Origin and Morphology of Ocean Margins

The continents are very old: they contain rocks aged thousands of millions of years. Ultimately, they are the product of selective accumulation of low density mantle material. Because of this low density they float on the mantle. The ocean floor, on the other hand, is geologically young, as we have seen. The basaltic rock which forms the ocean floor basement is rather close in composition to the mantle rock it came from. It is slightly heavier than continental rock (largely due to its high iron content, Appendix A6). The light-weight continental mass protrudes above the surrounding sea floor (Fig. 2.1a). Thick sediment piles accumulate at the boundary between continent and ocean, which build the actual margin (Fig. 2.1c). These sediments may be well layered or strongly deformed, depending on the tectonic forces active at the margin.