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The ocean basins were, at one time, believed to be permanent features on the Earth. But today we understand that these are relatively young features on the globe. Saltwater oceans and seas cover about 71 percent of the Earth.
The transition between the continents and ocean basins occurs at what is termed the continental margin, which can be subdivided into the continental shelf, the continental slope and rise. The deep ocean basins are floored by what is termed the abyssal plain that can be broken by mid-ocean ridges and oceanic trenches. Seamounts can also rise above the sea floor.
Sediments accumulating on the ocean floor are termed pelagic, and can consist of detrital muds or biogenic ooze.
For more information on the sea floor than you'd know what to do with you might like to visit the World Data Center-A Marine Geology and Geophysics.
Today, plate tectonics is the unifying theory of the earth sciences. It is recognized that the outer portion of the earth is segmented into rigid plates of various sizes, and are in constant motion relative to one another (although rather slowly - about the rate at which your fingernails grow).
Continental drift was originally proposed in 1912 by Alfred Wegener, a German meteorologist. In addition to the jigsaw puzzle fit of the continents, he cited evidence related to the distribution of fossils on the continents, similarities in rock sequences on various continents, and on evidence of past glaciations and ancient climates.
The geological community did not regard Wegener's evidence as unequivocal, and largely rejected his ideas until the period following World War II, when the ocean basins were finally studied in some detail. The notion of sea floor spreading and the discovery of paleomagnetism led to a new look at this old idea. Also, the discovery subduction zones additionally supported the notion of continental drift and the modern theory of plate tectonics was born.
Plate boundaries are marked by increased earthquake and volcanic activity. Three types of motions can occur at the boundaries: divergent, convergent and transform. Hot spots are interesting occurrences of volcanic activity within a plate, and the volcanic activity on the Hawaiian Islands and at Yellowstone National Park are good examples.
The driving mechanism for the plate tectonic engine seems to be convection within the mantle. This hotly (pardon the pun) debated idea is receiving a lot of attention by geophysicists today.
