A condition of gravitational balance, called isostasy, exists between different parts of the earth's crust. The theory of isostasy claims that the continental crust floats higher than the oceanic crust because the former is composed of a thick layer of lower density rocks while the latter is composed of a thin layer of higher density rocks. Isostatic adjustments to changes in mass distribution on the earth's surface associated with plate interactions may occur through flow of semifluid materials deep in the earth. These materials cause a compensatory uplift of mountains and plateau areas as erosion wears them down. The mass of eroded material is added to and thus depresses the continental shelves and the ocean floor.
Adjustments also accompany such changes as the growth and melting of continental ice sheets.
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.