African Rift Valley System

The African rift valley system is characterized by its elongated basins, which cut across a region dotted with domes that extend from South Africa to the Red Sea. Most of the region's lakes are located in the rift basin.

Rift Zone Characteristics

A continental rift is a linear topographic depression that may develop into an ocean as the bounding regions drift into two separate continental fragments. The African rift valley system is one of several continental rifts. Others include the Rio Grande, the Baikal, and the Rhine Graben. It is a long system that extends from South Africa to the Red Sea coast.

The African rifts are marked by depressions that cut across domes, such as the East African and the Afro-Arabian domes. The East African dome encompasses parts of Tanzania, Uganda, and Kenya and is dissected by two rift branches. The eastern branch is discontinuously traceable to the Main Ethiopian Rift, which is one of three rifts of the Afro-Arabian dome. The other two rifts are the Red Sea and the Gulf of Aden. The three rift arms of the Afro-Arabian dome meet in a triangular depression, the Afar. The central structures of the three rifts overlap in the Serdo Block, a roughly square area of about ninety kilometers per side.

The rift basin margins may be distinct and marked by cliffs in some places. At Dalol, about 100 kilometers north of the Serdo Block, the rift floor is about 3,000 meters below the rift rim, and there is a 5,000-meter-thick layer of salt that was deposited in the last four million years. Since salt forms at sea level, the Afar floor must have been slowly sinking (subsiding) about five kilometers in four million years. Clearly, the rift floors have formed by subsidence relative to the rim.

Subsidence occurs as the rock block on top of an inclined fracture surface slides downward, a movement that geologists call normal faulting. Rift basins are formed by a series of normal fault movements that generally produce grabens and half-grabens. A half-graben consists of an arcuate ridge that bounds a depression and is formed by normal faulting; it is the basic building unit of continental rifts. A typical half-graben is about 100 kilometers long and less than four kilometers wide. Half-grabens might be arranged facing or opposite or staggered in the rift basins. The thickness of the rock layer (lithosphere) at the rift basins ranges from twenty-one to thirty kilometers, less than one-half its thickness elsewhere on the continent.

Rift Volcanoes

Geologists believe the lithosphere lies above a partially molten layer, the asthenosphere. The melt, also known as magma, rises from the asthenosphere through fractures and flows to form layered basalt, or it oozes to form volcanic domes and cones, or it is ejected explosively to spray volcanic ash and fragmental rocks called pyroclastics. Some volcanic rocks, known as carbonatites, are formed from magmas that originate at great depths and that contain abundant carbon dioxide. Also, from deep within the mantle, sodium- and potassium-rich magma forms volcanic rocks termed alkaline igneous rocks. However, the dominant igneous rocks of the rift basins are tholeiites, which are comparatively rich in magnesium and the magma of which originated at comparatively shallow depths. Some of the rift volcanoes issue substantial quantities of volcanic gas. Volcanic gases and steam (hydrothermal fluids) also seep through fractures unrelated to volcanoes.

Igneous and volcanic activity is not restricted to rift basins. Huge volcanic edifices are formed outside the rift basins. A string of such volcanic structures is found on either side of the rift rims. Examples of such volcanoes include Mount Kilimanjaro in Tanzania and Ras Dashen in Ethiopia. The rift-related volcanic activity started some twenty-three million years ago outside the rift, although most of it has been limited to the rift basin in the last six million years.

In addition to igneous rocks, the rift basin is covered by sediments and sedimentary rocks. At cliffed rift basins, boulders, cobbles, gravel, and granules accumulate at the bottoms of the cliffs. Rivers descend into rift basins and wind along the rift until they empty into lakes. Such rivers transport detrital sediments such as sand, silt, and mud deposited within the river channel, overbank floodplains, and lakes. In addition to detrital sediments, evaporation of rift lakes produces chemical sediments such as salt and gypsum. Rift basins, particularly the parts close to the seas, can be flooded by the sea. Before volcanic rocks isolated it, the northern part of the Afar was covered by a shallow sea. Evaporation of that sea yielded copious amounts of salt deposits and underwater volcanic flows. Thus, rift deposits include chemical and detrital sedimentary rocks that may be interbedded with volcanic flows or pyroclastics.

Theory

Geologists have suggested that the origin of the rifts can be explained by plate tectonics theory, according to which the lithosphere is segmented into discrete plates. The plates move, and the boundary type of the plates is identified by the direction of movement of neighboring plates. In divergent plate boundaries, neighboring plates move away from each other. In transform boundaries, neighboring plates slide alongside each other. In convergent boundaries, the plates collide with each other.

The movement of the plates is guided by convection within the asthenosphere, which in turn results from the movement of molten material to equalize the temperature within the asthenosphere. Hot molten material from the deeper part of the asthenosphere rises, pushes on the lithosphere above, and then diverges beneath the lithosphere. The lithosphere is carried along by the diverging asthenosphere like luggage on a conveyor belt. Geologists believe that the African rift system is a divergent plate boundary at which Africa is tearing apart. According to this bulge-rift model, the rift borders are elevated from the surrounding areas because of the initial bulge before the rifting process was established. The discrete domes along the rift would then indicate areas at which randomly rising deeper and hot asthenosphere encountered the lithosphere, which was expanded and buoyed because of the heat.

