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Journal of the Geological Society; 1993; v. 150; issue.5; p. 885-896;
DOI: 10.1144/gsjgs.150.5.0885
© 1993 Geological Society of London

Papers

Incremental caldera collapse of Suswa volcano, Gregory Rift Valley, Kenya

I. P. SKILLING

Environmental Science Division, Lancaster University, Bailrigg, Lancaster LA1 4YQ, UK

Suswa volcano, located at 1°10'S, 36°20'E, is Quaternary in age (<0.4 Ma), dominantly trachytic-phonolitic in composition, and has two calderas. Regional extension was a fundamental control on caldera collapse, providing pathways for the siting, drainage and recharge of magma chambers. Caldera I collapse was associated with magmatic overpressure from volatile exsolution, magma-water interaction, influx of denser magma and magma drainage at depth. Trachybasalt ash, trachyte globular-ash ignimbrites, trachyte pumice lapilli air-fall tuffs and carbonate-trachyte ignimbrites characterize the initial subsidence. Air-fall tuffs, erupted during caldera collapse at Longonot, are interbedded, suggesting a regional collapse event. Incremental, but dominantly Valles-type, collapse continued with the eruption of trachyte agglutinate flows from concentric ring-fractures outside the caldera ring-fault (Ring-Feeder Zone) and trachyte pumice lapilli air-fall tuffs from west caldera I.

Following caldera I collapse, phonolite lava flows were erupted from the caldera floor. Centrally-erupted phonolite lava flows led to the construction of Ol Doinyo Onyoke lava cone. A pit-crater on the cone was a precursor to the collapse of caldera II, both of which were generated entirely by magma withdrawal. Regional decompression caused ring-fault bounded, block-resurgence of the caldera floor




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