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
Gulf of Aden through Ethiopia
Kenya into Northern Tanzania, passing over the local uplifts of
the Ethiopian and Kenyan domes. Ancient fossils of early hominins,
the ancestors of humans, have been found in the southern part of 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
Ol Doinyo Lengai
Ol Doinyo Lengai erupting in 1966
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. 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. 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. The Tanzanian
portion includes Mount Kilimanjaro, the highest mountain in Africa,
and the huge caldera of Ngorongoro. This portion also contains Ol
Doinyo Lengai, the world's only active carbonatite volcano.
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.
Joseph Thomson, first geologist to examine the region
The first geologist to explore the region was Joseph Thomson, a member
of an expedition in 1879–1880 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
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
Great Rift Valley is considered a classic. Gregory was
the first 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".
In 1913 the German geologist
Hans Reck made the first 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
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. 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.
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
Main Ethiopian Rift as the
opening of the Gregory rift caused the
Afar Triple Junction
Afar Triple Junction to
form. 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.
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. 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
^ a b c d Dawson 2008, p. 2.
^ Beccaluva, Bianchini & Wilson 2011, p. 38.
^ Frisch & Meschede 2010, p. 35.
^ Frisch & Meschede 2010, p. 36.
^ Anadón, Cabrera & Kelts 1991, p. 6.
^ Dawson 2008, p. 3.
^ Dawson 2008, p. 6.
^ a b Beccaluva, Bianchini & Wilson 2011, pp. 38-39.
^ Beccaluva, Bianchini & Wilson 2011, pp. 107.
^ Anadón, Cabrera & Kelts 1991, p. 5.
^ Beccaluva, Bianchini & Wilson 2011, pp. 108.
^ Bosworth, Burke & Strecker 2000.
Anadón, P.; Cabrera, L; Kelts, K. R. (1991). Lacustrine facies
analysis. John Wiley & Sons. ISBN 0-632-03149-2.
Beccaluva, Luigi; Bianchini, Gianluca; Wilson, Marjorie (2011).
Volcanism and Evolution of the African Lithosphere. Geological Society
of America. ISBN 0-8137-2478-3.
Bosworth, W.; Burke, K.; Strecker, M. (2000). M.W. Jessell and
J.L.Urai, ed. "Magma chamber elongation as an indicator of intraplate
stress field orientation: "borehole break-out mechanism" and examples
from the Late Pleistocene to Recent
Kenya Rift Valley. In: Stress,
Strain and Structure, A volume in honour of W D. Means". Journal of
the Virtual Explorer. 2. Retrieved 2011-12-27.
Dawson, John Barry (2008). The Gregory rift valley and Neogene-recent
volcanoes of northern Tanzania. Geological Society Memoir No. 33.
Geological Society of London. ISBN 1-86239-267-6.
Frisch, Wolfgang; Meschede, Martin (2010). Plate Tectonics. Springer.
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