Abstract

The Agua de Pau volcano on the island of Sã o Miguel is blanketed with many trachytic pumice deposits, two of which have been selected for detailed study. One, resulting from an eruption 4600 years B.P., has a volume of 0.6–0.7 km3 (dense rock equivalent) and is mostly a coarse-grained homogeneous pumice breccia produced by a continuous gas blast plinian eruption of great vigour. Pyroclastic flow deposits form fine-grained beds in this deposit, and diagnostic criteria are proposed for them. Such beds, forming thin layers interstratified with a coarser-grained deposit, are easily misinter-preted as air-fall ashes. The other deposit, that of a.d. 1563, has about one-quarter the volume and is well bedded partly resulting from fluctuations in vigour of the eruption and partly from rain-showers. Field grain-size measurements, and mechanical analyses of 100 samples from the two deposits, have been made to learn the value of such measurements and to understand the factors which influence the grain-size parameters. The Fogo A deposit shows striking variations in the proportions of pumice, crystals and lithic fragments. From simple field measurements it seems possible to determine inter alia the threshold diameter between ballistic fragments and fragments which have settled from the ash cloud.

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