Gregory Rift

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Gregory Rift in East Africa Gregory Rift Topographical.svg
Gregory Rift in East Africa

The Gregory Rift is the eastern branch of the East African Rift fracture system. The rift is being caused by the separation of the Somali plate from the Nubian plate, driven by a thermal plume. Although the term is sometimes used in the narrow sense of the Kenyan Rift, the larger definition of the Gregory Rift is the set of faults and grabens extending southward from the Gulf of Aden through Ethiopia and Kenya into Northern Tanzania, passing over the local uplifts of the Ethiopian and Kenyan domes. [1] Ancient fossils of early hominins, the ancestors of humans, have been found in the southern part of the Gregory Rift. [1]

Contents

Etymology

The Gregory Rift is named in honour of the British geologist John Walter Gregory who explored the geology of the rift in 1892–93 and 1919. [2]

Location

Ol Doinyo Lengai erupting in 1966 Oldoinyolengai.jpg
Ol Doinyo Lengai erupting in 1966

The Gregory Rift lies within the Mozambique belt, often considered to be the remains of an orogenic system similar to the Himalayas. This belt runs from Ethiopia through Kenya, Tanzania and Mozambique. [3] The rift is widest at the northern end in the Afar region, narrowing to a few kilometers in northern Tanzania, then splaying out in the North Tanzania Divergence. [1] The Gregory Rift has shoulders rising over 3,000 metres (9,800 ft) above sea level, 1,000 metres (3,300 ft) above the inner part of the graben. [4] The Tanzanian portion includes Mount Kilimanjaro, the highest mountain in Africa, and the huge caldera of Ngorongoro. [1] This portion also contains Ol Doinyo Lengai, the world's only active carbonatite volcano. [5]

Lakes in the rift other than Lake Turkana are mostly small and shallow, some with fresh water but many being saline. The thickness of lake sediments is mostly unknown. In Lake Turkana they seem to be at most 4 kilometres (2.5 mi) thick, in the Baringo – Bogoria half-graben from 500 metres (1,600 ft) to 1,000 metres (3,300 ft) thick and in the Afar depression up to 100 metres (330 ft) thick. [6]

Exploration

Joseph Thomson, first geologist to examine the region Joseph Thomson.png
Joseph Thomson, first geologist to examine the region

The first well-known European geologist to explore the region was Joseph Thomson, a member of an expedition in 18791880 sponsored by the Royal Geographical Society of Britain. From his observations he deduced the existence of a great fault.

Thomson returned in 1883, traveling through the rift valley in Kenya from Mount Longonot to Lake Baringo. Describing the valley around this lake he said: "Imagine if you can a trough or depression 3300 feet above sea level, and twenty miles broad, the mountains rising with very great abruptness on both sides to a height of 9000 feet". John Walter Gregory visited central Kenya in 1893 and again in 1919. His 1896 book The Great Rift Valley is considered a classic. Gregory was the first well-known European to use the term "rift valley", which he defined as "a linear valley with parallel and almost vertical sides, which has fallen owing to a series of parallel faults". [7]

In 1913 the German geologist Hans Reck made the first European study of the strata in the Olduvai Gorge to the west of the Crater Highlands. He brought a large collection of mammalian fossils back to Berlin. In 1928 Louis Leakey, the anthropologist, visited Berlin, where he saw that some of Reck's materials were artifacts. Leakey began exploring Olduvai in the 1930s and collecting material that has led to the site being recognized as an important center of early hominin occupation. [8]

Development

Volcanism and rifting started in Kenya in the northern region of Turkana between 40 and 35 million years ago and then spread north and south. To the south volcanism and rifting happened together, first in other parts of northern Kenya around 30 million years ago, then around 15 million years ago in the central part of the Kenyan Rift, 12 million years ago in southern Kenya and 8 million years ago in northern Tanzania. [9] When rifting reached the Tanzanian Craton, the rift split into the eastern Gregory Rift and the western Albertine Rift, which are separated by the 1,300 kilometres (810 mi) wide East African Plateau. Large shield volcanoes near the margins of the craton and in the adjacent Mozambique belt issued large volumes of basaltic to trachytic magmatism between five and one million years ago, with faulting around 1.2 million years ago. [10]

