An ocean basin is one of several structures formed by the oceanic crust. More specifically, it is a saucer-shaped depression under sea level, allowing the seawater to flow into it. It is bordered by continental margins, which extend seaward in varying degrees of inclination; this inclination determines the amount of water flowing into the basins. Ocean basins contain the earth’s oceans, which in turn hold around 97% of the earth’s supply of water. Similarly, the five major oceans of the world each have their corresponding basins similarly named.
Ocean basins are sometimes considered to be the undersea counterpart of continents, with the latter being the primary geological structure above sea level and the former being the one below it. Even features such as continental mountains, volcano chains, plains and valleys have their oceanic counterparts. Underwater mountains are called seamounts, volcanic chains are termed mid-ocean ridges, and valleys are known are oceanic trenches. The underwater counterparts, however, are usually several times larger than those above sea level. This is understandable, though, when it is taken into account that oceans cover roughly 71% of the earth’s surface, in contrast to continental landmass of only 29%.
Another similarity shared by a continent and an ocean basin is their tendency to change in size. There are different factors that contribute to the changing size of the earth’s basins, just as there are many factors that affect the sizes of continents. Erosion is often considered a major factor in contributing to the shrinkage of an ocean basin, along with sedimentation from ocean tributaries and tectonic plate movements. Some of the characteristics of an active basin include the presence of an elevated mid-ocean ridge, or a nearby subduction zone, or a boundary between two tectonic plates.
The Arctic and Atlantic Ocean basins, for example, are basins that are steadily growing due to the constant tectonic activity in their respective territories. The Pacific Ocean basin, on the other hand, is steadily shrinking. These, when combined with the same tectonic activity leading to continental borders shifting, are the primary factors behind what is called continental drift, or the process of different landmasses slowly moving closer or further away from each other. An ocean basin that lacks any of these factors does not change in size and is considered inactive. Several minor basins, such as those holding the Gulf of Mexico, the Sea of Japan, and the Bering Sea, have been inactive for hundreds, even thousands, of years.