Sketch and describe the movement along a divergent plate boundary that results in the formation of new oceanic lithosphere.
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 1). 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 2, 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 (2500 miles) kilometers.
Further, along the crest of some ridge segments is a deep canyonlike structure called a rift valley (Figure 3).
This structure is evidence that tensional (opposing) 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 15 centimeters (6 inches) per year have been measured along sections of the East Pacific Rise.
Although these rates of seafloor production are slow on a human time scale, they are nevertheless rapid enough to have generated all of Earth’s ocean basins 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 found 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 upwelling. Thus, it begins to cool and contract, thereby increasing in density. This thermal contraction accounts for the increase in ocean depths 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 it to become increasingly more rigid.
Thus, oceanic lithosphere is generated by cooling of the asthenosphere from the top down. Stated another way, the thickness of oceanic lithosphere is age dependent. The older (cooler) it is, the greater its thickness. Oceanic lithosphere that exceeds 80 million years in age is about 100 kilometers (60 miles) thick— approximately its maximum thickness.
Divergent boundaries can develop within a continent, in which case the landmass may 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 4A).
During this process, the lithosphere is thinned, while the brittle crustal rocks break 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 4B,C) and eventually a new ocean basin (Figure 4D).
An example of an active continental rift is the East African Rift (Figure 5). 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 (see Figure 4C).
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 (see Figure 4D).