Western cliffs of the Eastern Rift Valley near Iten with step faulting Elgeyo.jpg
Western cliffs of the Eastern Rift Valley near Iten with step faulting

Volcanic activity started in the central Ethiopian plateau around 30 million years ago, long before rifting began. The first period of activity deposited flood basalts and rhyolites from 500 metres (1,600 ft) to 2,000 metres (6,600 ft) thick. Uplift of the Ethiopian plateau began around this time or soon after. Between 30 million and 10 million years ago synrift shield volcanoes deposited from 1,000 metres (3,300 ft) to 2,000 metres (6,600 ft) of additional material over the Ethiopian flood basalts. Rifting in Ethiopia began about 18 million years ago in the southwest and 11 million years ago in northern parts of the Main Ethiopian Rift as the opening of the Gregory rift caused the Afar Triple Junction to form. [9] Volcanism from the Middle Pleistocene onward formed a chain of volcanoes along the floor of the rift throughout its length, dividing it into separate valleys. [11]

There are some indications that the lithosphere may have thinned below the Gregory rift, although based on basalt geochemistry the lithosphere is at least 75 kilometres (47 mi) thick below the south of Kenya. [12] The Gregory rift is oriented NS, and in the past the minimum horizontal tectonic stress direction was EW, the direction of extension. The alignment of rows of small vents, cones, domes and collapse pits in the Suswa, Silali and Kinangop Plateau regions support this theory. However, data from oil and gas exploration wells in Kenya, vents in volcanic shields to the east of the rift at Huri Hills, Mount Marsabit and Nyambeni Hills and recent small cones at Suswa and east of the Silali caldera all indicate that the minimum horizontal stress direction has changed to NW-SE within the last half million years. [13]

Related Research Articles

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Rift A linear zone where the Earths crust is being pulled apart, and is an example of extensional tectonics

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Suguta Valley

The Suguta Valley, also known as the Suguta Mud Flats, is an arid part of the Great Rift Valley in Kenya (Africa), directly south of Lake Turkana.

Northern Cordilleran Volcanic Province

The Northern Cordilleran Volcanic Province (NCVP), formerly known as the Stikine Volcanic Belt, is a geologic province defined by the occurrence of Miocene to Holocene volcanoes in the Pacific Northwest of North America. This belt of volcanoes extends roughly north-northwest from northwestern British Columbia and the Alaska Panhandle through Yukon to the Southeast Fairbanks Census Area of far eastern Alaska, in a corridor hundreds of kilometres wide. It is the most recently defined volcanic province in the Western Cordillera. It has formed due to extensional cracking of the North American continent—similar to other on-land extensional volcanic zones, including the Basin and Range Province and the East African Rift. Although taking its name from the Western Cordillera, this term is a geologic grouping rather than a geographic one. The southmost part of the NCVP has more, and larger, volcanoes than does the rest of the NCVP; further north it is less clearly delineated, describing a large arch that sways westward through central Yukon.

Namarunu

Namarunu is a shield volcano located in the Great Rift Valley, Kenya.

Volcanism of Canada

The volcanism of Canada is represented by many types of landform including lava flows, volcanic plateaus, lava domes, cinder cones, stratovolcanoes, shield volcanoes, submarine volcanoes, calderas, diatremes, and maars, along with examples of more less common volcanic forms such as tuyas and subglacial mounds. It has a very complex volcanic history spanning from the Precambrian eon at least 3.11 billion years ago when this part of the North American continent began to form.

Barrier Volcano

The Barrier Volcano is an active shield volcano located in the north of Kenya. It is last known to have erupted in 1921.

Afar Triple Junction Place where three tectonic rifts meet in East Africa

The Afar Triple Junction is located along a divergent plate boundary dividing the Nubian, Somali, and Arabian plates. This area is considered a present-day example of continental rifting leading to seafloor spreading and producing an oceanic basin. Here, the Red Sea Rift meets the Aden Ridge and the East African Rift. It extends a total of 6,500 kilometers (4,000 mi) in three arms from the Afar Triangle to Mozambique.

