• Earth is a planet dominated by oceans. Seventy-one percent of Earth’s surface consists of oceans and marginal seas. In the Southern Hemisphere, often called the water hemisphere, about 81% of the surface is water. Of the three major oceans, Pacific, Atlantic, and Indian, the Pacific Ocean is the largest, contains slightly more than half the water in the world oceans, and has the greatest average depth.

• Ocean depths are determined using echo sounders and multibeam sonars. These devices bounce sonic signals off the ocean floor. Delicate receivers intercept the reflected echos. The time interval of the signal must also be carefully measured. Ocean depths are plotted to produce ocean floor profiles. Recently, satellite measurements of the shape of the ocean surface have added data for mapping ocean floor features.

• Oceanographers studying the topography of the ocean basins have delineated three major units: continental margins, the ocean basin floor, and mid-ocean ridges.

• The zones that collectively make up a passive continental margin include the continental shelf (a gently sloping, submerged surface extending from the shoreline toward the deep-ocean basin); continental slope (the true edge of the continent, which has a steep slope that leads from the continental shelf into deep water); and in regions where trenches do not exist, the steep continental slope merges into a more gradual incline known as the continental rise. The continental rise consists of sediments that have moved downslope from the continental shelf to the deep-ocean floor.

  

• Active continental margins are located primarily around the Pacific Ocean in areas where the leading edge of a continent is overrunning oceanic lithosphere. Here sediment scraped from the descending oceanic plate is plastered against the continent to form a collection of sediments called an accretionary wedge. An active continental margin generally has a narrow continental shelf, which grades into a deep-ocean trench.

• Submarine canyons are deep, steep-sided valleys that originate on the continental slope and may extend to depths of 3 kilometers. Some of these canyons appear to be the seaward extensions of river valleys. However, most information seems to favor the view that many submarine canyons are excavated by turbidity currents (downslope movements of dense, sediment-laden water). Turbidites, sediments deposited by turbidity currents, are characterized by a decrease in sediment grain size from bottom to top, a phenomenon known as graded bedding.

• The ocean basin floor lies between the continental margin and the mid-ocean ridge system. The features of the ocean basin floor include deep-ocean trenches (long, narrow depressions that are the deepest parts of the ocean, and where moving crustal plates descend back into the mantle); abyssal plains (among the most level places on Earth, consisting of thick accumulations of sediments that were deposited atop the low, rough portions of the ocean floor by turbidity currents); and seamounts (isolated, steep-sided volcanic peaks on the ocean floor that originate near oceanic ridges or in association with volcanic hot spots).

• Mid-ocean ridges, the sites of seafloor spreading, are found in all major oceans and represent more than 20 percent of Earth's surface. These broad features are certainly the most prominent features in the oceans, for they form an almost continuous mountain range. Ridges are characterized by an elevated position, extensive faulting, and volcanic structures that have developed on newly formed oceanic crust. Most of the geologic activity associated with ridges occurs along a narrow region on the ridge crest, called the rift valley, where magma from the asthenosphere moves upward to create new slivers of oceanic crust.

  

• Coral reefs, which are confined largely to the warm, sunlit waters of the Pacific and Indian oceans, are constructed over thousands of years primarily from the accumulation of skeletal remains and secretions of corals and certain algae. A coral island, called an atoll, consists of a continuous or broken ring of coral reef surrounding a central lagoon. Atolls form from corals that grow on the flanks of sinking volcanic islands, where the corals continue to build the reef complex upward as the island slowly sinks.

• There are three broad categories of seafloor sediments. Terrigenous sediment consists primarily of mineral grains that were weathered from continental rocks and transported to the ocean. Biogenous sediment consists of shells and skeletons of marine animals and plants. Hydrogenous sediment includes minerals that crystallize directly from seawater through various chemical reactions.

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