The oldest continental rocks dated by radioactivity are 3.98 billion years old, which suggests that the continents and oceans are probably permanent features of the earth's surface. Although the continental regions have been periodically covered by shallow seas, they appear never to have been the sites of deep oceans. How the continents originated has been a major debate in geology. The 19th-century geologist J. D. Dana proposed the continent accretion theory where the continents have always been stationary, with the gradual addition of new material around a central nucleus. Another theory was called the continental assimilation hypothesis, where the ocean areas accumulate the denser elements, then subside to form basins. In the late 19th cent., George Darwin proposed that the moon was gravitationally extracted from the Pacific Ocean, with the earth eventually redistributing into oceanic and continental crusts. In 1925, the expansion of the earth hypothesis stated that the present continents split apart as the earth expanded, noting that the continents could cover a sphere half the surface area of the present earth. Accepted theory now points to continental drift and seafloor spreading as a result of plate tectonics.
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.