A rift valley is a linear zone of lowland where the Earth’s lithosphere—its rigid outer layer—is actively being pulled apart. This geological structure is a depression created by the sustained separation and thinning of the crust, often stretching for hundreds to thousands of miles across a continent or the ocean floor. Rift valleys are expressions of extensional tectonics, distinct from valleys carved by rivers or glaciers because their origin lies entirely in the deep movements of the Earth’s crust.
Tectonic Forces Driving Rift Formation
Rift valleys originate in areas subjected to extensional stress, where tectonic plates are moving away from one another at a divergent boundary. This pulling motion, known as tensional force, causes the Earth’s rigid lithosphere to stretch and thin. As the crust stretches, it weakens and fractures along a series of normal faults that are approximately parallel to the direction of the stress.
The normal fault is a specific type of break where the overlying block of crust moves downward relative to the block beneath it. Tensional forces cause these faults to form in pairs, dipping toward each other, which allows the central block of crust to drop down. This sunken block, called a graben, creates the trough-like depression characteristic of a rift valley.
The continued separation and thinning of the lithosphere can cause the underlying hot mantle material to rise closer to the surface. This change in pressure and temperature often leads to melting, resulting in associated volcanic activity and magma intrusion into the crust. If the rifting process continues for tens of millions of years, the continental crust may completely rupture, leading to the formation of a new ocean basin with seafloor spreading.
The Physical Landscape of a Rift Valley
The visible landscape of a rift valley is defined by the results of crustal stretching and subsidence. The most prominent feature is the deep, elongated basin of the valley floor, which is the graben structure. This depressed block can be tens of miles wide and thousands of miles long, subsiding vertically by thousands of feet relative to the surrounding land.
Flanking the graben are the uplifted blocks of crust known as horsts, which form high plateaus or mountain ranges on either side of the valley. The boundary between the graben and the horsts is marked by steep, cliff-like features called fault scarps. These scarps are the exposed faces of the normal faults, clearly outlining the boundaries of the sunken valley floor.
The low-lying nature of the graben often allows for the accumulation of water, leading to the formation of long, narrow, and deep bodies of water known as ribbon lakes. Lake Tanganyika and Lake Malawi are examples of these rift lakes, contained within the fault-bounded depressions. Additionally, the proximity of the mantle to the thinned crust often results in geothermal activity, such as hot springs and geysers along the valley floor.
Major Rift Systems Around the World
Rift systems are found globally, representing different stages of the crustal separation process. The most significant example of an active continental rift is the East African Rift System (EARS), where the African plate is slowly splitting into the Nubian and Somalian plates. This vast system extends over 3,700 miles, from the Afar Triple Junction in the north down toward Mozambique in the south.
The EARS is characterized by two main branches, the Eastern and Western Rifts, which contain numerous lakes and active volcanoes like Mount Kilimanjaro and Mount Kenya. If this process is sustained, the continent will eventually break apart and form a new ocean. Another notable continental rift is the Rio Grande Rift in the southwestern United States, extending through New Mexico and Colorado.
The largest and most extensive rift system globally is the Mid-Oceanic Ridge, which represents oceanic rifting where new seafloor is continuously created. This underwater mountain range stretches for over 40,000 miles around the Earth, with the Mid-Atlantic Ridge being a well-known segment. The rift valley sits along the crest of the ridge, marking the boundary where two oceanic plates diverge and magma rises to form new crust.