4.5: Divergent Plate Boundaries

The processes by which divergent plate boundaries are first formed are not fully understood. It is believed that rift zones in the continents may be at least partially caused by upwelling of magma accumulated under the continents, and that this magma may accumulate because the continental crust acts as an insulator and causes heat to build up in the mantle below it. However, the most prominent rift zone on the Earth today, the East African Rift Zone, is thought to have been caused by upwelling associated with the superplume that is believed to exist in the lower mantle beneath the African continent.

At oceanic ridges, new oceanic crust is continuously formed by upwelling of magma from the asthenosphere as the plates move apart, but this upwelling is not the cause of the plate divergence. Instead, the magma upwelling occurs in order to fill the gap created when sinking (downwelling) of lithosphere at a subduction zone elsewhere on one (or both) of the plates moves the entire plate toward this subduction zone, “pulling” the plate away from the oceanic ridge and creating a divergent boundary.

Oceanic Ridges

When two plates are pulled apart (diverge), the gap between them is filled by upwelled magma. The reduction of weight on the upper mantle that occurs as the plate edges are drawn apart causes solid mantle material to melt as pressure is reduced. This hot, low-density magma rises through the splitting crust and spreads on the seafloor from a series of volcanoes aligned along the gap. This is the primary process responsible for building the connected undersea mountain chains of the oceans (Fig. 3-4) and is often called seafloor spreading. The mountain chains are known as oceanic ridges and oceanic rises. Most are in the approximate center of their ocean basins and follow the general shape of the coastlines on either side of the ocean (Fig. 4-11). An example is the Mid-Atlantic Ridge, which bisects the Atlantic Ocean from near the North Pole to the southern tip of South America (Fig. 4-19, Fig. 3-4).

Figure 4-19. Topography of the Arctic Ocean floor. The Mid-Atlantic Ridge extends into the Arctic Ocean. This extension is named the Gakkel Ridge. Shallow sills restrict water circulation between the Arctic Ocean and other oceans. The sill between the Arctic Ocean and the Bering Sea is especially shallow, and the circulation between the Arctic and Pacific Oceans is more restricted than the circulation between the Arctic and Atlantic Oceans.

As the plate edges separate at an oceanic ridge, new oceanic crust is formed by a sequence of processes that cause oceanic crust to be layered. Molten magma upwells from below and forms a chamber of molten magma below the seafloor. Some magma cools and solidifies on the sides of this chamber to form a rock called “gabbro.” This becomes the lowest of four layers that are found consistently in almost all oceanic crust. The next layer up is formed by magma that rises through vertical cracks and solidifies in wall-like sheets called “dikes.” The third, and initially upper, layer is composed of pillow basalt, which is formed when erupting lava is cooled rapidly by seawater. The final layer of oceanic crust is composed of sediments deposited from the ocean water column.

New crust formed at an oceanic ridge or rise is hot and continues to be heated from below as long as it remains above the divergence zone. Because it is hot, it is less dense than older, cooler crust. Beneath the new crust, the solid mantle portion of the lithosphere is also thin. Therefore, the new crust floats with its base higher on the asthenosphere (CC2), and consequently, the seafloor is shallower. As the new crust is moved progressively farther from the divergent plate boundary, it cools by conduction of heat to the overlying water and becomes denser. In addition, the lithosphere slowly thickens as mantle material in the asthenosphere just below the base of the lithosphere cools, solidifies, and is added to the lithospheric plate. Therefore, the aging crust on the plate sinks steadily to float lower on the asthenosphere. Thus, oceanic ridge mountains slowly move away from the divergence zone and sink, while new mountains form at the divergence zone to take their place.

Oceanic Ridge Types

There are three basic types of oceanic ridges distinguished by spreading rate, the rate at which the two plates are being pulled apart. Two of these types constitute a majority of the length of the world’s oceanic ridge system. The third, where the spreading rate is very slow, is less well studied.

