2.4: Divergent Boundaries

At divergent boundaries, sometimes called constructive boundaries, lithospheric plates move away from each other. There are two types of divergent boundaries, categorized by where they occur: continental rift zones and mid-ocean ridges. Continental rift zones occur in weak spots in the continental lithospheric plate. A mid-ocean ridge usually originates in a continental plate as a rift zone that expands to the point of splitting the plate apart, with seawater filling in the gap. The separate pieces continue to drift apart and become individual continents.

Continental Rifting

Figure \(\PageIndex{1}\): Faulting that occurs in divergent boundaries.

In places where the continental plates are very thick, they reflect so much heat back into the mantle it develops strong convection currents that push super-heated mantle material up against the overlying plate, softening it. Tensional forces created by this convective upwelling begin to pull the weakened plate apart. As it stretches, it becomes thinner and develops deep cracks called normal faults in the diagram above. Eventually, plate sections located between large faults drop into deep depressions known as rift valleys, which often contain keystone-shaped blocks of down-dropped crust known as grabens. The shoulders of these grabens are called horsts. If only one side of a section drops, it is called a half-graben. Depending on the conditions, rifts can grow into very large lakes and even oceans.

Figure \(\PageIndex{2}\): The Afar Triangle (center) has the Red Sea ridge (center to upper left), Gulf of Aden ridge (center to right), and East African Rift (center to lower left) form a triple junction that are about 120° apart.

While seemingly occurring at random, rifting is dictated by two factors. 1) Rifting does not occur in continents with older and more stable interiors, known as cratons. 2) When continental rifting does occur, the break-up pattern resembles the seams of a soccer ball (in three directions from a central point). This is the most common surface-fracture pattern to develop on an evenly expanding sphere because it uses the least amount of energy [68].

Using the soccer ball model, rifting tends to lengthen and expand along a particular seam while fizzling out in the other directions. These seams with little or no tectonic activity are called failed rift arms. A failed rift arm is still a weak spot in the continental plate. One example of a failed rift arm is the Mississippi Valley Embayment, a depression through which the upper end of the Mississippi River flows. Occasionally connected rift arms do develop concurrently, creating multiple boundaries of active rifting. In places where the rift arms do not fail, for example the Afar Triangle, three divergent boundaries can develop near each other forming a triple junction.

Figure \(\PageIndex{3}\): NASA image of the Basin and Range horsts and grabens across central Nevada.

Rifts come in two types: narrow and broad. Narrow rifts are characterized by a high density of highly active divergent boundaries. The East African Rift Zone, where the horn of Africa is pulling away from the mainland, is an excellent example of an active narrow rift. Lake Baikal in Russia is another. Broad rifts also have numerous fault zones, but they are distributed over wide areas of deformation. The Basin and Range region located in the western United States is a type of broad rift.

Figure \(\PageIndex{4}\): The narrow East African Rift.

Rifts have earthquakes, although not of the magnitude and frequency of other boundaries. They may also exhibit volcanism.

Mid-Ocean Ridges

Figure \(\PageIndex{5}\): Progression from rift to mid-ocean ridge.

Mid-ocean ridges, also known as spreading centers, have several distinctive features. They are the only places on earth that create new oceanic lithosphere. Melting in the rift zone changes asthenosphere material into the new lithosphere, which oozes up through cracks in the oceanic plate. The amount of new lithosphere being created at mid-ocean ridges is highly significant. These undersea rift volcanoes produce more lava than all other types of volcanism combined. Despite this, most mid-oceanic ridge volcanism remains unmapped because the volcanoes are located deep on the ocean floor.

In rare cases, such as a few locations in Iceland, rift zones come to the surface and display the type of volcanism, spreading, and ridge formation found on the ocean floor.

Figure \(\PageIndex{6}\): Age of oceanic lithosphere, in millions of years. Notice the differences in the Atlantic Ocean along the coasts of the continents.

As the ridge continues to spread, the lithosphere material is pulled away from the area of volcanism and becomes colder and denser. This means the sections of the lithosphere furthest away from the mid-ocean ridges will be the oldest. Sediment accumulation patterns also confirm the idea of the ocean being younger near the mid-ocean ridges as sediment layers tend to be thinner near them, indicating it has had less time to build up.

References

• 68. Sears, J. W., St. George, G. M. & Winne, J. C. Continental rift systems and anorogenic magmatism. Lithos 80, 147–154 (2005).