A divergent boundary is a linear feature where two tectonic plates move away from each other. This process is sometimes called a constructive or extensional boundary because it creates a gap in the Earth’s lithosphere. The movement is driven by the slow, convective flow of heat and material within the mantle. This continuous pulling motion causes the crust to fracture and thin.
How New Crust is Formed
New crust formation begins with the upwelling of hot, solid rock from the underlying mantle (asthenosphere). As this rock rises, the pressure decreases significantly, which lowers the melting point—a process called decompression melting. This partial melting generates fluid basaltic magma.
This newly formed, less dense magma rises to fill the void created by the separating plates. The continuous injection of this molten rock into the fracture zone drives seafloor spreading. As the magma cools and solidifies, it forms new oceanic lithosphere, composed primarily of basalt and gabbro.
New crust forms symmetrically on both sides of the boundary, pushing older crust away from the spreading center. This means the crustal material is youngest directly at the boundary and progressively older further away. The total amount of lava produced at these deep-sea centers is substantial, exceeding the volume of all other types of volcanism combined.
Creation of Major Global Landforms
Divergent boundaries create two distinct geographical features, depending on whether separation occurs under the ocean or within a continent. Most active divergent boundaries are found beneath the ocean, forming Mid-Ocean Ridges. These ridges are massive underwater mountain ranges, representing the longest mountain chain on Earth, winding for over 65,000 kilometers globally.
The ridges are elevated because the newly formed crust is hot and less dense than the older, cooler crust that has moved away. A central rift valley runs down the center of the ridge crest, marking the exact line of plate separation. This valley is 25 to 50 kilometers wide and up to a kilometer deep. The Mid-Atlantic Ridge is a prime example, where the North American and Eurasian plates are slowly moving apart.
When divergence begins within a continental landmass, it forms a Continental Rift Valley, representing an early stage of separation. Tensional forces stretch and thin the continental crust, causing large blocks to drop down along faults. This faulting creates distinctive structures: down-dropped blocks (grabens) form the rift valley floor, flanked by uplifted blocks (horsts). The East African Rift Valley is an active example that, if rifting continues, will eventually thin the crust enough for an ocean basin to form.
Dynamic Geological Activity
Plate separation at a divergent boundary is accompanied by specific types of volcanic and seismic activity. The volcanism is effusive, meaning the lava is runny, basaltic, and flows easily rather than erupting explosively. Underwater, this magma rapidly cools upon contact with seawater, often forming rounded shapes known as pillow lavas.
On land, such as in Iceland where the ridge crest is exposed, eruptions often occur along long fractures known as fissure eruptions. The tectonic stress from the plates pulling apart also results in frequent seismic activity. However, the earthquakes are typically shallow and low in magnitude.
These mild earthquakes are caused by the brittle fracturing and tearing of the crust under tension. They occur along normal faults, where one block of crust slides down relative to the other. This contrasts with the deep, powerful earthquakes associated with compressional forces at converging plate margins. The combination of low-magnitude earthquakes and steady volcanism defines the dynamic environment of a spreading center.