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Journal of the Geological Society

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Geological evolution of the trachytic caldera volcano Menengai, Kenya Rift Valley

Menengai is a trachytic central volcano underlain by a high-level magma chamber. Activity started shortly before 0.18 Ma, with the growth of a low-angle trachyte lava shield having a volume of about 30 km³. Krakatau-style collapse to form a 77 km² caldera within an embayed ring-fracture was accompanied by the eruption of two ash-flow tuffs representing a combined magma volume of about 50 km³. The eruption of both tuffs was preceded by an air-fall phase. Both were emplaced as single flow units, and have intermediate aspect ratios of c. 1:4000. The tuffs cover an area of c.1350 km², including outcrops higher than the eruptive vents. During post-caldera times, some 25 km³ of magma was erupted, mostly as lava flows which now cover the caldera floor, but also as cinder cones and plinian-type tephra sheets. The chronology of syn- and post-caldera events is based on correlation with dated fluctuations in the levels of nearby lakes, suggesting that the two ash-flows may have been erupted at about 29 000 and before 12 850 years ago. Lake sediments inside the caldera provide evidence for a late intra-caldera lake from about 10 300 to 8300 years B.P.

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