5.5 The Mid-Atlantic Ridge and the Global Oceanic Ridge System: Main Features

5.5 The Mid-Atlantic Ridge and the Global Oceanic Ridge System: Main Features

Along well-developed divergent plate boundaries, the seafloor is elevated, forming a broad linear swell called the oceanic ridge, or mid-ocean ridge. Our knowledge of the oceanic ridge system comes from soundings taken of the ocean floor, core samples obtained from deep sea drilling, visual inspection using deep-diving submersibles, and even first-hand inspection of slices of ocean floor that have been displaced onto dry land along convergent plate boundaries. An elevated position, extensive faulting and associated earthquakes, high heat flow, and numerous volcanic structures characterize the oceanic ridge.

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The interconnected Oceanic Ridge system is the longest topographic feature on Earth’s surface, exceeding 43,000 miles in length. Representing more than 20% of Earth’s surface, the oceanic Ridge winds through all major oceans in a manner similar to the seam on a baseball. The crest of this linear structure typically stands 2 to 3 km above the adjacent deep ocean basins and marks the plate margins where the new oceanic crust is created.

Large sections of the oceanic ridge system have been named based on their locations within the various oceanic basins. Some ridges run along the middle of the ocean basins, where they are called mid-ocean ridges. This holds true for the Mid-Atlantic Ridge, which is positioned in the middle of the Atlantic, roughly paralleling the margins of the continents on either side. This is also true for the mid Indian Ridge. However, the East Pacific Rise is not a mid-ocean feature. Rather, as its name implies, it is located in the eastern Pacific, far from the center of the ocean. At its northern end there are two branches, one that points toward Central America and another that curves towards South America.

The term ridge may be misleading, because these features are not narrow and steep as that term implies, but have widths from 1,000 to 4,000 km and the appearance of a broad, elongated swell that exhibits various degrees of ruggedness. Furthermore, the ridge system is broken into segments that range from a few tens to hundreds of kilometers in length. Although each segment is offset from the adjacent segment, they are generally connected, one to the next, by a transform fault.

Oceanic ridges are as high as some mountains found on the continents. However, the similarity ends there. Whereas most continental mountains form when compressional forces fold and metamorphose thick sequences of sedimentary rocks along convergent plate boundaries, oceanic ridges form where tensional forces fracture and pull the ocean crust apart. The oceanic ridge consists of layers and piles of newly-formed mafic rocks that have been faulted into elongated blocks that are buoyantly uplifted.

Along the axis of some segments of the oceanic ridge system are deep, down-faulted structures called rift valleys. The name Rift Valley has been applied to these features because they are strikingly similar to the continental rift valleys that comprise the East African Rift. Some rift valleys, including many along the Mid-Atlantic Ridge, exceed 30 km in width and have walls that tower 2,000 meters above the valley floor. This makes them comparable to the deepest and widest part of Arizona’s Grand Canyon.

Why are Oceanic ridges elevated?

The primary reason for the elevated position of a ridge system is the fact that newly-created oceanic lithosphere is hot, occupies more volume, and is therefore less dense than cooler rocks of the deep ocean basin. As the newly-formed basaltic crust travels away from the ridge crest it is cooled from above as sea water circulates through the pore spaces and fractures in the rock. It also cools because it is moving away from the zone of upwelling, which is the main source of heat. As a result, the lithosphere gradually cools, contracts, and becomes more dense. This thermal contraction accounts for the greater ocean depths that exist away from the ridge. It takes almost 80 million years before cooling and contraction cease completely. By this time, rock that was once part of an elevated ocean ridge system is located in the deep ocean basin, where it may be covered by relatively thick accumulations of sediment.

As the lithosphere is displaced away from the ridge crest, cooling also causes a gradual increase in lithosphere thickness. This occurs because the boundary between the lithosphere and asthenosphere is temperature dependent. Recall that the lithosphere is Earth’s cool rigid outer layer, whereas the asthenosphere is a comparatively hot and weak zone. As material in the upper mantle ages or cools, it becomes rigid. Thus the upper portion of the asthenosphere is converted to lithosphere simply by cooling. Newly-formed oceanic lithosphere will continue to thicken for about 80 million years. Thereafter its thickness remains relatively constant until it is subducted.