When two tectonic plates move away from each other, a geological feature known as a divergent boundary forms. These areas represent locations where Earth’s lithosphere is actively separating. This pulling apart allows new material from the mantle to rise and generate fresh crustal rock.
The Dynamics of Plate Separation
The process of plate separation at a divergent boundary is driven by the upwelling of magma from Earth’s mantle. This hot material rises towards the surface, creating tensional forces that pull the plates apart. As the mantle material ascends, the pressure on it decreases, leading to decompression melting. This melting generates basaltic magma, which is less dense than the surrounding solid rock and thus buoyant.
Upward flow of magma pushes the plates away from the spreading center. The process is referred to as seafloor spreading when it occurs in oceanic settings. New crustal material is formed as the magma solidifies, widening the ocean basin or continental rift. This mechanism explains how continents move apart over geological timescales, shaping Earth’s geography.
Formation of New Crust and Topography
Divergent plate movement leads to the formation of geological features. In oceanic environments, upwelling magma and seafloor spreading create vast underwater mountain ranges known as mid-ocean ridges. These ridges are about 2,000 meters high and can span thousands of kilometers, forming the longest mountain chain on Earth. A central rift valley, 25 to 50 kilometers wide and up to 1 kilometer deep, runs along the crest of these ridges, marking the precise location where the plates are pulling apart.
New oceanic crust is generated at these ridges, and its age progressively increases with distance from the spreading center. When divergent boundaries occur within continents, they form continental rift valleys. As the continental lithosphere stretches and thins, it breaks along normal faults, leading to sections of crust dropping down to create these valleys.
Volcanic and Seismic Activity
Divergent boundaries are regions of geological activity, including both volcanism and earthquakes. The rise of magma from the mantle results in frequent volcanic eruptions. This volcanism is effusive and produces basaltic rock. Under the ocean, this rapidly cooling lava forms pillow basalts. Mid-ocean ridges, where most oceanic divergence occurs, account for a portion of Earth’s volcanism, generating more lava than all other types of volcanic activity combined.
Earthquakes are also common at divergent plate boundaries, though they are shallow and of lower magnitude compared to those at other plate boundary types. These seismic events occur as the plates pull apart, causing the newly formed crust to fracture and shift. The brittle nature of the thin lithosphere at these spreading centers, combined with tensional forces, leads to frequent, less powerful tremors.
Global Manifestations
Examples of divergent boundaries can be observed across the globe, showcasing both oceanic and continental rifting. The Mid-Atlantic Ridge is an oceanic divergent boundary that bisects the Atlantic Ocean, separating the North American and Eurasian plates in the north, and the African and South American plates in the south. This vast undersea mountain range generates new oceanic crust, causing the Atlantic Ocean to widen at a rate of 2 to 5 centimeters per year.
On land, the East African Rift Valley provides an illustration of continental divergence. Here, the African plate is splitting into two new plates, the Somalian and Nubian plates, at a rate of 6-7 millimeters per year. This extensive rift system, which includes the Afar triple junction in Ethiopia, is characterized by numerous lakes, active volcanism, and a series of subsiding valleys. Given enough geological time, this continental rift could evolve into a new ocean basin.