Most divergent plate boundaries (di = apart, vergere = to move) are located along the crests of oceanic ridges and can be thought of as constructive plate margins because this is where new ocean floor is generated (Figure 11). Here, two adjacent plates move away from each other, producing long, narrow fractures in the ocean crust. As a result, hot molten rock from the mantle below migrates upward to fill the voids left as the crust is being ripped apart. This molten material gradually cools to produce new slivers of seafloor. In a slow yet unending manner, adjacent plates spread apart, and new oceanic lithosphere forms between them. For this reason, divergent plate boundaries are also called spreading centers.

Oceanic Ridges and Seafloor Spreading

The majority of, but not all, divergent plate boundaries are associated with oceanic ridges, elevated areas of the seafloor characterized by high heat flow and volcanism. The global oceanic ridge system is the longest topographic feature on Earth’s surface, exceeding 70,000 kilometers (43,000 miles) in length. As shown in Figure 10, various segments of the global ridge system have been named, including the Mid-Atlantic Ridge, East Pacific Rise, and Mid-Indian Ridge.

Representing 20 percent of Earth’s surface, the oceanic ridge system winds through all major ocean basins, like the seams on a baseball. Although the crest of the oceanic ridge is commonly 2 to 3 kilometers (1 to 2 miles) higher than the adjacent ocean basins, the term ridge may be misleading because it implies “narrow” when, in fact, ridges vary in width from 1000 kilometers (600 miles) to more than 4000 kilometers (2500 miles). Further, along the crest of some ridge segments is a deep canyonlike structure called a rift valley (Figure 12). This structure is evidence that tensional (pulling apart) forces are actively pulling the ocean crust apart at the ridge crest.

The mechanism that operates along the oceanic ridge system to create new seafloor is appropriately called seafloor spreading. Spreading typically averages around 5 centimeters (2 inches) per year, roughly the same rate at which human fingernails grow. Comparatively slow spreading rates of 2 centimeters per year are found along the Mid-Atlantic Ridge, whereas spreading rates exceeding 16 centimeters (6 inches) per year have been measured along sections of the East Pacific Rise (refer to Figure 10). Although these rates of seafloor production are slow on a human time scale, they are rapid enough to have generated all of Earth’s current oceanic lithosphere within the past 200 million years.

The primary reason for the elevated position of the oceanic ridge is that newly created oceanic lithosphere is hot and, therefore, less dense than cooler rocks located away from the ridge axis. (Geologists use the term axis to refer to a line that follows the general trend of the ridge crest.) As soon as new lithosphere forms, it is slowly yet continually displaced away from the zone of mantle upwelling. Thus, it begins to cool and contract, thereby increasing in density. This thermal contraction accounts for the increase in ocean depth away from the ridge crest. It takes about 80 million years for the temperature of oceanic lithosphere to stabilize and contraction to cease. By this time, rock that was once part of the elevated oceanic ridge system is located in the deep-ocean basin, where it may be buried by substantial accumulations of sediment.

In addition, as the plate moves away from the ridge, cooling of the underlying asthenosphere causes its upper layers to become increasingly rigid and hence to become part of the lithosphere. Stated another way, the thickness of oceanic lithosphere is age dependent. The older (cooler) it is, the greater its thickness. Oceanic lithosphere that exceeds  million years in age is about 100 kilometers (60 miles) thick—approximately its maximum thickness.

Continental Rifting

Divergent boundaries can develop within a continent and may cause the landmass to split into two or more smaller segments separated by an ocean basin. Continental rifting begins when plate motions produce tensional forces that pull and stretch the lithosphere. This stretching, in turn, promotes mantle upwelling and broad upwarping of the overlying lithosphere (Figure 13A). This process thins the lithosphere and breaks the brittle crustal rocks into large blocks. As the tectonic forces continue to pull apart the crust, the broken crustal fragments sink, generating an elongated depression called a continental rift, which can widen to form a narrow sea (Figure 13B,C) and eventually a new ocean basin (Figure 13D).

An example of an active continental rift is the East African Rift (Figure 14). Whether this rift will eventually result in the breakup of Africa is a topic of ongoing research. Nevertheless, the East African Rift is an excellent model of the initial stage in the breakup of a continent. Here, tensional forces have stretched and thinned the lithosphere, allowing molten rock to ascend from the mantle. Evidence for this upwelling includes several large volcanic mountains, including Mount Kilimanjaro and Mount Kenya, the tallest peaks in Africa. Research suggests that if rifting continues, the rift valley will lengthen and deepen (refer to Figure 4.13C). At some point, the rift valley will become a narrow sea with an outlet to the ocean. The Red Sea, formed when the Arabian Peninsula split from Africa, is a modern example of such a feature and provides us with a view of how the Atlantic Ocean may have looked in its infancy (refer to Figure 13D).

• Seafloor spreading leads to the formation of new oceanic lithosphere at mid-ocean ridge systems. As two oceanic plates move apart from one another, tensional forces open cracks in the plates, allowing magma to well up and generate new slivers of seafloor. This process generates new oceanic lithosphere at a rate of 2 to 15 centimeters (1 to 6 inches) each year.

• As it ages, oceanic lithosphere cools and becomes denser. It, therefore, subsides as it is transported away from the mid-ocean ridge. At the same time, the underlying asthenosphere cools, adding new material to the underside of the plate, which consequently thickens.

• Divergent boundaries are not limited to the seafloor. Continents can break apart, too, starting with a continental rift (as in modern-day East Africa) and potentially producing a new ocean basin between the two sides of the rift.

• continental rift: A linear zone of divergence along which continental lithosphere stretches and pulls apart. Its creation may mark the beginning of a new ocean basin.

• divergent plate boundary: Regions where two adjacent rigid plates are moving apart, typified by the mid-ocean ridges. Also called spreading centers.

• oceanic ridge system: A continuous zone elevated 2–3 kilometers off the seafloor of all the major ocean basins and varying in width from 1000 to more than 4000 kilometers (600 to 2500 miles). The rift valleys at the crests of ridges represent divergent plate boundaries.

• rift valley: A long, narrow trough bounded by normal faults, found at divergent plate boundaries of the seafloor and on continents. It represents the region where divergence is taking place.

• seafloor spreading: The process of producing new seafloor between two diverging plates at average rates of around 5 centimeters (2 inches) per year.