Maitland Volcano

Maitland Volcano is a heavily eroded shield volcano in the Northern Interior of British Columbia, Canada. It is 83 km (52 mi) southeast of the small community of Telegraph Creek in what is now the Klappan Range of the northern Skeena Mountains. This multi-vent volcano covered a remarkably large area and was topped by a younger volcanic edifice. Little remains of Maitland Volcano today, limited only to eroded lava flows and distinctive upstanding landforms created when magma hardened within the vents of the volcano.

Volcanism of Northern Canada

Volcanism of Northern Canada has produced hundreds of volcanic areas and extensive lava formations across Northern Canada. The region's different volcano and lava types originate from different tectonic settings and types of volcanic eruptions, ranging from passive lava eruptions to violent explosive eruptions. Northern Canada has a record of very large volumes of magmatic rock called large igneous provinces. They are represented by deep-level plumbing systems consisting of giant dike swarms, sill provinces and layered intrusions.

Lokitaung Place in Turkana County, Kenya

Lokitaung is a settlement in Kenya's Turkana County, a few miles inland of northwest Lake Turkana. Lokitaung is the site of the 36-million year old Lokitaung Basalt lava flows, which lay atop Cretaceous sediments including dinosaur bones. The basalts are over 1 kilometer thick, and approximately 100 kilometers wide.

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The Great Rift Valley is part of an intra-continental ridge system that runs through Kenya from north to south. It is part of the Gregory Rift, the eastern branch of the East African Rift, which starts in Tanzania to the south and continues northward into Ethiopia. It was formed on the "Kenyan Dome" a geographical upwelling created by the interactions of three major tectonics: the Arabian, Nubian, and Somalian plates. In the past, it was seen as part of a "Great Rift Valley" that ran from Madagascar to Syria. Most of the valley falls within the former Rift Valley Province.

Tanzania Craton An old and stable part of the continental lithosphere in central Tanzania

The Tanzania Craton is an old and stable part of the continental lithosphere in central Tanzania. Some of the rocks are over 3 billion years old.

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The greater Turkana Basin in East Africa determines a large endorheic basin, a drainage basin with no outflow centered around the north-southwards directed Gregory Rift system in Kenya and southern Ethiopia. The deepest point of the basin is the endorheic Lake Turkana, a brackish Soda lake with a very high ecological productivity in the Gregory Rift.

Lake Suguta

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Gharyan volcanic field

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Geology of Tanzania

The geology of Tanzania began to form in the Precambrian, in the Archean and Proterozoic eons, in some cases more than 2.5 billion years ago. Igneous and metamorphic crystalline basement rock forms the Archean Tanzania Craton, which is surrounded by the Proterozoic Ubendian belt, Mozambique Belt and Karagwe-Ankole Belt. The region experienced downwarping of the crust during the Paleozoic and Mesozoic, as the massive Karoo Supergroup deposited. Within the past 100 million years, Tanzania has experienced marine sedimentary rock deposition along the coast and rift formation inland, which has produced large rift lakes. Tanzania has extensive, but poorly explored and exploited natural resources, including coal, gold, diamonds, graphite and clays.

References

  1. 1 2 3 4 Dawson 2008, p. 2.
  2. http://sp.lyellcollection.org/content/6/1/1.full.pdf
  3. Beccaluva, Bianchini & Wilson 2011, p. 38.
  4. Frisch & Meschede 2010, p. 35.
  5. Frisch & Meschede 2010, p. 36.
  6. Anadón, Cabrera & Kelts 1991, p. 6.
  7. Dawson 2008, p. 3.
  8. Dawson 2008, p. 6.
  9. 1 2 Beccaluva, Bianchini & Wilson 2011, pp. 38–39.
  10. Beccaluva, Bianchini & Wilson 2011, pp. 107.
  11. Anadón, Cabrera & Kelts 1991, p. 5.
  12. Beccaluva, Bianchini & Wilson 2011, pp. 108.
  13. Bosworth, Burke & Strecker 2000.

Sources