An Introduction to Plate Tectonics
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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