Ocean basins are the part of Earth’s surface that extends seaward from the continental margins. The ocean basins constitute one of the two major topographic features of Earth’s surface, the other being the continents. They range in depth from an average of about 6,500 ft (2,000 m) down into the deepest trenches. Ocean basins cover about 70% of the total sea area and about half of the planet’s total surface area.
In contrast to the landforms of the continents familiar to humans—features such as mountains, plateaus, hills, and rivers—the various topographic features of the ocean basins are still not well understood. The ocean basins are thousands of meters below the water’s surface, and they can be explored only with remote measuring equipment or in special research submarines known as submersibles.
The familiar landscapes of continents are mirrored, and generally magnified, by comparable features in the ocean basin. The largest underwater mountains, for example, are higher than those on the continents, and underwater plains are flatter and more extensive than those on continents.
The basins of Earth’s four ocean basins, the Atlantic, Pacific, Indian, and Arctic, differ from each other in many respects. Yet, they all contain certain common features such as oceanic ridges, trenches, and fracture zones and cracks, abyssal plains and hills, seamounts and guyots.
Some of the most impressive topographic features of the ocean basins are the enormous mountain ranges, or oceanic ridges. The Mid-Atlantic Ridge, for example, begins at the tip of Greenland, runs down the center of the Atlantic Ocean between the Americas on the west and Africa on the east, and ends at the southern tip of the African continent. At that point, it continues around the eastern edge of Africa, where it becomes the Mid-Indian Ridge.
The Mid-Indian Ridge then divides along the center of the Indian Ocean basin, with a second arc curving away south of the Australian continent. As that ridge continues eastward from Australia, it eventually heads northward on the floor of the Pacific Ocean, along the western coastline of South and Central America. In this region, the ridge is known as the East Pacific Ridge. Because of all these interconnections, one may consider the ridge as a single oceanic feature that encircles Earth and stretches more than 40,000 mi (65,000 km).
In most locations, the oceanic ridges are 6,500 ft (2,000 m) or more below the surface of the oceans. In a few places, however, they actually extend above sea level and form islands. Iceland, the Azores, and Tristan de Cunha are examples of such islands.
Running along the middle of an oceanic ridge, there is often a deep crevice known as a rift, or median valley. This central rift can plunge as far as 6,500 ft (2,000 m) below the top of the ridge that surrounds it. According to the theory of plate tectonics, ocean ridges are formed when molten rock, or magma, escapes from Earth’s interior to form the lithospheric plates (which include the seafloor) of Earth, a process known as seafloor spreading. Rifts may be the specific parts of the ridges where the magma escapes.
Trenches are long, narrow, canyon-like structures, most often found adjacent to a continental margin. They occur much more commonly in the Pacific than in any of the other oceans. The deepest trench on Earth is the Marianas trench, which runs from the coast of Japan south and then west toward the Philippine Islands. Its deepest spot is 36,152 ft (11,022 m) below sea level and it runs a distance of about 1,580 m (2,550 km). The longest trench is located along the coast of Peru and Chile. Its total length is 3,700 m (5,900 km) and it has a maximum depth of 26,420 ft (8,055 m).
Earthquakes and volcanic activity are commonly associated with trenches. In fact, the trenches that encircle the Pacific Ocean are sometimes called the Ring of Fire because of the volcanic activity located there. According to the plate tectonic theory, trenches form at sites where one lithospheric plate is forced beneath another, or subducted, as a result of seafloor spreading elsewhere. Friction between the two plates is responsible for the associated earthquakes and volcanic activity.
Fracture zones are the source of additional seismic activity in ocean basins. These are regions where, along numerous faults, sections of the ocean floor slide past each other, relieving tension created by seafloor spreading at the ocean ridges. Ocean crust in a fracture zone looks like it has been sliced up by a giant knife. The faults usually cut across ocean ridges, often nearly at right angles to the ridge. A map of the North Atlantic Ocean basin, for example, shows the Mid-Atlantic Ridge traveling from north to south across the middle of the basin, with dozens of fracture zones cutting across the ridge from east to west.
Some of the largest fracture zones are located along the eastern edge of the Pacific Ocean. The Clipperton And Clarion Fracture Zones, for example, originate along the western coast of Mexico and extend up to 3,300 mi (5,300 km) to the west. At their maximum, these zones may be almost 30 mi (50 km) wide and 10,500 ft (3,150 m) deep.
Abyssal plains and hills
Abyssal plains are relatively flat areas of the ocean basin with slopes of less than one part in a thousand. They tend to be found at depths of 13,000-16,000 ft (4,000-5,000 m). Oceanographers believe that abyssal plains are so flat because they are layered with sediments that have washed off the surface of the continents for thousands of years. On the abyssal plains, these layers of sediment have now covered up any irregularities that may exist in rock of the ocean floor beneath them.
Abyssal plains found in the Atlantic and Indian Ocean tend to be more extensive than those in the Pacific Ocean. The majority of the world’s largest rivers empty into the Atlantic and Indian Oceans, supplying both ocean basins the sediments from which abyssal plains are made.
Abyssal hills are irregular structures on the ocean floor that average about 825 ft (250 m) in height. They often occur over very wide stretches of the ocean floor and are especially common in the Pacific Ocean. Abyssal hills are probably just smaller versions of the volcanic features known as seamounts.
Largely unseen by human eyes, the ocean basins are alive with volcanic activity. Magma flows upward from the mantle to the ocean bottom not only through rifts, but also through numerous volcanoes and other openings in the ocean floor. Seamounts are submarine volcanoes and can either be active or extinct. Guyots are extinct volcanoes that were once above sea level, but have since subsided below the surface. As they subsided, wave or current action eroded the top of the volcano to a flat surface.
Seamounts and guyots typically rise about 0.6 mile (1 km) above the ocean floor. One of the largest known seamounts is Great Meteor Seamount. It extends to a height of more than 1,300 ft (4,000 m) above the ocean floor in the northeastern part of the Atlantic Ocean.
See also Continental shelf; Hydrothermal vents; Oceanography.
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David E. Newton