Harry Hammond Hess propounded in his theory of seafloor spreading that the mid-oceanic ridges were situated on the rising thermal convection currents coming out from the mantle. The oceanic crust moves in opposite directions from mid-oceanic ridges.
These molten lavas cool down and solidify to form new crust along the trailing ends of divergent plates. Thus, there is continuous creation of new crust along the mid-oceanic ridges and the expanding crusts (plates) are destroyed along the ocean trenches.
Verification of Seafloor Spreading:
The validity of the theory of seafloor spreading by Harry Hess was confirmed most notably by paleomagnetism and ocean floor mapping.
When any rock containing iron is formed — such as iron-rich ocean floor basalt — it is magnetized so that the iron-rich grains become aligned with earth’s magnetic field. This orientation then becomes a permanent record of the polarity of earth’s magnetic field at the time of rock solidified.
Over the last 100 million years the polarity of Earth’s magnetic field has reversed itself more than 170 times — with the north magnetic pole becoming the south magnetic pole.
In 1963, Vine and Mathews used paleomagnetism to test the theory of seafloor spreading by studying paleomagnetic data from a portion of the mid-oceanic ridge system. If the seafloor has spread laterally by the addition of new crust at the oceanic ridges, they should be as a relatively symmetrical pattern of magnetic orientation — normal polarity, reversed polarity, normal polarity and so on- on both sides of the ridges.
The rocks equidistant on either side of the crest of mid-oceanic ridges show remarkable similarities in terms of the period of formation of chemical compositions and magnetic properties. The age of the rocks increases as one moves away from the crest.
The oceanic crust rocks are much younger than the continental rocks.
Thus, the seafloors can be linked to gigantic conveyor belts, moving ever outward from the mid-ocean ridges toward the trenches. Oceanic lithosphere has a relatively short life at earth’s surface. New crust is formed at the oceanic ridges and within 200 million years is returning to the mantle by subduction, because lower density continental lithosphere cannot be subducted, once it forms it is virtually permanent.
So, as it turns out, Alfred Wegener was wrong about one important detail in his theory of continental drift. It is not just the continents that are drifting. The continents are embedded in the thicker lithospheric plates, carried along by the action of seafloor spreading.