Fall vs flow activity during the 1991 climactic eruption of Pinatubo Volcano (Philippines)
Fall vs flow activity during the 1991 climactic eruption of Pinatubo Volcano (Philippines)
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1991 年皮纳图博火山（菲律宾）顶峰喷发期间的秋季与水流活动
DOI：
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发表时间：
2001
期刊：
影响因子：
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通讯作者：
T. Bacolcol
中科院分区：
文献类型：
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作者：
M. Rosi;M. Paladio;A. Di Muro;Roberto Leoni;T. Bacolcol
摘要。1991年皮纳图博火山喷发六年后，火山周围堆积的火山碎屑沉积物的深侵蚀切口使我们能够研究山谷底部和相邻山脊的气候沉积物的地层学。Marella水系中下降、流动和涌动沉积物之间的地层关系表明，在气候喷发期间，发生了一个渐进的转变，从早期的对流状态，到一个由普林尼对流柱和大部分稀释的密度流组成的过渡状态，再到一个完全崩溃的状态，产生大部分密集的火山碎屑流。新普林尼期的稀密度流（涌浪）沿着山谷底部和马雷拉山谷的连续山脊从火山口传播了~10公里，而后普林尼期的火山碎屑流有更大的跳动（~13公里），局限于山谷，与明显的涌浪无关。地层研究和粒度分析可以识别出三种类型的平顺层内沉积：(a)较低且通常是粗粒度的涌积沉积，在沉降层最粗部分的堆积过程中，每隔16-24分钟沉积一次；(b)上部细粒冲涌沉积，与落层细粒部分成层间，侵位时间间隔较短，约为3 ~ 13 min；(c)细粒上落层中与上部涌浪互层的小体积、受河道约束的块状浮质流沉积。岩屑（ML）和浮石（MP）的最大粒径等线分别为1.6 cm和0.8 cm和2.0 cm和4.0 cm，在火山口周围几乎对称分布，表明台风云雅在火山喷发期间的经过对高雷诺数碎屑的轨迹影响不大。然而，在3.2 cm的ML和6.0 cm的MP近端等长条面上观察到明显的扭曲，其模式可能受到从柱缘落下的碎屑与上升的共烟灰羽的相互作用的影响。凯里和斯帕克斯（1986）模型对伞状云产生的未受干扰的等等值线的应用得出最大柱高约42公里，与卫星测量结果非常吻合。结合卫星资料和目击者的描述，对Marella山谷的平直落层沉积物进行了系统的地层学和垂直粒度研究，表明对流柱的承载能力和相关的沉降活动在喷发早期达到顶峰，在13:41之前开始，逐渐下降，直到3小时后停止。大部分浮质火山碎屑流沉积是在沉降活动结束后（约16:30）、山顶破火山口坍塌前（约19:11）就位的。在最后的破火山口崩塌之后，只有少量的浮石流沉积。Holasek et al.（1996）先前的重建将卫星数据记录的柱的逐渐降低解释为由于质量喷发率的降低，与此相反，我们认为从一个斜柱到一个大的共火成岩柱的逐渐转变也可以解释这种变化。
Abstract. Six years after the 1991 Mt. Pinatubo eruption, deep erosion incisions into the pyroclastic deposits accumulated around the volcano enabled us to investigate the stratigraphy of the climactic deposits both in valley bottoms and on contiguous ridges. Stratigraphic relationships between fall, flow, and surge deposits in the Marella drainage system indicate that during the climactic eruption a progressive shift occurred from an early convective regime, to a transitional regime feeding both the plinian convective column and mostly dilute density currents, to a fully collapsing regime producing mostly dense pyroclastic flows. Syn-plinian dilute density currents (surges) propagated up to ~10 km from the crater, both along valley bottoms and on contiguous ridges of the Marella Valley, whereas post-plinian pyroclastic flows had greater runout (~13 km), were confined to valleys and were not associated with significant surges. Stratigraphic study and grain-size analyses allow the identification of three types of intra-plinian deposits: (a) lower and often coarse-grained surge deposits, emplaced during the accumulation of the coarsest portion of the fallout bed at time intervals of ~16–24 min; (b) upper fine-grained surge deposits, interstratified with the fine-grained portion of the fall bed and emplaced at shorter time intervals of ~3–13 min; and (c) small-volume, channel-confined, massive pumiceous flow deposits interbedded with the upper surges in the upper fine-grained fall bed. Maximum clast size isopleths of 1.6 and 0.8 cm for lithics (ML) and 2.0 and 4.0 cm for pumices (MP) show almost symmetrical distribution around the vent, indicating that the passing of the typhoon Yunya during the climactic eruption had little effect on trajectories of high-Reynold-number clasts. Significant distortion was, however, observed for the 3.2-cm ML and 6.0-cm MP proximal isopleths, whose patterns were probably influenced by the interaction of the clasts falling from column margins with the uprising co-ignimbrite ash plumes. Application of the Carey and Sparks (1986) model to the undisturbed isopleths generated by the umbrella cloud yields a maximum column height of ~42 km, in good agreement with satellite measurements. Systematic stratigraphic and vertical grain-size studies of the plinian fall deposit in the Marella Valley, combined with satellite data and eyewitness accounts, reveal that the carrying capacity of the convective column and related fallout activity peaked in the early phase of the eruption, beginning slightly before 13:41 and gradually declined until its cessation 3 h later. Most of the pumiceous pyroclastic flow deposits were emplaced after the end of the fallout activity at ~16:30 but before the summit caldera collapse at approximately 19:11. Only a small volume of pumiceous flow deposits accumulated after the final caldera collapse. In contrast to the previous reconstruction of Holasek et al. (1996), which interpreted the progressive lowering of the column, documented by satellite data, as due to a decreasing mass eruption rate, we suggest that a progressive shift from a plinian column to a large co-ignimbrite column could also account for such a variation.