SEDIMENTATION OF TEPHRA BY VOLCANIC PLUMES .1. THEORY AND ITS COMPARISON WITH A STUDY OF THE FOGO-A PLINIAN DEPOSIT, SAO-MIGUEL (AZORES)
SEDIMENTATION OF TEPHRA BY VOLCANIC PLUMES .1. THEORY AND ITS COMPARISON WITH A STUDY OF THE FOGO-A PLINIAN DEPOSIT, SAO-MIGUEL (AZORES)
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DOI：
10.1007/bf00301486
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发表时间：
1992-04-01
影响因子：
3.5
通讯作者：
CAREY, SN
中科院分区：
文献类型：
--
作者：
BURSIK, MI;SPARKS, RSJ;CAREY, SN
在亚速尔群岛圣米格尔的Fogo a平原沉积中，研究了火山柱喷出物的沉积作用与离源距离的关系。根据浮石颜色、正长岩附属岩屑的丰度和分布特征，可将福古A粗面浮石矿床分为两部分。下部贫正长岩部分向南分散，受风的影响明显。上部富正长岩部分粗粒，在通风口周围呈近对称分布。等压线向东延伸表明受弱风影响。研究了上部粗晶部分的晶、晶组分的粒度变化。总蓄积量和单位面积蓄积量（以kg/m2表示）在大于约7 km的距离上，对于直径小于2 cm的颗粒，表现出良好的高斯函数拟合。这些结果与在静止流体上运动的径向扩散湍流的理论模型一致。高斯系数是颗粒大小和进入喷发柱伞形区域的物质流速的函数。因此，系数也是列高的函数。根据这些资料推算出的柱高为21 km，这与Carey和Sparks（1986）用最大碎屑扩散法推算出的柱高27 km基本一致。大于2厘米的碎屑堆积符合一个理论，即从上升的喷发柱边缘的碎屑沉降物，该理论将羽流视为一连串的大漩涡，它们的颗粒质量随着时间的指数函数而减少。还计算了周围空气径向流入塔内对碎屑沉积的影响。这些计算将福戈A火山喷发后期的风速限制在每秒最多几米。这项研究允许在喷发柱中识别出四种不同的动态类型的碎屑行为。
Sedimentation of ejecta from volcanic plumes has been studied as a function of distance from the source in the Fogo A plinian deposit, Sao Miguel, Azores. The Fogo A trachytic pumice deposit is reversely graded and can be divided into two parts on the basis of pumice colour, abundance of syenite accessory lithic clasts and distribution. The lower syenite-poor part was dispersed to the south and was clearly influenced by wind. The upper syenite-rich part is coarse-grained and has a nearly symmetrical distribution around the vent. Elongation of isopachs to the east indicate a weak wind influence. The grain-size variations of lithic and crystal components in the upper coarse part were studied. Total accumulation and accumulation per unit area (expressed in kg/m2) show good fits to a gaussian function at distances greater than approximately 7 km for grain diameters less than 2 cm. These results agree with a theoretical model for a radially spreading turbulent current moving over a quiescent fluid. The gaussian coefficient is shown to be a function of grain size and the flow rate of material into the umbrella region of the eruption column. The coefficient is therefore also a function of column height. The column height deduced from these data is 21 km, which is in broad agreement with the column height of 27 km deduced from maximum clast dispersal using the method of Carey and Sparks (1986). The accumulation of clasts larger than 2 cm agrees with a theory for the fallout of clasts from the margins of the ascending eruption column, which treats the plume as a succession of large eddies that decrease their mass of particles as an exponential function of time. Calculations are also presented for the influence of the radial inflow of surrounding air into the column on the deposition of clasts. These calculations constrain the wind speed during the later part of the Fogo A eruption to be at most a few metres per second. The study has allowed four different dynamic categories of clast behaviour to be recognised in eruption columns.