Dictionary of Human Evolution and Biology

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Paleo-Lake Olduvai

Depressed area of water collection that formed initially 3.6 mya when the eruption of the Sadiman volcano just west of the Ngorongoro crater covered the area with a thick layer of ash that was compressed and stabilized by rain (the same eruption that captured the footprints at Laetoli). The Ngorongoro volcano then erupted at 2.5 mya, depositing another layer of volcanic ash. Finally, at 1.89 mya, the Olmoti volcano, north of Ngorongoro, erupted and covered the whole area with a thick, nonporous black basalt. This layer formed the bottom of the gorge, just beneath Olduvai Bed I, and formed the basin of a brackish permanent lake in which Plio-Pleistocene sediments collected for the next 400 ky. Paleo-lake Olduvai was a semi-arid, enclosed basin in which seasonal rainfall patterns controlled sediment deposition at the lake margin: waxy clays deposited during wet periods and earthy sediments characteristic of dry periods. The perennial (minimum) diameter of the lake was about 5-10 km, but episodically the maximum expansion was to about 15-20 km. Finally, owing to tectonic faulting and uplifting about 1.5 mya, the paleolake drained, although a stream remained that eventually cut today’s Olduvai Gorge.

See western Olduvai Gorge.

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