The two faster-spreading types of oceanic ridges have certain common characteristics. Each has a string of volcanoes aligned along the axis of the ridge. The volcanoes create a continuously spreading layer of new oceanic crust on the ocean floor. The most common type of oceanic ridge, exemplified by most of the Mid-Atlantic Ridge, has a well-defined, steep-sided central rift valley extending down its center (Fig. 4-20, Fig. 3-4). The valley is the gap formed by the two plates pulling apart. Numerous earthquakes occur beneath the valley, and volcanic vents erupt magma to create the new oceanic crust. Central rift valleys are generally 25 to 50 km wide and 1 to 2 km lower than the surrounding peaks. The oceanic ridge mountains stand 1 to 3 km above the surrounding ocean floor and are extremely rugged.

Figure 4-20. Oceanic ridge divergent plate boundaries are characterized by a ridge with a central rift valley and by fracture zones. On slowly spreading ridges, such as the Mid-Atlantic Ridge, the rift valley may be 30 km wide and up to 3 km deep, and the ridge is rugged with steep slopes. On rapidly spreading ridges, such as the East Pacific Rise, the rift valley is typically 2 to 10 km wide and only 100 km or so deep, and the ridge is broad with low-angle slopes and relatively smooth topography.

The second type of oceanic ridge, exemplified by the East Pacific Rise, is much broader and less rugged than the Mid-Atlantic type, and the central rift valley is absent or poorly defined (Fig. 3-3). The lack of a central rift valley is thought to be due to a faster spreading rate. The less rugged topography of this type of ridge is reflected in the seafloor of the surrounding plate, which is flatter than the seafloor surrounding more slowly spreading ridges.

The third type of oceanic ridge occurs where the spreading rate is extremely slow. This type of oceanic ridge has very few and widely spaced volcanoes. In addition, much of the new seafloor generated at these ridges is missing the characteristic basalt layer found elsewhere. This layer is thought to be missing because magma at these slowly spreading ridges has time to cool and solidify before it rises to the seafloor. The entire process appears to be different at these very slowly spreading ridges than at other oceanic ridges. At very slowly spreading ridges, the seafloor is apparently just cracked apart between volcanoes, and warm but solid rock then rises to become new seafloor. The Gakkel Ridge beneath the Arctic Ice Cap, northeast of Greenland, and the Atlantic-Indian Ridge as it snakes around the southern tip of Africa are examples. By some estimates, up to 40% of the oceanic ridge system worldwide may be of this extremely slowly spreading type.

Oceanic Ridge Volcanoes

Where the peaks of oceanic ridge volcanoes approach the ocean surface, as they do near Iceland, eruptions can be violent. In such cases, numerous small steam explosions are created at the volcano vent when hot erupting magma and seawater come in contact (you may have seen video footage of such eruptions). However, most eruptions at oceanic ridge volcanoes are quiet and smooth-flowing, despite the magma–seawater contact. There are several reasons for the lack of explosiveness. First, unlike the magma of subduction zone volcanoes, the magma rising into oceanic ridge volcanoes does not include sediments and water. Second, water movements carry heat away from the erupting magma, minimizing or preventing the production of steam. Third, the boiling point of water is several hundred degrees Celsius at the high pressures present at the depths of most oceanic ridge volcanoes.

Because they are not explosive, and they occur deep within the ocean, most oceanic ridge volcanic eruptions occur without being noticed. Nonetheless, they can be at least as large and as frequent as eruptions on land. For example, a sidescan sonar survey of part of the East Pacific Rise in the early 1990s revealed a huge lava field formed by an eruption believed to have occurred within the previous 20 years. The lava field covers an area of 220 km² and has an estimated average thickness of 70 m. The volume of lava is estimated to be about 15 km³— enough to cover 1000 km of four-lane highway to a depth of 600 m, or to repave the entire U.S. interstate highway system 10 times. The lava field is the largest flow known to be generated during human history (much larger lava fields from prehistoric eruptions are known). Only a small fraction of the 60,000 km of oceanic ridge has been studied by sidescan sonar surveys or by submersibles. Hence, the huge young lava field on the East Pacific Rise may be dwarfed by undiscovered flows from other undersea eruptions.

