Thickness variations and volume estimates of tephra fall deposits: the importance of particle Reynolds number
Thickness variations and volume estimates of tephra fall deposits: the importance of particle Reynolds number
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DOI：
10.1016/s0377-0273(98)00007-9
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发表时间：
1998-05-01
影响因子：
2.9
通讯作者：
Sparks, RSJ
中科院分区：
文献类型：
--
作者：
Bonadonna, C;Ernst, GGJ;Sparks, RSJ
保存完好的火山灰瀑布沉积物显示的厚度变化比简单的指数变薄更复杂。在测井厚度与等厚线所围面积平方根的关系图上，许多矿床显示出两个或多个近似直线段，在某些情况下还显示出弯曲区域。我们表明，主要的变化，变薄率发生的颗粒尺寸随距离的通风口，作为一个结果的变化，从高到低雷诺数的沉降行为的预测W.I.玫瑰横向扩展羽流沉积的计算机模型预测，在高雷诺数（Re> 500）下，粗喷出物（火山砾和粗火山灰）会出现指数变薄的陡峭近端段。在更远的距离，更细的喷出物预计将显示幂律变薄。识别出两个远端节段。最远段由低雷诺数颗粒组成，可以用指数变薄律近似，但用幂律更好地描述。远段和近段由包含中间（0.4 <Re <500）和低雷诺数颗粒（Re <0.4）的混合群体的弯曲段连接。许多史前火山灰沉积物没有保存超过更近端的高雷诺数段和存款量，使用一个简单的指数变薄法，将大大低估，如果大量的火山灰喷发。数据库（定义的等厚线数量）通常过于稀疏，无法识别中间区域和曲率，因此数据通常被解释为两条直线段。不同柱高的坡折距离的模型预测显示出良好的一致性与观察到的坡折范围从12到45公里的喷发柱。数据表明，近端段的衰减常数（厚度半距B（t））也与柱高相关，如模型所预测，并且仅弱依赖于总粒度分布。富含极细火山灰的火山羽（如共熔凝灰岩云）产生大量B（t）值很大的沉积物。(C)1998 Elsevier Science B.V.保留所有权利。
Well-preserved tephra fall deposits display thickness variations which are more complex than simple exponential thinning. On plots of log thickness against square root of area enclosed by an isopach contour, many deposits show two or more approximately straight-line segments and in some cases regions of curvature. We show that major changes in thinning rate occur as the particle size decreases with distance from the vent, as a consequence of the change of settling behaviour from high to low Reynolds number as predicted by W.I. Rose. Computer models of sedimentation from laterally spreading plumes predict a steep proximal segment with exponential thinning for coarse ejecta (lapilli and coarse ash) with high Reynolds number (Re > 500). At greater distance finer ejecta are predicted to show power-law thinning. Two distal segments are identified. The most distal segment is composed of low Reynolds number particles and can be approximated by an exponential thinning law, but is better described by a power law. The distal and proximal segments are connected by a curved segment containing mixed populations of intermediate (0.4 < Re < 500) and low Reynolds number particles (Re < 0.4). Many prehistoric tephra deposits are not preserved beyond the more proximal high Reynolds number segment and deposit volume, using a simple exponential thinning law, would be substantially underestimated if large amounts of ash were erupted. The database (number of defined isopach contours) is usually too sparse to recognise the intermediate region and curvature, so that the data will often be interpreted as two straight-line segments. The model predictions of break-in-slope distances for different column heights show good agreement with observed breaks-in-slope for eruption columns ranging from 12 to 45 km. Data indicate that the decay constant (the thickness half-distance b(t)) for the proximal segment is also correlated with column height, as predicted by the models, and is only weakly dependent on total grain size distribution. Volcanic plumes rich in very fine ash (such as co-ignimbrite clouds) produce extensive deposits with large values of b(t). (C) 1998 Elsevier Science B.V. All rights reserved.