DataSheet3_Complex styles of phreatomagmatic explosions at Kīlauea Volcano, Hawaii, controlled by magma structure.pdf

Explosive eruptions at basaltic volcanoes remain poorly understood. Kīlauea Volcano is a type locality for basaltic eruptions and is well-known for effusive activity. However, more than 7 m of phreatomagmatic Keanakākoʻi Tephra unit D deposits from explosive eruptions crown the southern rim of the summit caldera and provide a stark reminder of Kīlauea’s explosive past and future potential. We used detailed field observations as well as granulometric and morphological analysis of 100 samples from two proximal sections to assess the eruption style and fragmentation mechanism. The deposits can be divided into four subunits, six different lithofacies, and contain three juvenile tephra components. Each juvenile component shows distinct shape variability resulting from molten fuel-coolant interaction (MFCI) explosions of magma of variable vesicularity. Fragmentation of dense glass generates olive-green ash, fragmentation of low to moderately vesicular magma generates a dark gray ash-lapilli component, and fragmentation of highly vesicular magma generates light-yellow pumice. Our work shows that magma structure impacts MFCI explosion efficiency. Small-scale planar bedding throughout most of the deposit points to a general eruption style of small, frequent explosions generating low plumes. Thicker beds of accretionary lapilli of fine-extremely fine ash are related to very efficient magma-water mixing. Pyroclastic density current (PDC) deposits in the upper part of the stratigraphy contain at least three flows but show no significant dune or cross-bedding structures. We suggest that this is a function of the vent being situated in a caldera that was then ∼600 m deep, where the caldera wall acted as a barrier and changed the flow dynamics to very dilute overspills and co-PDC plume falls over the wall. Deconvolution modeling of the polymodal grain size distributions is used to assess grain size changes of each juvenile component for this deposit, which greatly improves interpretation of lithofacies generation and eruption dynamics. Size-correlated shape parameters show that shape data across a wide size range are needed to accurately track grain shapes. This study demonstrates how careful examination of grain size and shape of juvenile tephra clasts can help volcanologists understand how effusive basaltic volcanoes can become violently explosive.

玄武岩火山爆发性喷发仍处于理解不足的状态。基拉韦厄火山是玄武岩喷发的典型地区，以其溢流活动而闻名。然而，超过7米的喷泉式-爆炸式基安纳卡科伊火山灰单元D沉积物，由爆发性喷发形成，覆盖了山顶火山口的南部边缘，成为基拉韦厄火山过去爆炸性喷发及未来潜在爆炸性的鲜明提醒。我们利用详细的实地观察以及来自两个邻近剖面100个样品的粒度学和形态学分析，评估了喷发式样和破碎机制。沉积物可分为四个亚单元、六个不同的岩性层，并含有三个幼年期火山灰成分。每个幼年期成分都显示出由熔融燃料-冷却剂相互作用（MFCI）爆炸导致的熔岩多孔性变化所引起的形状多样性。致密玻璃的破碎生成橄榄绿色火山灰，低至中多孔熔岩的破碎生成深灰色火山灰-角砾成分，而高度多孔熔岩的破碎则生成浅黄色火山弹。我们的研究显示，熔岩结构影响着MFCI爆炸效率。在整个沉积物中，小型平面层理的广泛存在表明普遍的喷发式样为小型、频繁的爆炸，产生低矮的火山羽流。较厚的堆积层状角砾，由细至超细火山灰组成，与非常有效的熔岩-水混合有关。在层序的上部，火山碎屑密度流（PDC）沉积物至少包含三个流层，但未显示出显著的风成沙丘或交错层理结构。我们认为这可能是由于喷口位于当时约深600米的火山口内，火山口壁作为屏障，改变了流动动力学，形成了非常稀释的溢流和共PDC羽流。使用多模态粒度分布的反卷积建模来评估该沉积物每个幼年期成分的粒度变化，极大地提高了对岩性层形成和喷发动力学的解释。尺寸相关的形状参数显示，需要广泛尺寸范围内的形状数据来准确追踪颗粒形状。这项研究展示了仔细检查幼年期火山灰碎屑的粒度和形状如何帮助火山学家理解溢流玄武岩火山如何变为剧烈的爆炸性火山。