In 2009, the ROV Jason II captured video of an eruption of the West Mata volcano, located 1100 m deep on the Tonga Ridge. Several more underwater volcano eruptions have now been captured on video and studied extensively by scientists (watch an eruption here). It appears these underwater volcanic eruptions are far more frequent and numerous than volcanic eruptions on land.

Rift Zones

Rift zones occur where a continent is being pulled or is breaking apart. If the rifting continues long enough, an ocean basin is created between two separate, smaller continents. A rift zone may be created when a continental crustal block remains in one place for a prolonged period. In such instances, heat beneath the continent is partially trapped by the insulating effect of the continental rocks. The mantle temperature may increase until the rock at the base of the lithosphere melts, at which time hot magma could rise and split the continental block. Rift zones may also be formed by variations in the convection processes operating within the mantle that cause the mantle upwelling to become located under the continent (CC3). As yet, we have no detailed understanding of the relative roles of mantle convection and the insulating effect of continental crust, or of how these processes interact to split continents and create new oceans. In contrast, the processes that occur once a rift zone has been initiated are much better understood.

New Oceans

A new ocean may form in a location where the temperature of the mantle below the continent becomes elevated (Fig. 4-21). The process may occur as follows: As the mantle temperature increases, its density decreases and it rises isostatically (CC2), causing the crust to be thrust upward to form a dome. The upward thrust stretches the crust, causing fractures that extend outward from the center of the dome. As such a fracture widens, the lithosphere beneath the bulge begins to melt. Eventually, blocks of crust break off and slip down into the rift valley formed by the fractures. This occurs unevenly along the length of the rift, with some sections of the rift valley being deeper than others.

Figure 4-21. The history of new ocean formation at a rift zone. (a) Rising magma beneath a continent pushes the crust upward and creates cracks and fractures. (b) The crust is stretched and thinned. Collapses occur along a series of fault blocks, leading to the formation of a rift valley. Lakes may be formed in the rift valley. (c) The rift widens, allowing ocean water to enter and creating a narrow sea, such as the Red Sea. The rift continues to widen, and an oceanic ridge is formed down its center, eventually creating a new ocean.

Initially, the rift valley floor may be well above sea level. In wet climates, the deeper areas of the valley may fill with rainwater, forming long, narrow lakes. Volcanoes form in the rift valley and on its sides as magma upwells through cracks left by the fracturing and slipping blocks of crust. The sides of the rift move steadily apart, and magmatic rocks accumulate at the bottom of the valley. Because magmatic rocks, when they cool, are denser than the continental rocks being displaced to the sides of the rift, the rift valley floor sinks until it is eventually below sea level. At this point, the rift may fill with seawater and become an arm of the ocean. However, new volcanoes and landslide debris falling from the rift valley flanks may fill and temporarily increase the elevation of the rift valley floor. Thus, the connection with the ocean may be made and broken numerous times before the rift valley becomes a “permanent” marginal sea.

As the rift continues to widen, the original continent splits into two separate continents, separated by a widening ocean with an oceanic ridge at its center. The rift valley provides a route through which the excess heat built up below can be released. If the quantity of heat is limited, the rifting process may stop before an ocean is formed. However, if excess heat is supplied at a high rate, the spreading may continue, and a new ocean may be formed.

Of the several rift zones on the Earth today, the East African Rift is among the newest (Fig. 3-4). The African continent appears to have remained at its present location for more than 100 million years. It is elevated several hundred meters higher than most of the other continents and is believed to be directly above a mantle superplume. The East African Rift displays a rift valley, rift valley lakes, and rift volcanoes typical of newly formed continental divergent plate boundaries. The Red Sea is probably a more advanced rift zone, where a connection with the ocean, perhaps a permanent one, has already been made.