The presence of older oceanic floor along the trenches was used to infer that oceanic lithosphere was being consumed at the trenches. The ocean floor was being pulled into the mantle were it was heated to form magma which in turn generated volcanoes. The destruction of the oceanic lithosphere caused earthquakes down to depths of 800 km, explaining the presence of the deepest earthquakes adjacent to oceanic trenches. 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 I llustration of the plate tectonic cycle. Oceanic lithosphere created by magma rising from asthenosphere.
Plates move away from oceanic ridge and descend beneath a trench at the subduction zone.
The rates and directions of plate motions have were originally determined by computing the distance of oceanic floor of known age from the oceanic ridge system. Some volcanic islands in the interiors of plates form above fixed plumes of magma rising from the mantle. The locations of these mantle plumes are known as hot spots . The islands form as the plate moves over the magma source, much like a tectonic conveyer belt. Islands are progressively older with increasing distance from the hot spot.