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.