Gregory Rift
The Gregory Rift (Ufa la Gregori, in
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
The Gregory Rift lies within the
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
The first well-known European 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 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
Development
Volcanism and rifting started in Kenya in the northern region of
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
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
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]References
- ^ a b c d Dawson 2008, p. 2.
- ^ http://sp.lyellcollection.org/content/6/1/1.full.pdf[bare URL PDF]
- ^ 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.
Sources
- Anadón, P.; Cabrera, L; Kelts, K. R. (1991). Lacustrine facies analysis. John Wiley & Sons. ISBN 978-0-632-03149-8.
- Beccaluva, Luigi; Bianchini, Gianluca; Wilson, Marjorie (2011). Volcanism and Evolution of the African Lithosphere. Geological Society of America. ISBN 978-0-8137-2478-2.
- 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. doi:10.3809/jvirtex.2000.00008. Archived from the originalon 2012-03-28. 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 978-1-86239-267-0.
- Frisch, Wolfgang; Meschede, Martin (2010). Plate Tectonics. Springer. ISBN 978-3-540-76503-5.