A Detailed Field study of a Silicic Tuff ring in Southwest Tenerife, Deposited During Small, Phonolitic Eruptions.

Abstract

Caldera del Rey is a small, double-crater, silicic tuff ring situated on Tenerife’s southern coastline, located close to the diffuse southern rift zone. The Caldera del Rey formation displays extensive exposures of the proximal and medial deposits of the tuff ring that grew as a result of interactions between trachy-phonolitic magma and shallow groundwater within an underlying basaltic aquifer. Detailed fieldwork, laboratory work and the analysis and correlation of 39 sections through the deposits show that the eruption repeatedly alternated between eruption styles. The formation is divided into an upper and lower sequence that both initiated with a period of pumice fall from eruption plumes of almost subplinian dimensions. Over time the eruptive processes transitioned towards dominantly wetter phreatomagmatic eruptions. As the eruptions reached the climactic stage, numerous powerful, single-surge pyroclastic density currents dispersed radially up to 4 km from the vent. The depletive currents show various downcurrent lithofacies transitions that record transformations of the depositional flow-boundary zone with distance. In some cases, transformations from granular fluid-based to fully dilute currents occurred as a result of loss of granular fluid by deposition. The ash aggregation deposits share the same couplet structure seen in other ignimbrite sheets formed elsewhere on Tenerife, with variations from this occurring due to the rapid, pulsatory nature of phreatomagmatic eruptions. Tuff ring explosive activity so close to densely populated urban areas is rarely recognised, but it remains a possibility that should be factored into hazard assessments.