needles would point toward the magnetic south pole. Paleomagnetism is the record of the earth's magnetic field preserved in igneous rocks. The age of the ocean floor varies with location and is consistently younger at the oceanic ridges and older along the ocean margins. The age of the oceanic crust increases symmetrically with distance from the ridge system in the Atlantic Ocean. The plates are in motion, moving away from ridges and towards trenches.