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首页> 外文期刊>Journal of Volcanology and Geothermal Research >Spatiotemporal evolution of a marine caldera-forming eruption, generating a low-aspect ratio pyroclastic flow, 7.3 ka, Kikai caldera, Japan: Implication from near-vent eruptive deposits
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Spatiotemporal evolution of a marine caldera-forming eruption, generating a low-aspect ratio pyroclastic flow, 7.3 ka, Kikai caldera, Japan: Implication from near-vent eruptive deposits

机译：火山口形成火山喷发的时空演化，产生低纵横比的火山碎屑流，7.3 ka，日本Kikai火山口：来自临近喷发沉积的隐含意义

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The VEI 7, 7.3 ka caldera-forming eruption of the Kikai caldera occurred in a shallow sea, and caused devastating damage to the prehistoric human settlements of southern Kyushu, Japan. This Holocene activity at this volcano records eruptions with compositional range of 56 to 74 wt.% in SiO_2 spanning with the climactic eruption. In this study, stratigraphy, component, and lithology of the pyroclastic deposits were analyzed at near-vent islands (Satsuma Iwo-jima and Take-shima) in order to reconstruct this eruption. Stratigraphic sections are characterized by plinian pumice-fall deposits (Unit A), intraplinian flow deposits (Unit B), climactic pyroclastic flow deposits (Units C1-C3), and co-ignimbrite ash-fall deposits (Unit D). In total the estimated magma volume in the system was 70-80 km~3 (DRE) and the eruption therefore represents the evacuation of a major silicic magmatic system. The plinian stage (Phase 1) is subdivided into an initial small phase and a second large one. The column height in the second phase was estimated to be 40-43 km. The total tephra volume of this stage was estimated to be 40 km~3. The magma discharge rate has been calculated from the column height data to be 2 × 10~8 kg/s. The eruption duration is also estimated to be a minimum of approximately 28 h. Collapse of the column (Phase 2) produced Unit B, which consists of multiple thin lithic-rich or pumice-rich layers or pods, including welded pumice-fall layers. The deposits are characterized by stratified or cross-stratified facies and display various degree of welding. These sedimentary characteristics indicate that, during the plinian column collapse, high temperature turbulent density currents were generated where dense pyroclasts were well segregated, resulting in the lithic-rich layers or pods. Phase 3 is characterized by Units C1-C3. Unit C1 shows nonwelded stratified facies, which consist of lithic and crystals, including quenched juvenile materials as a minor constituent. Unit C2 displays welded stratified facies, which consist of lithic-rich layers and pumice-rich layers. These two subunits occur only in topographic lows in Satsuma Iwo-jima. Unit C3 is thickest and poorly-sorted non-welded massive deposit, which includes fragments of welded tuff from underlying units in proximal regions. These facts indicate that multiple pyroclastic density currents produced Units C1 and C2 in the near-vent area, and were followed by the main sustained current producing Unit C3, a low-aspect ratio ignimbrite, distributed over a wide area of southern Kyushu across the sea. Varying extents of magma-water interactions started during Phase 2, continuing during the early stages of Phase 3, and diminished during the climactic C3 ignimbrite stage. In addition, collapse of the caldera may have started before Unit C deposition, based on the evidence of a fault overlain by Unit C on the caldera rim. The collapse may have initiated water access to the magma. The source appears to have been biased toward the western side of the caldera. The Holocene evolution of the Kikai volcano records the existence of a large silicic magma system at depths of about 7 km that coexisted with or was regularly recharged with mafic magma.
机译：Kikai破火山口的VEI 7、7.3 ka破火山口喷发发生在浅海中，对日本九州南部的史前人类住区造成了毁灭性破坏。该火山的全新世活动记录了伴随着气候喷发的SiO_2组成范围为56至74 wt。％的喷发。在这项研究中，为了重建这种喷发，分析了在近排风岛（萨摩硫磺岛和竹岛）的火山碎屑沉积的地层，成分和岩性。地层剖面的特征是：普利尼亚浮石沉积物（A单元），内斜岩流沉积物（B），高热火山碎屑流沉积物（C1-C3单元）和共着火灰分沉积物（D单元）。估计系统中的岩浆总量为70-80 km〜3（DRE），因此喷发代表了主要硅质岩浆系统的疏散。普利尼阶（阶段1）细分为初始小阶段和第二大阶段。第二阶段的柱高估计为40-43 km。该阶段的总提夫拉体积估计为40 km〜3。根据柱高数据计算出的岩浆排出速率为2×10〜8 kg / s。估计喷发持续时间至少约为28小时。色谱柱的塌陷（第2阶段）产生了单元B，该单元B由多个薄的富含石屑的层或富含浮石的层或豆荚（包括焊接的浮石降层）组成。沉积物的特征是分层或交叉分层的相，并显示出不同程度的焊接。这些沉积特征表明，在p系柱塌陷期间，高温湍流密度流在致密的火山碎屑充分隔离的地方产生，从而形成了富含岩性的层或荚。阶段3的特征是单元C1-C3。 C1单元显示了非焊接的分层相，它由石块和晶体组成，包括淬火的少年材料作为次要成分。 C2单元显示焊接的分层相，由富岩层和富浮石层组成。这两个亚基仅出现在萨摩硫磺岛的地形低点。 C3单元是最厚且分类不良的非焊接块状沉积物，其中包括来自近端区域下层单元的焊接凝灰岩碎片。这些事实表明，多次火山碎屑密度流在附近的通气区产生了C1和C2单元，随后是主要的持续电流产生单元C3，这是一个低纵横比的火成岩，分布在整个九州南部的大范围海域上。岩浆-水相互作用的程度有所不同，从阶段2开始，持续到阶段3的早期，而在C3的高火成岩阶段则减弱。此外，基于C单元在破火山口边缘上覆盖的断层证据，C单元沉积之前可能已开始破火山口塌陷。坍塌可能导致水进入岩浆。来源似乎偏向破火山口的西侧。 Kikai火山的全新世演化记录了在约7 km深度处存在着大型硅质岩浆系统，该系统与镁铁质岩浆共存或定期充注。

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