Ocean Basin

Some of the most impressive topographic features of the ocean basins are the enormous mountain ranges, or oceanic ridges, that cover the ocean floor. 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, some scientists say that a single oceanic ridge encircles Earth, one that stretches a total of 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.