A rift valley is a large, linear depression in the Earth’s crust that forms as the land slowly pulls apart. These geological features are characterized by steep, parallel walls and a flat floor, sometimes filled by lakes or sedimentary deposits. Understanding their formation requires looking at the movement of massive crustal plates and the specific boundary where this separation occurs.
Divergent Plate Boundaries
The type of tectonic setting responsible for forming rift valleys is the divergent plate boundary, where two lithospheric plates move away from each other. This motion generates extensional stress, or a pulling force, within the Earth’s rigid outer layer. Divergent boundaries contrast with convergent boundaries, where plates collide, and transform boundaries, where plates slide past one another.
The pulling action of divergence creates the necessary conditions for the crust to stretch and ultimately fracture. This process, known as rifting, is the initial stage of continental breakup. As the plates separate, the lithosphere thins and weakens, setting the stage for the collapse that defines a rift valley.
The Mechanism of Continental Rifting
The process of continental rifting begins when heat from the underlying mantle causes the crust to dome upward and thin, a process called attenuation. This weakening makes the lithosphere susceptible to the tensional forces acting horizontally across the region. The stretching of the brittle upper crust causes it to fracture along a series of faults.
These fractures are known as normal faults, where the hanging wall block moves downward relative to the footwall block due to the pulling-apart motion. As extension continues, the central block of crust drops down between two parallel normal faults, creating the characteristic valley floor. This depressed block is called a graben, which is the geological term for the rift valley itself.
The crustal blocks on either side of the sunken valley remain uplifted, forming elevated ridges or plateaus known as horsts. The resulting topography is an alternating pattern of lowered valleys (grabens) and raised ridges (horsts). The vertical displacement along these faults can be thousands of meters, creating the steep walls that define the valley. Rift valleys are typically zones of high seismic activity and often include volcanism, as magma can rise more easily through the thinned crust.
Major Global Rift Systems
The most recognized example of an active continental rift is the East African Rift System (EARS). This immense feature extends for thousands of miles, splitting the African Plate into the smaller Somali and Nubian plates. The EARS is a textbook example of continental breakup in progress, expected to result in the formation of a new ocean basin millions of years in the future.
Another prominent example is the Rio Grande Rift, which cuts through the southwestern United States, primarily in New Mexico and Colorado. Other noteworthy continental rifts include the Baikal Rift Zone in Siberia, which contains the world’s deepest continental rift lake, and the Basin and Range Province in western North America. These active systems showcase the ongoing process of continental extension and fragmentation.