The greatest shortcoming, at least in the eyes of American geologists, was the lack of an adequate mechanism for moving the continents. Wegener proposed that the Earth's spin caused the continents to move, plowing through the oceanic plate and producing mountains on their leading edges. Geologists at that time understood enough about the strength of rocks to know that this was highly unlikely. Wegener's work was largely unaccepted in the northern hemisphere. In the southern hemisphere, where geologists were familiar with the rocks that Wegener used to support his hypothesis, continental drift was generally accepted.
A mechanism to move continents was proposed by Arthur Holmes, Scottish geologist in 1928. He believed heat trapped in the Earth caused convection currents, areas where fluids beneath the Earth's crust rise, flow laterally, and then fall. The currents would rise beneath continents, spread laterally, then plunge beneath the oceans. (Geologists now know that solid rock, not fluids, convect in the mantle). Unfortunately, Wegener died in 1930 while exploring the Greenland ice cap. He never had the opportunity to adapt Holmes' ideas to his views of continental drift.
During the 1940s and 1950s, great advances were made in our knowledge of the sea floor and in the magnetic properties of rocks. Both of these fields of study provided new evidence to support continental drift.
Geologists have known for over a century that a ridge exists in the middle of the Atlantic Ocean. The Mid-Atlantic Ridge is 6,500 feet (2,000 m) above the adjacent sea floor, which is at a depth of about 20,000 feet (6,000 m) below sea level. In the 1950s, a seismologist, a scientist who specializes in the study of earthquakes, showed that the global system of mid-ocean ridges was also an active seismic belt, or zone of earthquakes. An international group of geologists proposed that the seismic belt corresponded to a trough, or rift, system similar to the trough known at the crest of the Mid-Atlantic Ridge. The rifts are about 20 miles (30 km) wide and 6,500 feet (2,000 m) deep. In all, the oceanic ridges and their rifts extend for more than 37,500 miles (60,000 km) in all the world's oceans.
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.
