The East African Rift formed 22-25 million years ago. The Rift was formed as a result of plate tectonics, with the Somalian and Nubian plates moving away from the Arabian Plate. Because of these forces, significant sections of the crust sank between the two parallel fault lines, forcing molten rocks to erupt as volcanic eruptions. The process (rifting) is still ongoing, as evidenced by the numerous semi-active and active volcanoes found along the Rift. There are several hot springs located across the Rift Valley, indicating active volcanic activity along the Rift (Ring, 2014). The East African Rift begins at Afar Triple Junction in Ethiopia through several East African countries and ends in Mozambique. It consists of two arms; the Eastern Rift Valley (Gregory Rift) which consists of the Ethiopian Rift, Tanzanian Rift and Kenyan Rift, the Western Rift Valley begins at Mozambique up through Malawi, Zambia, Tanganyika Rift, Burundi, Rwanda and terminates at the Albertine Rift in Uganda (See Fig. 1).
Fig 1: East African Rift Valley Satellite image Fig 2: Map of the East African Rift Valley
Geologic provinces are areas that have similar geologic attributes; they may include single dominant elements such as a basin or several contiguous related elements. The East African Rift is home to some of the largest igneous and metamorphic provinces.
Geologists have constantly said that the Nubian Plate (which is the larger Africa), the Somalian plate (the horn of Africa) which is the smaller Plate and the Arabian Plate are pulling away from each other hence creating a new plate when the old ones split apart. The three Plates form the oldest rift which is the triple junction at the Ethiopian Afar region which is the oldest Rift. Down South, the Western Rift valley led to the formation of Albertine rift which contains of the East African Great Lakes; Lake Albert, Lake Edward, Lake Kivu and Lake Tanganyika (Ring, 2014).
Fig 3: East African Rift Valley Cross-Section
After a long debate by geologists and geophysicist, one model explains how rifts are formed. The flow of heat from the mantel is constantly causing two thermal bulges; central Kenya and another in the Ethiopian Afar region. The bulges come out as highlands in the two regions and as they form they cause stretching of the crust forming faults. Magma pushes up the weak faults and comes out as huge volcanic eruptions covering large stretches of land with lava (See Fig. 3). As the stretching continues it leads to the formation of a new plate tectonic crust (Wolfenden et al., 2006)
Fig 4: Danakil depression Lava fields in Ethiopia
The East African Rift is quite interesting. The fact that it has two branches makes it even more complicated. The Eastern Rift dies at Tanzania. The Western rift then goes round Lake Victoria through the great lakes. Lake Victoria is found between the two Rift branches. It is thought that due to an ancient metamorphic rock (Tanzanian Craton) the rift found it hard to tear straight through the area hence the rift diverged around creating a second branch (See Fig. 5).
Fig 5: Branches of the East African Rift
Though the rift is still currently above sea level, rifting is a continuous process taking place along the East African Rift and as the rift grows deeper it might finally drop below sea level letting the ocean waters flow into it. This forms a narrow, shallow sea inside the rift. A good example of a completely developed rift is the Red Sea.
Ethiopia
The Great Rift Valley stretches about 2800 km. long through Ethiopia. It stretches into the Danakil depression (116 meters), which is one of the hottest places on earth. The volcanic floor has an average of 30-80 km. wide and 4000 meters high. The Ethiopian Rift Valley floor consists of seven lakes which are between the two Ethiopian highlands. However, most of the lakes have no outlet hence their alkaline nature. They include: Lake Zeway, Shala, Abyata, Langano, Awassa, Chmao and Abaya (Wolfenden, et al., 2006).
The Afar Depression consists of lowland plains which are split by faults and existing shield volcanoes. It is surrounded by large discontinuous faults that formed major tectonic Ethiopia escarpments. The faults separated the rift floor from surrounding plateaus to the east and west, Danakil block to the northeast, Ali-Sabieh block to the south-east and the southern Red Sea rift to the north (See Fig. 6).
Fig 6: The Afar depression of Ethiopian Rift
Kenya
The Kenyan Rift is a continuation of the Ethiopian Rift which extends from Lake Turkana. The northern rift in Kenya has deep raben basins that are up to 7 km. thick which are surrounded by deep faults along the west of the rift which run all the way from Lake Turkana to Elgayo Escarpments and Tugen hills. The faults in the Turkana area are narrower than those found in Ethiopia but wider (25-40km) than the rest of the rift as it decreases southwards approximately 5-10 km. in the south. The age of volcanism, crustal thinning and the fault timing to the south, points towards the theory that the rift propagated southwards.
The Rift Valley in Kenya is developed in a complex lithospheric area related to Proterozoic orogenic events influenced by patterns of faulting uplift, subsidence and magmatism. Here being so much tectonic and volcanic activity along the rift, the East African Rift especially in Kenya is a potential power source.
