The Granadilla pumice deposit of Southern Tenerife, Canary Islands

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The Granadilla pumice, the product of a large Plinian eruption, covers over 800 km2 of southern Tenerife. It is, for the most part, reversely graded, and is divided into four fall-units which characterise specific phases in the eruption. Isopach and isograde maps indicate a north-westerly palaeo-wind direction. The results of the granulometric analysis of 170 samples are presented, together with criteria which allow pyroclastic-fall and pyroclastic-flow deposits to be differentiated. The Granadilla eruption took place over 32,000 years B.P. after a considerable repose period. It began with intermittent pumice showers and increased to a continuous high intensity gas blast. Reduced activity during the early part of the eruption allowed rain-flush beds to form while the volcano was quietly venting large volumes of fine ash. The eruption concluded with a pyroclastic-flow which is absent higher on the volcano, having failed to adhere to the steep slopes. The deposits are composed of pumice lapilli, lithic fragments, and a small proportion of sanidine crystals. Each component shows better sorting characteristics than complete samples, the frequency curves of which are asymmetrically skewed by the admixture of varying percentages of lithic fragments. In the finer grades, high lithic percentages are the result of wind drift removing the fine-grained pumice from the eruption cloud. The Granadilla pumice and associated pyroclastic deposits originated in the caldera of Las Canadas to the north-east. The formation of this caldera cannot be explained simply by northward Iandslipping as has been suggested previously. The southward dispersal of many large volume ignimbrites undermines the theory for the formation of Las Canadas by simple landslipping, while the total volume of all pyroclastic deposits in southern Tenerife fully accounts for the volume of the caldera and the pre-Teide volcano. It is therefore believed the formation of the caldera was gradual, in response to the progressive emptying of the magma chamber during a series of largescale Plinian eruptions. © 1973, The Geologists' Association. All rights reserved.

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  • Basil Booth

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