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The Face of the Earth: The Ocean Floor. Source: Wikipedia

The Face of the Earth: The Ocean Floor

The Face of Earth

The two principal divisions of Earth’s surface are the ocean basins and the continents. A significant difference between these two areas is their relative levels. The elevation difference between the ocean basins and the continents is primarily due to differences in their respective densities and thicknesses.

Ocean basins

The average depth of the ocean floor is about 3.8 kilometers (2.4 miles) below sea level, or about 4.5 kilometers (2.8 miles) lower than the average elevation of the continents. The basaltic rocks that comprise the oceanic crust average only 7 kilometers (about 4.5 miles) thick and have an average density of about 3.0 g/cm3.

The face of Earth major surface features of the geosphere. Source: Earth: An Introduction to Physical Geology, 12th Edition, Tarbuck et al.

The continents are remarkably flat features that have the appearance of plateaus protruding above sea level. With an average elevation of about 0.8 kilometer (0.5 mile), continental blocks lie close to sea level, except for limited areas of mountainous terrain. Recall that the continents average about 35 kilometers (22 miles) thick and are composed of granitic rocks that have a density of about 2.7 g/cm3.

The thicker, less dense continental crust is more buoyant than the oceanic crust. As a result, continental crust floats on top of the deformable rocks of the mantle at a higher level than oceanic crust for the same reason that a large, empty (less dense) cargo ship rides higher than a small, loaded (denser) one.

Major Features of the Ocean Floor

If all water were drained from the ocean basins, a great variety of features would be seen, including chains of volcanoes, deep canyons, plateaus, and large expanses of monotonously flat plains. In fact, the scenery would be nearly as diverse as that on the continents.

During the past 65 years, oceanographers have used modern depth-sounding equipment and satellite technology to map significant portions of the ocean floor. These studies have led them to identify three major regions: continental margins, deep-ocean basins, and oceanic (mid-ocean) ridges.

The continental margin is the portion of the seafloor adjacent to major landmasses. It may include the continental shelf, the continental slope, and the continental rise.

Although land and sea meet at the shoreline, this is not the boundary between the continents and the ocean basins. Rather, along most coasts, a gently sloping platform of material, called the continental shelf, extends seaward from the shore. Because it is underlain by continental crust, it is clearly a flooded extension of the continents. For example, it is broad along the east and Gulf coasts of the United States but relatively narrow along the Pacific margin of the continent.

The boundary between the continents and the deep-ocean basins lies along the continental slope, a relatively steep dropoff that extends from the outer edge of the continental shelf to the floor of the deep ocean. Using this as the dividing line, we find that about 60 percent of Earth’s surface is represented by ocean basins and the remaining 40 percent by continents.

In regions where trenches do not exist, the steep continental slope merges into a more gradual incline known as the continental rise, a thick wedge of sediment that moved downslope from the continental shelf and accumulated on the deep-ocean floor.

Deep-Ocean Basins

Situated between the continental margins and oceanic ridges are deep-ocean basins. Parts of these regions consist of incredibly flat features called abyssal plains. The ocean floor also contains extremely deep depressions, some more than 11,000 meters (36,000 feet) deep. Although these deep-ocean trenches are relatively narrow and represent only a small fraction of the ocean floor, they are nevertheless very significant features. Some trenches are located adjacent to young mountains that flank the continents. For example, the Peru–Chile trench off the west coast of South America parallels the Andes Mountains. Other trenches parallel island chains called volcanic island arcs.

Dotting the ocean floor are submerged volcanic structures called seamounts, which sometimes form long, narrow chains. Volcanic activity has also produced several large lava plateaus, such as the Ontong Java Plateau located northeast of New Guinea. In addition, some submerged plateaus are composed of continental-type crust. Examples include the Campbell Plateau southeast of New Zealand and the Seychelles Bank northeast of Madagascar.

Oceanic Ridges

The most prominent feature on the ocean floor is the oceanic ridge, or mid-ocean ridge. The Mid-Atlantic Ridge and the East Pacific Rise are parts of this system. This broad elevated feature forms a continuous belt winding more than 70,000 kilometers (43,500 miles) around the globe, in a manner similar to the seam of a baseball. Unlike most continental mountains that consist of highly deformed rock, the oceanic ridge system consists of layer upon layer of igneous rock that has been fractured and uplifted.

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