2.4: Divergent Boundaries (2023)

continental rift

Where the tectonic plates are very thick, they reflect so much heat back into the mantle that they develop strong convection currents that push the superheated mantle material against the overlying plate, softening it. The tensile forces created by this convective buoyancy begin to pull the weakened plate apart. As it stretches, it thins and develops deep cracks called normal or stretch defects. Eventually, sections of plates plunge between large faults into deep depressions known as rift valleys, often containing keystone-shaped blocks of downward-sloping crust known as grabens. The shoulders of these ditches are called horst. When only one side of a section falls, it is called a half ditch. Depending on the conditions, the rifts can become very large lakes and even oceans.

Although seemingly random, the eruption is dictated by two factors. The fracture does not occur on continents with older, more stable interiors, the cratons. When a continental rift occurs, the fracture pattern resembles the seams of a soccer ball, also known as a truncated icosahedron. This is the most common surface fracture pattern that develops on a uniformly expanding sphere because it consumes the least amount of energy.68].

Using the soccer ball model, the tear tends to lengthen and expand along a particular seam while fading in the other directions. These seams with little or no tectonic activity are referred to as failed fault arms. A broken fracture arm remains a weak point in the continental plate; even without the presence of active extension errors, it can become an aukogen. An example of a failed fault arm is the Mississippi Valley Inlet, a depression through which the upper end of the Mississippi flows. Occasionally, connected breakout arms will develop simultaneously, creating multiple active breakout boundaries. In places where the arms of the rift do not fail, for example in the Afar triangle, three diverging boundaries can develop close to each other, forming a triple junction.

Cracks come in two types: narrow and wide. Narrow rifts are characterized by a high density of highly active divergent boundaries. The East African Rift Zone, where the Horn of Africa is retreating from the continent, is an excellent example of an active narrow rift. Lake Baikal in Russia is another. Wide cracks also have numerous failure zones, but these are distributed over a wide range of deformation. The Basin and Range region of the western United States is a type of wide rift. The Wasatch Fault, which also created the Wasatch Ridge in Utah, forms the divergent eastern boundary of this broad graben (Animation 1jAnimation 2).

The rifts have earthquakes, although not of the magnitude and frequency of other limits. They can also exhibit volcanism. Unlike the flow-molten magma found in subduction zones, fracture-zone magma is formed by decompression melting. As the tectonic plates pull apart, they create a low-pressure region that melts and pulls the lithosphere upward. When this molten magma reaches the weakened, fault-ridden fault zone, it migrates to the surface, either breaching the plate or escaping through an open fault. Examples of young rift volcanoes can be found in the Basin and Range region of the United States. The activity of the rift zone is responsible for the formation of unique volcanism such as B. Ol Doinyo Lengai in Tanzania. This volcano erupts lava composed primarily of carbonatite, a relatively cold, liquid carbonate mineral [69].

South America and Africa separate and form the Atlantic.Videoby Ask Atwater.

mid-ocean ridges

As rifting and volcanic activity progress, the continental lithosphere becomes more mafic (see Chapter 4) and thinner, with the end result that the plate beneath the rift area is transformed into oceanic lithosphere. This is the process that creates a new ocean, much like the narrow Red Sea that formed with Arabia's movement away from Africa. As the oceanic lithosphere continues to diverge, a mid-oceanic ridge forms.

Mid-ocean ridges, also called spreading centers, have several distinctive features. They are the only places on Earth that create new oceanic lithosphere. Decompression melting at the rift zone converts material from the asthenosphere into new lithosphere, which exits through cracks in the oceanic plate. The amount of new lithosphere formed at mid-ocean ridges is very significant. These submarine rift volcanoes produce more lava than all other types of volcanism combined. Despite this, most volcanism of mid-ocean ridges remains uncharted because the volcanoes are deep on the sea floor.

In rare cases, as in some places in Iceland, rift zones show the type of volcanism, spreading, and ridge formation found on the sea floor.

The ridge feature is formed by the accumulation of hot lithospheric material that is lighter than the underlying dense asthenosphere. This isostatically buoyant piece of lithosphere is partially submerged and partially exposed to the asthenosphere, like an ice cube floating in a glass of water.

As the ridge continues to spread, lithospheric material moves away from the volcanic area, becoming cooler and denser. As it continues to expand and cool, the lithosphere settles into broad swaths of relatively featureless topography, referred to as abyssal plains of lower topography.70].

This model of ridge formation suggests that the sections of lithosphere furthest from mid-ocean ridges will be the oldest. Scientists have tested this idea by comparing the ages of rocks at different locations on the sea floor. Rocks near the ridges are younger than those far from the ridges. Sediment accumulation patterns also support the idea of ​​seafloor spreading. Sedimentary layers tend to be thinner near mid-ocean ridges, indicating they have had less time to accumulate.

As mentioned in the section on paleomagnetism and the development of plate tectonics theory, scientists found that mid-oceanic ridges contained unique magnetic anomalies that appear as symmetrical fringes on either side of the ridge. The Vine-Matthews-Morley Hypothesis [20] proposes that these alternating inversions are produced by the Earth's magnetic field imprinting on the magma after it emerges from the ridge [71]. Very hot magma has no magnetic field. As oceanic plates move apart, magma cools below the Curie point, the temperature below which a magnetic field becomes trapped in magnetic minerals. Alternating magnetic reversals in rocks reflect the periodic exchange of the Earth's north and south magnetic poles. This paleomagnetic pattern provides a great historical record of seafloor movement and is used to reconstruct past tectonic activity and determine ridge propagation rates.72].

Videothe breakup of Pangea and the formation of the North Atlantic. By Tanja Atwater.

Thanks to their distinctive geology, the mid-ocean ridges host some of the most unique ecosystems ever discovered. Ridges are often dotted with hydrothermal vents, deep fissures that allow seawater to circulate over the tops of the oceanic plate and interact with hot rock. Superheated seawater rises back to the surface of the plate, carrying with it dissolved gases, minerals, and small particles. The released hydrothermal water looks like black smoke underwater.

Scientists have known about these geothermal areas on the sea floor for some time. However, it wasn't until 1977 when scientists piloting a deep-sea vehicle, the Alvin, discovered a thriving community of organisms congregating around these hydrothermal vents.73]. These unique organisms, which include tubeworms that are 10 feet taller than humans, live in the total darkness of the ocean floor without oxygen or sunlight. They use the vents' geothermal energy and a process called bacterial chemosynthesis to feed on sulfur compounds. Before this discovery, scientists believed that life on Earth could not exist without photosynthesis, a process that requires sunlight. Some scientists suggest that this type of environment could have been the origin of life on Earth [74] and perhaps even extraterrestrial life in other parts of the galaxy, such as on Jupiter's moon Europa [75].

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