DataSheet4_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点，以其溢流活动而闻名。然而，超过7米的喷泉式喷发形成的Keanakākoʻi Tephra单元D沉积物，由爆发性喷发所致，覆盖在火山口南缘，成为基拉韦厄火山爆炸过去和未来潜在能力的鲜明提醒。我们利用详尽的实地观测以及来自两个邻近部位的100个样本的粒度学和形态特征分析，以评估喷发方式和碎裂机制。这些沉积物可分为四个亚单元、六个不同的岩性相，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）爆炸造成的形状可变性，这些爆炸是由不同泡孔率的岩浆引起的。致密的玻璃碎裂产生橄榄绿色灰，低至中泡孔率的岩浆碎裂产生深灰色灰-角砾成分，而高泡孔率的岩浆碎裂产生浅黄色火山弹。我们的研究显示，岩浆结构影响着MFCI爆炸效率。沉积物中的大部分小型平面层理表明，一般喷发方式为小规模、频繁的爆炸，产生低矮的羽流。较厚的积聚角砾层，由细至超细灰组成，与非常有效的岩浆-水混合有关。在层状地层的上部，火山碎屑密度流（PDC）沉积物至少包含三个流，但未显示出明显的沙丘或交错层理结构。我们推测，这是由于喷口位于当时深度约为600米的火山口内，火山口壁作为屏障改变了流动动力学，导致非常稀释的超溢和共PDC羽流跨越壁面。通过去卷积模型对多模态粒度分布的分析，评估了该沉积物中每个幼年期成分的粒度变化，这大大改善了岩性相生成和喷发动态的解释。与尺寸相关的形状参数显示，需要跨广泛尺寸范围的形状数据，以准确追踪颗粒形状。本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得极具爆炸性。