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. Plates descend into the mantle below trenches in regions termed subduction zones.
Plates typically contain both oceanic and continental lithosphere.
Oceanic lithosphere is continually created and destroyed. Continental lithosphere can not be destroyed but continents can be subdivided and assembled into super-continents. Interactions at plate boundaries cause volcanic activity and earthquakes. The depth of earthquakes becomes progressively deeper underlying ocean trenches. Earthquakes occur due to the break up of the descending plate as it pushes into the asthenosphere.