Divergent Boundary: Definition, Features, Examples

Divergent boundary, also known as a constructive boundary or an extensional boundary, is a linear geologic feature that exists between two tectonic plates that are moving away from each other. The separation of plates creates an opening at the surface, which is filled with new igneous rock material rising from the Earth's mantle. This continuous upwelling of magma is what forms new oceanic crust.

Divergent boundaries can form within continents or between continents and oceans. When they form within continents, they initially produce rifts, which are valleys formed by the ground subsiding between two diverging plates. Over time, these rifts can widen and deepen, eventually becoming rift valleys like the East African Rift Valley.

Most active divergent plate boundaries occur between oceanic plates and exist as mid-oceanic ridges. The Mid-Atlantic Ridge, for example, is a massive underwater mountain range that runs along the center of the Atlantic Ocean. It marks the location where the North American and Eurasian plates are diverging.

Divergent Boundary Features

Continental rifting

Continental rifting is the initial stage in the process of continental breakup, where the lithosphere (Earth's crust and uppermost mantle) thins and stretches. This stretching creates a rift valley, a long, linear depression formed by the subsidence of the land between uplifted fault blocks.

Continental rifting is primarily driven by plate tectonics, where convection currents in the mantle cause tectonic plates to move in different directions. As two plates diverge, the tensional stress pulls the continent apart.

Continental rifting process

Diverging Tectonic Plates: The initial driving force behind continental rifting is the movement of tectonic plates. As the plates diverge, they stretch the overlying continental crust.

Lithosphere Thinning: The stretching forces cause the lithosphere, the Earth's rigid outermost layer, to become thinner and weaker.

Faulting and Subsidence: As the lithosphere thins, it eventually cracks along normal faults, where the crustal blocks move down relative to each other. This faulting creates a valley along the rift zone.

Volcanic Activity: The thinning of the lithosphere allows hot material from the mantle to rise closer to the surface. This can lead to volcanic activity along the rift valley.

Ocean Basin Formation: Over millions of years, continued rifting and seafloor spreading can widen the rift valley and eventually lead to the formation of a new ocean basin.

Breakup: With continued divergence, the thinned continental crust eventually ruptures, and a new ocean basin begins to form.

Examples: The East African Rift System is a prominent example of an active continental rift. It stretches for thousands of kilometers across eastern Africa, with deep valleys, volcanic activity, and geothermal features marking the zone of divergence. The Red Sea Rift is another well-known example, where the Arabian Plate is slowly separating from the African Plate.

Rift valley in southwest Iceland is indeed part of the boundary between the Eurasian and North American continental tectonic plates.

Geological Features of Rifting

Grabens: Keystone-shaped blocks of down-dropped crust within rift valleys.

Horsts: The uplifted shoulders of grabens.

Half-Grabens: When only one side of a section drops

Outcomes of Rifting

Lakes and Oceans: Depending on the extent of rifting, these valleys can become large lakes or even new oceans.

Divergent plate boundaries features: Aulacogens, Triple Junctions, Half-Grabens.

Failed Rifts and Aulacogens

Failed Rift Arms

These are zones where continental rifting began but did not progress to complete continental breakup and seafloor spreading. They form when the tensional forces separating the plates weaken and stop acting, leaving behind a stretched and thinned region in the continent.

Example: The West European Rift System

Aulacogens

Aulacogens are a specific type of failed rift. It's a broader depression formed by a failed rift arm that has undergone further subsidence (sinking) and erosion.

These depressions are often filled with sediments eroded from the surrounding uplifted areas.

Aulacogens can be identified by their distinct geophysical signatures, like gravity anomalies caused by denser rock beneath the depression.

They can be important sources of hydrocarbons (oil and natural gas) because the organic matter deposited in these basins gets buried and cooked by the heat from the Earth's interior.

The Mississippi Embayment mentioned as a failed rift is also an example of an aulacogen.

Multiple Rifting and Triple Junctions

Multiple Rifting

This occurs when interconnected rift arms develop concurrently, creating multiple boundaries of active rifting along a continent. These rifts can be parallel or radiate from a central point.

These rift arms may eventually fail or evolve into successful rifts with ocean formation, depending on the ongoing tectonic forces.

The East African Rift System is a prominent example of multiple rifting, where the African Plate is being pulled apart along several branches.

Triple Junctions

When three tectonic plates meet at a divergent boundary (where plates move apart), a triple junction is formed. In some cases, all three arms of a triple junction can be active rift zones, leading to a very concentrated area of extension and volcanic activity.

Triple junctions can play a significant role in shaping continental margins and influencing the distribution of volcanic activity and earthquakes.

The Afar Triangle in East Africa is a well-known example of a triple junction with three active rift arms.

Divergent Boundary diagram

Mid-oceanic Ridges

Mid-ocean ridges are massive underwater mountain ranges located along the center of divergent boundaries. They are formed by the continuous upwelling of hot, buoyant mantle material as the tectonic plates diverge.

