The Earth’s outer shell consists of large, rigid pieces known as tectonic plates. These large slabs of rock constantly move across the planet’s surface. Most plate boundaries, where these pieces interact, are hidden beneath the oceans. Divergent plate boundaries are one type of interaction, characterized by plates moving away from each other.
How They Form
Divergent plate boundaries arise where two tectonic plates pull apart. This separation is driven by the slow motion of Earth’s mantle, known as convection currents. Heat from the planet’s interior causes mantle material to warm, become less dense, and rise towards the surface. As this material ascends, it creates an upward flow that exerts tensional forces on the overlying lithospheric plates.
This tension causes the Earth’s crust to stretch and thin, leading to the formation of cracks. Magma from the underlying mantle then rises to fill these gaps. As the magma reaches the surface, it cools and solidifies, creating new crustal material. This process of new crust formation and outward plate movement is known as seafloor spreading.
Seafloor spreading expands ocean basins. The newly formed crust is hot and less dense, contributing to the elevated topography at these boundaries. As the new crust moves away from the spreading center, it cools and becomes denser, causing it to sink to lower elevations. This balances crust destroyed at other plate boundaries, ensuring Earth’s total surface area remains relatively constant.
Landforms and Events
Divergent plate boundaries are responsible for distinct features. In oceanic settings, upwelling magma and spreading create extensive underwater mountain ranges known as mid-ocean ridges. These vast systems represent the longest mountain ranges on Earth, stretching for tens of thousands of kilometers.
A prominent feature along the crest of many mid-ocean ridges is a central rift valley. This valley forms as the crust pulls apart and sections subside. In continental settings, where plates diverge within a landmass, the stretching and thinning of the crust can form rift valleys. These are long, narrow depressions, often marking the initial stages of a continent breaking apart.
Volcanic activity is common at divergent boundaries, characterized by effusive basaltic eruptions. Magma rises through the weakened crust, leading to less explosive eruptions compared to other boundary types due to its low silica content and high fluidity. This volcanism often manifests as fissure eruptions, where lava flows out of long cracks, or as pillow lavas when it erupts underwater.
Earthquakes also occur frequently at divergent plate boundaries. They are generally shallow and less powerful than those at other boundary types. These seismic events result from the stretching and fracturing of the crust as the plates pull apart. While some earthquakes occur directly along spreading segments, many are concentrated along transform faults that offset mid-ocean ridge segments.
Major Locations
The Mid-Atlantic Ridge is a key example of an oceanic divergent plate boundary. This submerged mountain range extends from the Arctic Ocean to beyond the southern tip of Africa, bisecting the Atlantic Ocean. It separates the North American Plate from the Eurasian Plate in the North Atlantic, and the South American Plate from the African Plate in the South Atlantic.
The plates along the Mid-Atlantic Ridge are moving apart at a slow rate. This slow spreading rate contributes to a deep rift valley along its crest, comparable in depth and width to the Grand Canyon. In some locations, such as Iceland, the Mid-Atlantic Ridge rises above sea level, providing a unique opportunity to observe a divergent boundary on land.
Another example of a divergent boundary is the East African Rift Valley, representing continental divergence. Here, the African Plate is slowly splitting into two smaller plates: the Somalian Plate and the Nubian Plate. This ongoing rifting process has created a vast system of interconnected valleys, lakes, and volcanoes across eastern Africa.
The East African Rift Valley is characterized by active volcanism and shallow earthquakes. If rifting continues over geological timescales, it could eventually lead to the formation of a new ocean basin, similar to how the Atlantic Ocean formed. The Red Sea, between the Arabian Plate and the African Plate, represents a more advanced stage of continental rifting expected to widen into a new ocean.