The rift floor is covered by volcanic rocks. The oldest volcanic of the Lower Miocene age in the Kenyan rift are comprised of pyroclastic piles found in South Nyanza and eroded lavas. In the late Miocene age, more eruptions occurred; the lava flowed to form the current Yatta and Kapitiplateue. As faulting accompanied by eruptions followed, it gave rise to the volcanic piles of Mount Elgon, Mount Kenya and Mount Kilimanjaro. Within the rift itself where volcanism was mostly experienced, it gave rise to the cider cones and craters in the valley's floor e.g. Suswa, Menengai and Longonot.
Repeated faulting and volcanic eruptions at the Rifts floor gave rise to many seasonal basins which have been accumulated by fluviatile and lacustrine sediments. Olorgesaillie and Kriandusi lake beds contain a series of lacustrine sediments which are of the middle Pleistocene age. The beds are filled with diatomite, interesting mammalian fossils and artifacts.
The floor of the valley is also covered by a series of lakes that were formed as depressions due to the pulling and pushing of the Earth's crust, which later were filled with water. The lakes include Turkana, Baringo, Bogoria, Elementaita, Naivasha, Magadi and Nakuru. Some of the lakes are fresh water while others are alkaline hence huge deposits of soda ash.
Tanzania
In Tanzania, we have both the Eastern Rift which extends from Kenya and the Western Rift which extends from Mozambique and passes through Tanzania at its borders to the west and ends in Uganda. Polyphase rift successions diagenetic events are controlled by tectonic evolution, provenance geology, weathering, topographic effects and climate effects. In Tanzania, diagenetic process led to the formation of several different rocks and minerals. Due to renewed tectonic activity during the formation of the rift valley, new generations of calcite, clay minerals, hematite, feldspar and quartz were formed (Omenda, 2013).
Due to the tectonic movements and volcanic eruptions on the floor of the Tanzanian rift, several shallow lakes were formed, they include; Lake Eyasi, Makati, Manyara and Lake Natron. All the lakes are alkaline. Recent discoveries by scientist show that there is a helium gas field seated in the Tanzanian Rift Valley. The volcanoes found in the Rift valley are crucial in the formation of helium reserves. These volcanic activities provide the head needed to release helium that has accumulated in ancient crustal rocks.
Mozambique Belt
The Mozambique Belt extends all the way from the coast of Mozambique through Tanzanian and Kenyan Rift to Ethiopia. The Mozambique Belt in Kenya is characterized by large areas of Quaternary and Neogene volcanic rocks. The main volcanic area to the South West is the Eyasi half graben to the South East is the Pangani graben. Coming into the Northern Tanzanian Rift, the volcanic province is much younger than the greater Ethiopian Rift that stretches to Kenya. The volcanic province of this area stands in between two rocks; the oregenic fold belt of the north-south Mozambique and the Archaean rocks that belong to the Tanzanian Craton. These rocks are divided into three major formations, the oldest being Dodoman, Nyanzian and the Kavirondian. However, there is a brief craton that comprises of the greenstones belts and a little bit of granite intrusion.
The Mozambique belt of southern Tanzania volcanoes overlies cratonic rocks which are surrounded by metamorphic rocks. These rocks extend all the way across Serengeti Plains all the way to the west of the volcanic province and spill over to the East to parts of the Maasai Block. The Mozambique Belt was confirmed recently in central Tanzania to consist an ancient crust of Archaean which is 2970-2500Ma. This is an eye opener that the Tanzanian Craton is a foreland. The Mozambique Belt contains polycyclic orogenic complex which involves a basement and rocks cover of more than a single orogenic event, the last having taken place around 650 Ma. Similar proof of the belt's polycyclic nature is seen in Kenya (Delvaux, et al., 1992).
The Usagaran in Tanzania is basically divided into two major series; one is the Crystalline Limestone found in the higher grounds. This series comprises of dolomitic marbles, quartizea, mica schist, graphite schist, graphite marble, and the rare metabasites and kyanite gneisses. The other series is the lower Masasi which comprises of mainly hornblende-biotite gneisses, charnockites and quartzo-felspathic.
The Usagaran rocks range from upper amphibolites facies to biotite facies while the Pare mountain rocks which are of the Pan-African age are made up of granulite facies. The same rocks, granite and anorthosite are also found in the core and mantle of the Usagaran metamorphic rocks as seen in Loliondo and Longido which is near the Kenyan border. It is now clear that the Tanzanian Volcanic province seats between Archaean rocks and the Mozambique orogenic fold. Therefore on the surface geology, it appears as that the rift structure caused by volcanism are situated in a tectonic crust regime that has thermal and mechanical contacts between cold, thick, thinner anisotropic lithosphere belt and rigid Archaean lithosphere (Chorowicz, 2005)
In southern Kenya, there is a P-wave of velocity which is one km thick and extends to the west of the rift at Lake Magadi for 120 km. The layer is highly fractured. This has been interpreted by geologists to be caused by the westward overthrust of the Mozambique Belt rocks which led to a margin of the craton being buried beneath the Mozambique Belt rocks (Delvaux, et al., 1992). This explains the granulites found in northern Tanzania dated at 2.0 Ga, they prevail as xenoliths at Lashine volcano.
