Abstract

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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