Divergent plate boundaries represent zones where the Earth’s tectonic plates pull away from each other. This separation results in the formation of new crustal material.
The Dynamics of Divergent Boundaries
At divergent boundaries, tectonic plates experience tensional stress, causing them to stretch and pull apart. This stretching thins the Earth’s lithosphere, allowing hot mantle material to rise to the surface. As the mantle rock rises, the pressure on it decreases, leading to a process called decompression melting. This melting produces basaltic magma, which is less dense than the surrounding rock and ascends to fill the void created by the separating plates.
The magma cools and solidifies, adding new crustal material between the diverging plates. This continuous process, known as seafloor spreading, creates new oceanic lithosphere. The rate at which plates diverge varies, ranging from 1 to 20 centimeters per year. This ongoing formation of new crust at divergent zones drives the expansion of ocean basins and the movement of continents.
Prominent Landforms Created
Divergent plate boundaries are responsible for distinct geological features in oceanic and continental settings. In the ocean, mid-ocean ridges are prominent landforms: vast submarine mountain ranges stretching for tens of thousands of kilometers across the ocean floor. These ridges are elevated because the newly formed crust is hotter and less dense, causing it to sit higher on the mantle. A central rift valley, typically 25-50 kilometers wide and about 1 kilometer deep, often runs along the crest of these ridges, marking the point of divergence.
On continents, divergent boundaries initiate the formation of continental rift valleys. Here, the continental lithosphere stretches and thins, developing fractures and faults. As rifting progresses, the land between these faults subsides, forming elongated valleys. Continued divergence can lead to the continental crust becoming so thin that new oceanic crust begins to form, eventually creating a new ocean basin.
Volcanic and Seismic Manifestations
Divergent boundaries are characterized by volcanic and seismic activity. Magma continuously rises to the surface, primarily forming basaltic lava flows. In oceanic settings, this lava often erupts underwater, rapidly cooling to form rounded masses known as pillow lavas. Volcanism at these boundaries is typically effusive, meaning the lava flows readily rather than erupting explosively, due to the magma’s low silica content and low viscosity, which allows volcanic gases to escape easily.
Seismic activity at divergent boundaries consists of shallow earthquakes. These quakes occur as the brittle lithosphere fractures and adjusts to the tensional forces of the separating plates. While common, the earthquakes along mid-ocean ridges are less intense compared to those at convergent boundaries. Ecosystems thrive around hydrothermal vents, often called “black smokers,” found along mid-ocean ridges. These vents release superheated, mineral-rich fluids that support chemosynthetic life forms, independent of sunlight.
Real-World Examples
Several locations around the world showcase the features of divergent plate boundaries. The Mid-Atlantic Ridge is an example of an oceanic divergent boundary, extending approximately 16,000 kilometers from the Arctic Ocean to beyond the southern tip of Africa. Here, the North American and Eurasian plates, along with the South American and African plates, are pulling apart at an average rate of about 2.5 centimeters per year. This separation led to the formation of the Atlantic Ocean over millions of years.
On land, the East African Rift Valley illustrates a continental divergent boundary. This system of valleys and volcanoes stretches for thousands of kilometers, indicating where the African Plate is actively splitting into two. Iceland offers an example where a segment of the Mid-Atlantic Ridge emerges above sea level. This makes it one of the few places where seafloor spreading and associated volcanic activity can be observed directly on land.