The detailed mapping of the mid-Atlantic ridge together with the discoveries of the presently forming pillow lavas or ‘black smokers’ 17 and the mirror imaged magnetic reversal changes either side of the mid-Atlantic ridge 28 during the latter part of the 1950’s and early 60’s showed that the age of oceanic crust gradually increased from the present day at the ridge to approx 160 My (Jurassic era) at the western and eastern edges of the Atlantic Ocean. Taken in conjunction with the detailed mapping of the Pacific Ocean floor by Hess (during his tenure in the U.S. Navy during the 1939-1945 war) thus led him to put forward his theory of ‘Ocean Floor Spreading’ as the mechanism to describe the forces to explain Wegener’s Theory of Continental Drift.
Fig. 2. Hess model showing principal forces of 'ridge-push' and 'slab pull' associated with tectonic movement
This hypothesis suggests the convection currents in the earth’s mantle will create new oceanic crust at divergent current boundaries (Ridge- Push) and destroy and partially recycle crustal mass by sub-duction at convergent current boundaries. Fig.2 shows the Hess model in diagrammatic form. The downward movement (sub-duction) of the colder and denser lithosphere into the asthenosphere, which is referred to as the ‘Slab Pull’ force, is described as being the major force responsible for the creation of the trenches and the orogenic and volcanic activity on the uplifted plate. Park 34, Hamblin 16 and Davies 8 describe the Hess model as well as more recent work on convection currents in great detail.