Downward-propagating eruption following vent unloading implies no direct magmatic trigger for the 2018 lateral collapse of Anak Krakatau
Downward-propagating eruption following vent unloading implies no direct magmatic trigger for the 2018 lateral collapse of Anak Krakatau
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DOI：
10.1016/j.epsl.2021.117332
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发表时间：
2021-12
影响因子：
5.3
通讯作者：
K. Cutler;S. Watt;M. Cassidy;A. Madden-nadeau;S. Engwell;M. Abdurrachman;Muhammad Edo Marshal Nurshal;D. Tappin;S. Carey;A. Novellino;C. Hayer;J. Hunt;S. Day;S. Grilli;I. A. Kurniawan;N. Kartadinata
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作者：
K. Cutler;S. Watt;M. Cassidy;A. Madden-nadeau;S. Engwell;M. Abdurrachman;Muhammad Edo Marshal Nurshal;D. Tappin;S. Carey;A. Novellino;C. Hayer;J. Hunt;S. Day;S. Grilli;I. A. Kurniawan;N. Kartadinata
2018年12月印度尼西亚Anak Krakatau火山的横向坍塌突出了火山建筑物不稳定的潜在破坏性影响。尽管如此，Anak Krakatau崩盘的导火索仍然不明。火山在过去六个月里一直在喷发，尽管在失败之后发生了强烈的爆炸活动，但正是紧接着坍塌之前的那段时间，可能是提供可识别的坍塌前预警信号的关键。在这里，我们综合了横跨崩塌时期的特弗拉矿床的物理、微观结构和地球化学特征。我们论证了第一个崩塌后喷发阶段(喷发的幼年碎屑具有较低的微晶面数密度和相对较大的微岩，反映了晶体生长主导的状态)，最好的解释是相对停滞的上层管道的瞬时卸荷。紧随其后的是第二个崩塌后阶段，时间尺度为几个小时，这一阶段依次挖掘更热和更深的岩浆批次，反映在斜长石钙长岩含量和更多镁铁质玻璃成分的增加，以及计算出的更高的上升速度和减压速率。崩塌后产物的特征意味着岩浆储存系统向下传播的不稳定作用是对崩塌的响应，而不是崩塌前岩浆上升触发的破坏。重要的是，这表明坍塌是与建筑物生长和不稳定有关的长期过程的结果，岩浆系统的任何指示性变化都不可能预示着早期崩溃的可能性。因此，监测工作可能需要侧重于整合短期和长期的建筑物生长和变形模式，以确定侧向倒塌的易感性增加。坍塌后的喷发模式还表明，岩浆加压制度对地表驱动的扰动高度敏感，导致坍塌后岩浆流量增加，并迅速重新生长。快速再生不仅可能掩盖过去坍塌的证据，而且它还强调了建筑物加载和地壳岩浆储存之间的微妙平衡关系。
The lateral collapse of Anak Krakatau volcano, Indonesia, in December 2018 highlighted the potentially devastating impacts of volcanic edifice instability. Nonetheless, the trigger for the Anak Krakatau collapse remains obscure. The volcano had been erupting for the previous six months, and although failure was followed by intense explosive activity, it is the period immediately prior to collapse that is potentially key in providing identifiable, pre-collapse warning signals. Here, we integrate physical, microtextural and geochemical characterisation of tephra deposits spanning the collapse period. We demonstrate that the first post-collapse eruptive phase (erupting juvenile clasts with a low microlite areal number density and relatively large microlites, reflecting a crystal-growth dominated regime) is best explained by instantaneous unloading of a relatively stagnant upper conduit. This was followed by the second post-collapse phase, on a timescale of hours, which tapped successively hotter and deeper magma batches, reflected in increasing plagioclase anorthite content and more mafic glass compositions, alongside higher calculated ascent velocities and decompression rates. The characteristics of the post-collapse products imply downward propagating destabilisation of the magma storage system as a response to collapse, rather than pre-collapse magma ascent triggering failure. Importantly, this suggests that the collapse was a consequence of longer-term processes linked to edifice growth and instability, and that no indicative changes in the magmatic system could have signalled the potential for incipient failure. Therefore, monitoring efforts may need to focus on integrating short- and long-term edifice growth and deformation patterns to identify increased susceptibility to lateral collapse. The post-collapse eruptive pattern also suggests a magma pressurisation regime that is highly sensitive to surface-driven perturbations, which led to elevated magma fluxes after the collapse and rapid edifice regrowth. Not only does rapid regrowth potentially obscure evidence of past collapses, but it also emphasises the finely balanced relationship between edifice loading and crustal magma storage.