Western Rift
The Eastern Rift is documented to have more significantly greater volcanism compared to the younger Western Rift which is characterized by four restricted spatially distinct provinces; Kivu and Virunga provinces at the DRC border with Rwanda, Uganda and Burundi, Rungwe volcanic field of South West Tanzania and Toro-Ankole of Western Uganda. It runs for a span of over 2100 km from the north at Lake Albert, through Lake Edward, down through Lake Kivu basins where it then turns toward the South East where it includes lakes Tanganyika and Rukwa basins and further south to Lake Malawi.
The Western Branch volcanic activity is much younger (13-12Myr). This is proven by the sediment thickness in the lakes found in the four volcanic provinces (Cohen et al., 1993). Western Branch is characterized by half grabens of very elevated angle rift faults. It is linked in the north to the Eastern Rift by Aswa fracture zone (See Fig. 6). Fault trends in the western rift are less continuous than the Rifts in the Eastern Rift; however, it has experienced one of the greatest subsidence on the Earth's surface. This is clearly shown by the rift lakes found in the Western branch whose depth is actually below sea level. The rifts are sharp and more pronounced in some parts with a striking example being the Ruwenzori Mountains which rise up to more than 5 km. above sea level. The Ruwenzori Mountains are actually the highest rifts on the planet (Ring, 2014).
Fig 6: Two branches of the East African Rift converging in South West Tanzania where they create a triple junction along the Rungwe volcanic complex. Triple junction creates favorable state for enthalpy resource.
The Albertine Graben basins are pre-dominated by a series of thick deposits and are mostly non-magmatic apart from the southern basins which record volcanic products. The (TRM) section of the Western Rift i.e. Tanganyika Rukwa-Malawi consists basement structures from the Proterozoic period. The lakes found within the TRM section, Malawi and Tanganyika have deep sedimentary basins however most of the area is also on-magmatic just like the Albertine Graden. The only volcanic fields experienced are at Rungwe which is between Tanganyika and Rukwa (Omenda. 2013). The area is however characterised by fumaroles and hot springs with temperatures up to 86°C.
The Neoproterozoic Mozambique Belt is dominated by argillaceous rocks, quartzites varying in metamorphic grade. The rocks are host to nickel, coltan, cobalt and gold minerals. The Karagwe-Ankolean is commonly known for porphyritic granites that are host to tin, iron plus coltan, lithium, bismuth, tungsten and alluvial gold.
The north parts of the Western branch are calcium rich, highly undersaturated silica, potassic, carbonatites and kama fungites indicating deep melting.
Fig 7: The total alkali content which indicates deeper degree melts in the West Rift compared to smaller degree melts at the Eastern Rift, Wt% Sio2. Southern Kenya Rift (SKR), Northern Kenya Rift (NKR), Main Ethiopian Rift (MER), Western Rift (WE) (Furman,, 2007).
The East African Rift is generally characterised with a lot of igneous rocks and geothermal activity which exist in form of fumaroles, hot springs and hot grounds. The igneous rocks and geothermal activities are associated with the Quaternary volcanic systems found at the rifts axis. This basically proves that there are deposits of hot magma bodies lying shallow under the volcanoes mostly in the Eastern branch (Omenda, 2013).
Works Cited
Cohen, Andrew S., Michael J. Soreghan, and Christopher A. Scholz. "Estimating the age of formation of lakes: an example from Lake Tanganyika, East African Rift system." Geology, no. 21.6, 1993, pp. 511-514.
Chorowicz J., 2005: The East African rift system. J. African Earth Sciences, no. 43, pp.379-410.
Delvaux, Damien, et al. "Cenozoic paleostress and kinematic evolution of the Rukwa-North Malawi rift valley (East African Rift System)." Bulletin des Centres de Recherche Exploration-Production ElfAquitaine, no. 16,992, p. 383-406.
Harðarson, Björn Sverrir. "Structural geology of the western branch of the East African Rift: tectonics, volcanology and geothermal activity." 001374011 (2014).
Ring, U., 2014: The East African Rift System. Australian J. of Earth Sciences, no. 107, pp. 132-146.
Omenda, Peter A. "The geology and geothermal activity of the East African Rift." Short Course V on Exploration for Geothermal Resources, organized by UNU-GTP, GDC and KenGen, at Lake Bogoria and Lake Naivasha, Kenya, Oct (2010)
Wolfenden, Ellen, et al. "Evolution of the northern Main Ethiopian rift: birth of a triple junction." Earth and Planetary Science Letters, no. 224.1, 2004, pp. 213-228.
