Sea-Floor Spreading

In 1962, a geologist presented an explanation for the global rift system. Harry Hess proposed that new ocean floor is formed at the rift of mid-ocean ridges. The ocean floor, and the rock beneath it, are produced by magma that rises from deeper levels. Hess suggested that the ocean floor moved laterally away from the ridge and plunged into an oceanic trench along the continental margin.

A trench is a steep-walled valley on the sea floor adjacent to a continental margin. For example, ocean crust formed at the East Pacific Rise, an oceanic ridge in the east Pacific, plunges into the trench adjacent to the Andes Mountains on the west side of the South American continent. In Hess' model, convection currents push the ocean floor from the mid-ocean ridge to the trench. The convection currents might also help move the continents, much like a conveyor belt.

As Hess formulated his hypothesis, Robert Dietz independently proposed a similar model and called it sea floor spreading. Dietz's model had a significant addition. It assumed the sliding surface was at the base of the lithosphere, not at the base of the crust.

Hess and Dietz succeeded where Wegener had failed. Continents are no longer thought to plow through oceanic crust but are considered to be part of plates that move on the soft, plastic asthenosphere. A driving force, convection currents, moved the plates. Technological advances and detailed studies of the ocean floor, both unavailable during Wegener's time, allowed Hess and Dietz to generate the new hypotheses.



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