Two Plinian-type eruptions in the Azores
Two Plinian-type eruptions in the Azores
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DOI：
10.1144/gsjgs.127.1.0017
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发表时间：
1971-01
影响因子：
2.7
通讯作者：
G. Walker;R. Croasdale
中科院分区：
文献类型：
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作者：
G. Walker;R. Croasdale
圣米格尔岛上的阿瓜德保罗火山覆盖着许多粗面浮石沉积物，其中两个已被选中进行详细研究。一种是公元前4600年的一次喷发，体积为0.6-0.7 km 3（致密岩石当量），主要是由连续的气体爆破强烈的plinian喷发产生的粗粒均质浮石角砾岩。火山碎屑流沉积在该存款中形成细粒层，并提出了细粒层的诊断标准。这种层与粗粒存款形成薄层间层，很容易被误认为是气降灰。另一个存款，即公元1563年的存款，大约有四分之一的体积，并且是很好的层状，部分原因是火山爆发的活力波动，部分原因是阵雨。对两个矿床的100个样品进行了现场粒度测量和力学分析，以了解这些测量的价值，并了解影响粒度参数的因素。Fogo A存款显示浮石，晶体和岩屑的比例惊人的变化。通过简单的实地测量，似乎有可能确定弹道碎片和从灰云沉降下来的碎片之间的阈值直径。
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.