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Distribution of volcanic rocks about mid-ocean ridges and the Kenya Rift Valley

Published online by Cambridge University Press:  01 May 2009

J. B. Wright
J. B. Wright
Affiliation:
Department of Geology, Ahmadu Bello University, Zaria, Nigeria
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Summary

Quartz-trachytic differentiates characterise volcanic islands on or near mid-ocean ridges, while phonolitic trends are found on islands rising from ocean basins. A large part of the Kenya Rift Valley is dominated by Plio-Pleistocene quartz-trachytes, which are underlain and flanked by variably nepheline-rich Miocene and Plio-Pleistocene lavas.

Phonolitic and nephelinitic lavas dominate the assemblages of Miocene age, trachytes and olivine basalts those of Plio-Pleistocence age. This change in petrographic character with time is attributed to two stages of sub-crustal pressure relief corresponding to Lower Miocene and late Pliocene elevations of the Kenya dome. The result was a change in partial melting products from less to more silicic, especially along the axial rift valley.

The doming movements and related vulcanism are believed to have originated because of lateral compression, induced by sea-floor spreading movements in the Atlantic and Indian oceans.

Type
Articles
Information
Geological Magazine , Volume 107 , Issue 2 , March 1970 , pp. 125 - 131
Copyright
Copyright © Cambridge University Press 1970

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