The Afro-Arabian dome, with its higher elevation, shape, and triple rifts, has attracted the attention of many geologists. Much as the crust of a pie placed in an oven would bulge up and form fractures, the rifts of the Afro-Arabian dome are considered to have formed as a result of the heat of the ascending plume. This theory is proposed to explain why triple rifts are present in many places before a continent is split apart and an ocean fills the gap. According to this dome-rift model, the Red Sea and Gulf of Aden, now occupied by seas, have begun to open up as Saudi Arabia is splitting and drifting away from Africa, whereas the third rift, the Main Ethiopian Rift, is not. While the dome-rift model requires that the Main Ethiopian Rift be a failed arm of a rift, evidence shows that it is a divergent boundary with a spreading rate of about one centimeter per year. Moreover, the exact triple point, the Serdo Block, is on land. The Serdo Block is linked by a line of volcanoes to the Red Sea and the Gulf of Aden, which mark plate boundaries between the Saudi Arabian plate to the north, the Ethiopian plate to the southwest, and the Somalia plate to the southeast.

The central rift system of the Gulf of Aden is connected to a mid-oceanic rift system, the Carlsberg Ridge, in the Indian Ocean southeast of Saudi Arabia. Scientists have found that the average age of igneous rocks along the central Gulf of Aden Rift is progressively younger toward the Serdo Block. They suggest that the origin of the Gulf of Eden Rift is to be explained by a rift-propagation model. Under this model, a rifting, once begun somewhere, will move laterally. According to geologists, the submarine Carlsberg Ridge might have initiated the “burn” of the African continent on the east side of where the Gulf of Aden is currently located, and that rift has propagated toward the Serdo Block.

In the northern Red Sea, geologists have found that sedimentary rift deposits contain older rock particles in younger strata. It is as though initially young rocks from a source area were eroded, and their particles were deposited in the rift. Subsequently, the rim of the rift was uplifted, older rocks were exposed in the source area, and their particles were deposited in the rift basin atop sedimentary layers that contain younger rock particles. Geologists propose that rifting probably arose because the Arabian plate collided with Eurasia to form the Zagros Mountains in the north and that the lithosphere is tearing apart to form the Red Sea as a consequence. The comparatively thin lithosphere at the Red Sea is then heated by the asthenosphere and made buoyant so that the rift borders rise to higher elevations. This is called the rift-bulge model.

Study of the African Rift System

Scientists have used a diverse set of instruments to study the different aspects of the African rift system. Aspects studied include the shape and structure of the landscape, the thickness of the lithosphere, the magnetic properties of the rocks, and the mineral and chemical composition of the rocks.

Ground surveys using various instruments are taken from different ground stations until a whole region is surveyed. The data can then be used to produce contour maps showing the landscape on two-dimensional paper. Mapping of remote areas awaited the use of aerial photographs. Overlapping photographs of a region are taken by cameras mounted on airplanes that fly along parallel lines. The overlapping photographs are then viewed under stereoscopes with suitably mounted mirrors and lenses that permit viewing the region in its three dimensions. The locations and heights of the ground are measured from the photographs. Topographic maps are made from these measurements after checking some of them by actual field examination, a process also called ground truing.

Satellite images of various sorts help delineate structures and textures. Energy-sensing devices mounted on satellites are used to detect the energy that is emitted from the ground. The type of emitted energy is identified by its wavelength, and the data are converted into numbers, or digitized. The digitized data and the coordinates of the source region are transmitted to receivers on the ground. Maps produced from these data are enhanced by false colors or shading and patterning to highlight particular features. Linear arrangements of volcanic cones and their relationships to other structures, the presence of major fault zones, and other features across a region can be identified from satellite images.

A primary instrument used to study the thickness of the lithosphere is the gravimeter, which is a mass suspended by a spring and encased in a suitable container. The attraction between the mass in the gravimeter and the earth helps determine the gravity values of a region. Since rocks have lower densities than the asthenosphere, a thick lithosphere has a lower gravity value than a thin lithosphere. Scientists have found that the rift basins have high gravity values, and they have used these values in conjunction with suitably devised models to estimate the thickness of lithospheres. The African continent has a lithosphere generally seventy kilometers thick, but in the African rift basins the lithosphere is between thirty and twenty-one kilometers thick, the thinnest part being in the Afar.

Principal Terms

asthenosphere: the layer of the earth that lies beneath the lithosphere and is partly composed of melt

carbonatite: an igneous rock with abundant carbon in its makeup

cinder cone: a cone-shaped mound made of volcanic granules

fault: a fracture in a rock associated with rock movement or sliding

graben: a linear topographic depression caused by subsidence along faults

half-graben: a structural element by which a rift system is formed, consisting of an arcuate ridge that bounds a depression and is formed by normal faulting

hydrothermal fluid: a natural hot steam that seeps through the ground

lithosphere: the top rock layer of the earth, ranging from seventy kilometers in depth in the African continent to twenty-one kilometers in the African rift

magma: a melt from which igneous rocks are formed

normal fault: a fault in which the rock block on top of an inclined fracture surface, also known as a fault plane, slides downward

pyroclastics: fragmentary igneous rocks that are formed by the forceful ejection of volcanic materials into the atmosphere

rift propagation: the lateral movement of a rifting process that leads to the prying open of a section of the lithosphere, accompanied by the formation of igneous rocks

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