Pyroclastic flows and surges over water: an example from the 1883 Krakatau eruption
Pyroclastic flows and surges over water: an example from the 1883 Krakatau eruption
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火山碎屑流和水面涌动：以 1883 年喀拉喀托火山喷发为例
DOI：
10.1007/bf00304435
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发表时间：
1996
影响因子：
3.5
通讯作者：
S. Bronto
中科院分区：
文献类型：
--
作者：
S. Carey;H. Sigurdsson;C. Mandeville;S. Bronto
1883年喀拉喀托火山爆发时的火山碎屑沉积物被描述为火山东北部的Sebesi岛、Sebuku岛和Lagoendi岛以及苏门答腊岛的东南海岸。大块且分层不良的单元主要由火山碎屑流和浪涌形成，这些火山碎屑流和浪涌在海上移动了80公里。沉积物的粒度和岩性特征表明，它们是位于喀拉喀托群岛及其附近的近陆和海底火山碎屑流沉积物的补充。随着离喀拉喀托的距离增加，远端沉积物的分选系数、中位粒度和厚度均呈下降趋势。晶体分选与远端相相一致，远端相来源于重力分选火山碎屑流的上部，其中晶体富集的相对量和致密岩屑的丰度向上递减。这些矿床与8月27日上午10点左右发生的一次主要火山碎屑流相有关。能量流以超过100公里/小时的速度从火山向外扩散，并从源头传播了80公里。水流保持的温度高到足以烧伤苏门答腊岛西南海岸的受害者。巽他海峡船只的历史记录将受水流影响的区域限制在至少4 × 103平方公里。在该区域的远端边缘，水流相对较稀且湍流，但携带了足够的物质，沉积了几十厘米厚的tephra。从上午10点开始，喀拉喀托流的巨大流动性可能是海上跳动增强的结果。有人提出，在流动/海水界面上产生的蒸汽可能导致了颗粒沉降的减少，从而延迟了流动停止横向运动并成为浮力对流的时间。这些充满火山灰和蒸汽的云的远端浮力上升到大气中，导致冷却、凝结和泥雨。
Pyroclastic deposits from the 1883 eruption of Krakatau are described from areas northeast of the volcano on the islands of Sebesi, Sebuku, and Lagoendi, and the southeast coast of Sumatra. Massive and poorly stratified units formed predominantly from pyroclastic flows and surges that traveled over the sea for distances up to 80 km. Granulometric and lithologic characteristics of the deposits indicate that they represent the complement of proximal subaerial and submarine pyroclastic flow deposits laid down on and close to the Krakatau islands. The distal deposits exhibit a decrease in sorting coefficient, median grain size, and thickness with increasing distance from Krakatau. Crystal fractionation is consistent with the distal facies being derived from the upper part of gravitationally segregated pyroclastic flows in which the relative amount of crystal enrichment and abundance of dense lithic clasts diminished upwards. The deposits are correlated to a major pyroclastic flow phase that occurred on the morning of 27 August at approximately 10 a.m. Energetic flows spread out away from the volcano at speeds in excess of 100 km/h and traveled up to 80 km from source. The flows retained temperatures high enough to burn victims on the SW coast of Sumatra. Historical accounts from ships in the Sunda Straits constrain the area affected by the flows to a minimum of 4x103 km2. At the distal edge of this area the flows were relatively dilute and turbulent, yet carried enough material to deposit several tens of centimeters of tephra. The great mobility of the Krakatau flows from the 10 a.m. activity may be the result of enhanced runout over the sea. It is proposed that the generation of steam at the flow/seawater interface may have led to a reduction in the sedimentation of particles and consequently a delay in the time before the flows ceased lateral motion and became buoyantly convective. The buoyant distal edge of these ash-and steam-laden clouds lifted off into the atmosphere, leading to cooling, condensation, and mud rain.