Turkana, Kenya

Location: 3.30N, 35.00E

The Turkana depression is the lower ground between the Kenyan and Ethiopian highlands. A series of horsts and grabens make up Turkana. It is 155 miles (250 km) wide. The east and west sides of the depression are gentle slopes. Its floor is cut by large faults running to the east. This causes three oddly shaped basins. These basins are also cut by faults. Most faults are found in the eastern part of the depression. Farther south, faults make the depression look like it has steps. In the east, lava slopes 35 degrees to the west. This dip decreases the higher you go. The rift valley that holds Turkana is not currently active. It opened about eight million years ago. Turkana is one of the best places in the world to find many fossils.

Turkana is divided into two areas. North Turkana is an erosion surface that was fractured and flooded by fissural basalts. This event probably represents the most important volcanic and tectonic episode in the creation of the Afro-Arabian Rift System.

Seven Pliocene volcanoes exist in southern Turkana. They have shield volcano forms. These volcanoes have produced lavas, pumice tuffs and ash-flow tuffs. Each volcano has a complex source area in which plugs, dikes, and pumice tuffs are found. Clearly defined craters and calderas are not common in these zones. The sides of this area have slopes of about five degrees. They are composed of thin lavas and ash flow sheets. The volcanoes range up to 31 miles (50 km) in diameter. They are stretched parallel to the direction of the rift. This shows that tectonic factors determined where the vents and their products were located.

Sources of Information:

Baker, B.H., et al, "The Surface Structure of Rift Zones," manuscript 9916, 39 pp., 1982.

Bellieni, G. et al, "Oligocene Transitional Tholeitic Magmatism in Northern Turkana (Kenya): Comparison with the Coeval Ethiopian Volcanism," Bulletin of Volcanology, v 44, n 3, pp 411-428, 1981.

Cerling, Thure E. and Powers, Dennis W., "Paleorifting between the Gregory and Ethiopian Rifts," Geology, v. 5, pp. 441-4, July 1977.

Savage, Robert and Williamson, Peter, "Geological Background to Fossil Man," Scottish Academic Press, pp. 375-394, 1978.

Webb, P.K. and Weaver, S.D., "Trachyte Shield Volcanoes: a New Volcanic Form from South Turkana, Kenya," Bulletin of Volcanology, v. 39, n. 2, pp. 294-312, 1975.

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