Formation: As the plates spread apart at a divergent boundary, the decompression melting of mantle rock occurs. This molten rock, being less dense than the surrounding solid mantle, rises towards the surface as magma. This ongoing volcanic activity erupts onto the seafloor, creating new oceanic crust and causing the mid-ocean ridge to spread further. The Mid-Atlantic Ridge is a prominent example of a mid-ocean ridge.

Key features of mid-oceanic ridges

Divergent Plate Boundary: Mid-ocean ridges mark the boundaries between diverging tectonic plates. As the plates move away from each other, the upwelling of mantle material creates a long, continuous mountain range at the seafloor.

Seafloor Spreading: The seafloor on either side of a mid-ocean ridge is constantly moving away from the ridge due to seafloor spreading. This process is driven by convection currents in the Earth's mantle.

Magma and Volcanic Activity: The upwelling of mantle material at mid-ocean ridges is accompanied by decompression melting, where the rising mantle rock partially melts due to the pressure decrease. This molten rock, or magma, erupts on the seafloor, forming new oceanic crust and contributing to the growth of the ocean basins.

Hydrothermal Vents: Hydrothermal vents are common features along mid-ocean ridges.

Examples: The Mid-Atlantic Ridge, spanning nearly the entire length of the Atlantic Ocean, is the longest mountain range on Earth. The East Pacific Rise is another major mid-ocean ridge system, winding its way through the Pacific Ocean floor.

Hydrothermal Vents

Hydrothermal vents are underwater fissures on the seabed that spew out superheated, mineral-rich water. They are essentially underwater hot springs, and form around areas where tectonic plates are moving apart, like mid-ocean ridges, or near areas of volcanic activity. These vents are often found at depths of 2,000 to 4,000 meters (6,500 to 13,000 ft) below the sea surface, and can reach temperatures exceeding 400 °C (752 °F).

Hydrothermal vents form when seawater seeps down through cracks in the Earth's crust and comes into contact with hot magma. The hot magma heats the water, which then dissolves minerals from the surrounding rock. This mineral-rich water is less dense than the surrounding seawater, and so it rises back up to the seafloor through vents.

Hydrothermal vents were first discovered in 1977 and have since been found in all the world's oceans.

There are two main types of hydrothermal vents:

Black smokers: These vents spew out extremely hot, black, mineral-rich fluid, often reaching temperatures exceeding 400°C (752°F). The black color comes from the presence of metal sulfides, such as iron sulfide (FeS), which precipitates as the hot water cools.

White smokers: These vents emit cooler, less acidic fluids, typically around 350°C (662°F). The white color is due to the presence of lighter-colored minerals, such as barium sulfate (BaSO₄) and calcium sulfate (CaSO₄).

Earthquakes and Volcanism

Earthquakes: Earthquakes are another common feature of divergent boundaries, particularly in areas where there is significant movement of tectonic plates. As the plates pull apart, the stress can cause fractures and faults within the crust, leading to seismic activity. These earthquakes are typically shallow and relatively low in magnitude compared to those associated with other types of plate boundaries.

Volcanism: Volcanic activity is common along divergent boundaries, especially at mid-ocean ridges. As plates pull apart, magma from the mantle rises to fill the gap, leading to volcanic eruptions. The magma solidifies upon reaching the surface, forming new oceanic crust. The volcanic activity associated with divergent boundaries is often characterized by basaltic lava flows and the formation of volcanic cones.

Divergent boundaries shown as red lines

Divergent Boundary Examples

Mid-Atlantic Ridge: This is the longest mountain range on Earth, stretching for about 60,000 kilometers (37,000 miles) along the floor of the Atlantic Ocean. The ridge is constantly being created as the North American and Eurasian plates move apart.

East Pacific Rise: This is another major mid-ocean ridge located in the Pacific Ocean. It runs along the west coast of North and South America and separates the Pacific Plate from the Nazca Plate and the Cocos Plate.

Red Sea Rift: This is a divergent boundary located between the Arabian Plate and the African Plate. It is an example of a continental rift valley that is in the early stages of development. The Red Sea is expected to widen and eventually become a new ocean basin.

East African Rift: This is another continental rift valley where the African Plate is slowly splitting into two pieces. The valley is home to numerous volcanoes and lakes, and it is expected to eventually become a new ocean basin.

Conclusion

In summation, divergent boundaries epitomize a fundamental aspect of plate tectonics, delineating regions where lithospheric plates undergo separation. Characterized by features such as mid-ocean ridges, rift valleys, volcanic activity, earthquakes, and hydrothermal vents, divergent boundaries exemplify the dynamic interplay between geological processes and the Earth's lithospheric structure. Through their manifestations, divergent boundaries contribute significantly to the ongoing evolution of Earth's crust and the broader geodynamic framework of our planet.