Plate tectonic theory has provided us with a basic understanding of the differences between the geology of continents and the geology of ocean basins. Away from continental margins, the deep seafloor has no folded and faulted mountains like those on the continents. Instead, deformation is largely restricted to the faulting and volcanism found at mid-ocean ridges and subduction zones. Moreover, the weathering and erosion processes described in previous chapters are much less important in the oceans than on land because the oceans lack efficient fragmentation processes, such as freezing and thawing, and major erosive agents, such as streams and glaciers. Deep-sea currents can erode and transport sediments, but cannot effectively attack the plateaus and hills of basaltic rock that form the oceanic crust.
Because deformation, weathering, and erosion are minimal over much of the seafloor, volcanism and sedimentation dominate the geology of ocean basins. Volcanism creates mid-ocean ridges, island groups (such as the Hawaiian Islands) in the middle of an ocean, and island arcs near deep-sea trenches. Sedimentation shapes much of the rest of the seafloor. Soft sediments of mud and calcium carbonate blanket the low hills and plains of the seafloor. Sediments begin to accumulate on oceanic crust as soon as it is formed at mid-ocean ridges. As the crust spreads farther and farther from the ridge, it accumulates more and more sediments. Deep-sea sedimentation is more continuous than the sedimentation in most continental environments, and it therefore preserves a better record of geologic events—for example, as we have seen, it provides a more detailed history of Earth’s climate changes.
The marine sediment record is limited, however, because subduction is continually recycling oceanic crust, thereby destroying marine sediments by metamorphism and melting. On average, it takes only a few tens of millions of years for the crust created at a mid-ocean ridge to spread across an ocean and come to a subduction zone. As we saw in Chapter 2, the oldest parts of today’s seafloor were formed in the Jurassic period, about 180 million years ago; they are currently found near the western edge of the Pacific Plate (see Figure 2.15). In the next 10 million years or so, the sedimentary record that lies atop this crust will disappear into the mantle.
The five major oceans (Atlantic, Pacific, Indian, Arctic, and Southern) form a single connected body of water sometimes referred to as the world ocean. The term sea is often used to refer to smaller bodies of water set off somewhat from the oceans. The Mediterranean Sea, for example, is narrowly connected with the Atlantic Ocean by the Strait of Gibraltar and with the Indian Ocean by the Suez Canal. Other seas are more broadly connected, as is the North Sea with the Atlantic Ocean. Seawater—the salty water of the oceans and seas—is remarkably constant in its general chemical composition from year to year and from place to place. The chemical equilibrium maintained by the oceans is determined by the composition of river waters entering the oceans, the composition of the sediments they transport to the oceans, and the formation of new sediments in the oceans.