Divergent plate boundaries are areas of the Earth’s lithosphere where tectonic plates slowly move away from each other. This continuous separation leads to the generation of new crustal material. They are distinct from convergent boundaries, where plates collide, and transform boundaries, where plates slide past one another. The constant pulling apart at divergent boundaries drives significant geological activity, creating unique topographical features both on continents and beneath the oceans.
The Forces Driving Plate Separation
The movement of tectonic plates at divergent boundaries is primarily driven by internal forces within the Earth. Mantle convection is a fundamental mechanism, where heat from the Earth’s core causes molten rock within the mantle to circulate. Hot, less dense mantle material rises towards the surface, while cooler, denser material sinks, creating slow-moving convection currents. This rising material exerts an upward force, helping to push the overlying plates apart and contributing to the initiation of rifting.
Another significant force contributing to plate separation is “ridge push.” At mid-ocean ridges, continuous upwelling of magma creates new, hot, and buoyant oceanic crust. This newly formed crust is elevated compared to the older, colder crust further away from the ridge. Gravity then causes this elevated oceanic lithosphere to slide down and away from the ridge crest, pushing the plates apart. This gravitational sliding, combined with mantle convection, drives the continuous movement and separation at divergent boundaries, as new seafloor forms when melt rises as magma and emerges as lava, creating new oceanic crust upon cooling.
Landforms and Geological Activity
Divergent plate boundaries are characterized by distinct landforms and geological activities. Underwater, mid-ocean ridges are prominent features, forming extensive submarine mountain ranges. These ridges form as magma rises from the mantle to fill the gap created by separating plates, solidifying to create new oceanic crust. This process, known as seafloor spreading, continuously adds new material to the ocean floor, causing oceanic plates to grow and move away from the ridge axis. The rate of spreading can influence the ridge’s shape; slower spreading rates result in steeper, more irregular topography, while faster rates produce wider, more gentle slopes.
On continents, divergent boundaries form rift valleys. These linear lowlands are created as continental crust stretches and thins, causing land to subside between faults. As rifting progresses, magma can rise to the surface, leading to volcanic activity. Volcanism at divergent boundaries is common due to decompression melting of the mantle as plates pull apart, producing typically basaltic magma that results in effusive lava flows rather than explosive eruptions. Earthquakes are also common along divergent boundaries, generally occurring at shallow depths and being relatively low in magnitude compared to those at other plate boundaries, as a direct result of tensional stress and faulting as the plates pull apart.
Where Divergent Boundaries Occur
Divergent plate boundaries are found in various locations across the globe. The most extensive examples are beneath the oceans, forming vast mid-ocean ridge systems. The Mid-Atlantic Ridge is a prime example, running down the center of the Atlantic Ocean and separating the North American and Eurasian plates, as well as the South American and African plates. This ridge is an immensely long underwater mountain chain, approximately 16,000 km (10,000 miles) in length, and is characterized by a deep rift valley along its crest where new crust is actively forming. The Atlantic Ocean is steadily widening due to the continuous seafloor spreading at this boundary, at a rate of approximately 2.5 centimeters (1 inch) per year.
On land, divergent boundaries manifest as continental rift zones. The East African Rift Valley is a prominent example where the African Plate is gradually splitting into two new plates, the Somalian and Nubian plates. This system extends for over 6,000 kilometers (3,700 miles) and is characterized by elongate valleys, volcanic activity, and shallow earthquakes. The rifting process in East Africa is occurring at a rate of about 6–7 millimeters (0.24–0.28 inches) per year, and if it continues, it could eventually lead to the formation of a new ocean basin. Other continental rift zones include the Rio Grande Rift in North America and the Baikal Rift in Russia.