To explain this data, Wegener proposed the theory of continental drift in a book (he wrote to notify the public) called ‘The Origins of the Continents and the Oceans’, published in Germany in 1915. His book stated that all of the continents had originally been joined together in a supercontinent called Pangaea. About 200 million years ago, the book continued, Pangaea broke apart and the continents slowly drifted to their present positions.
His theory was not accepted, but in 1926 he was invited to an international symposium in New York called to discuss his theory. Though he found some supporters, many speakers were sarcastic to the point of insult. The main problen with Wegeners theory was finding a force or forces that could make the continents "plow around in the mantle," as one critic put it. Wegener tentatively suggested two candidates: centrifugal force caused by the rotation of the Earth, and tidal-type waves in the Earth itself generated by the gravitational pull of the sun and moon. He realized these forces were inadequate, however, he noted that one thing was certain: The forces that displace continents are the same as those that produce great fold-mountain ranges. Continental drift, faults and compressions, earthquakes, volcanicity, ocean transgression cycles and polar wandering are undoubtedly connected on a grand scale.
Wegener's final revision came to the supporting evidence from many fields, including testimonials from scientists who found his hypothesis resolved difficulties in their problems much better than the old theories. Wegener considered such paleoclimatic validation one of the strongest proofs of his theory. Conversely, continental drift has since become the organizing principle of paleoclimatology and other paleosciences.
Despite general rejection, Alfred’s compelling concept continued to attract a few advocates over the next several decades. Then, beginning in the mid-1950s, a series of confirming discoveries in paleomagnetism and oceanography finally convinced most scientists that continents do indeed move and, just as Wegener had predicted, the movement is part of a grand scale process that causes mountain-building, earthquakes, volcanic eruptions, sea-level fluctuations, and apparent polar wandering as it rearranges Earth's geography.
Geologists now call the process "plate tectonics," after the large moving plates that form the planet's outer shell. These plates carry both continents and sea floor, but unlike the sea floor, the less-dense, buoyant continents resist subduction into the mantle.
Although there was no driving force evidently shown in Wagener’s theory, it has been universally accepted. Today there are still many new thereos of the driving force behind plate tectonics which include:
* Convection Cells below the lithosphere - Thought to be primary driving force for plate motion. Unequal heat distribution in the mantle may produce convection cells below the lithosphere. Hot material rises (correlates to spreading center), spreads laterally, cools and sinks deeper into the mantle to be reheated. Two convection cell models:
1. Shallow Convection Cell Model - Convection cells are restricted to the asthenosphere. Difficult to explain the source of heat for convection and the reason convection is restricted to the asthenosphere.
2. Deep Convection Cell Model - Entire mantle is involved in convection. Outer mantle is source of heat. Problem explaining how convection involved both the asthenosphere and lower mantle and how heat is transferred from the outer core to the mantle.
* Mantle Plumes - Hot spots, or plumes, of hot rising mantle material are known to exist around the world. Hot spots occur primarily at spreading centers (black smokers), although a few occur in the centers of oceanic plates and result in the formation of volcanic island chains. Hot plume upwarps overlying lithosphere which cracks and moves laterally away from the plume. Downward flow of the mantle must occur somewhere to balance the upward flow in the plumes.
* Push-Pull Model - Lithospheric plates are pushed apart at hot spreading centers. Cold lithospheric plates are dense and tend to sink into the mantle, pulling the rest of the plate with it. Each part of the model can operate independently and are gravity driven.
* Expanding Earth - Model holds that the Earth has expanded through its history, so that overall new crust is being created at spreading centers. Has few supporters. Would require a 50% increase in the volume of the Earth over the last 200 million years.