The August 2019 Tongan 18.325oS/174.365oW submarine volcanic eruption: eruptive processes and pumice raft formation and evolution

2019年8月7日，汤加附近的一座浅海底火山火山0403-091爆发，产生了一个200平方公里的浮石筏-一种高度多孔，浮力很大的火山岩，可以在水中漂浮数周，在此期间，它随着风和水流的方向漂移而分散在海洋周围。这个漂浮的浮石筏已经到达斐济，预计在未来几个月内将到达瓦努阿图、新喀里多尼亚，最终到达澳大利亚。浮石筏为众多海洋物种提供了家园，这些海洋物种开始在漂流的浮石上定居，这个木筏引起了全球的兴趣，因为它有可能将包括珊瑚在内的物种运送到澳大利亚大堡礁。浮石筏也引起了科学家和社会的兴趣，因为它们有可能损坏船只：过去的筏已经证明它们可以磨损船体，堵塞或损坏螺旋桨和方向舵，并阻塞进水口（导致发动机故障）-即使是大型货船。因此，未来进入主要航道的浮石筏可能会造成严重的经济破坏，就像2010年欧洲航空火山灰危机一样。为了为这样的事件准备减灾策略，我们需要更好地了解什么样的火山爆发（因此是哪些火山）可以产生漂浮的浮石，并提高我们预测木筏将通过洋流到达哪里的能力。 这次喷发为我们提供了一个独特的机会，以获得关键的数据，这将有助于我们了解浮石筏是如何形成的，它们的物理特性如何变化，因为它们漂过海洋，以及如何浮石筏生产的喷发影响生态系统和生物地球化学过程都在喷发现场和木筏到达的遥远地区。我们提议访问该火山，利用遥控潜水器对喷发口和浅火山山顶进行海底调查，记录视频和收集样本，并利用附带摄像机的挖掘系统从更深的地方收集更多样本。这将使我们能够绘制火山的新形态和海底沉积物，并记录生活在火山上的物种以及它们如何受到火山爆发和热液活动的影响。来自喷口附近（从未漂浮）和6公里以外（在下沉前漂浮了一段距离）的海底喷发产物样品将与水手在喷发一周内收集的浮筏浮石样品进行比较。通过分析这些不同样品的化学成分（岩浆成分，气体含量）和比较这些不同样品的物理特性（碎屑大小，气泡连通性），我们可以研究控制火山是否可以产生浮动浮石筏的喷发过程。我们还打算在漂流数周后，在附近的海岸线（如斐济，新喀里多尼亚）冲洗筏浮石时对其进行采样。这将揭示浮石特征（例如尺寸、形状、浮力）在散布过程中如何变化，这将帮助我们改进浮石筏散布和危险评估的模型。它还将揭示在木筏上殖民的海洋物种的数量和类型的时间变化，使我们能够评估哪些物种可能被运送到澳大利亚大堡礁。必须在火山爆发后，在浅水喷口可能被波浪/气旋作用（可能侵蚀和重新分配火山喷发产物）改变之前，在海底生态系统仍处于对火山爆发和热液活动作出反应的状态时，尽快进行这一调查和取样。这次喷发和它的木筏已经有了非常好的记录，例如，我们有清晰的木筏分散的卫星图像，而过去的木筏已经被云层所掩盖。因此，这是一个极其难得和宝贵的机会，可以在严格的限制条件下获得海底和漂浮样品;这种机会在未来几十年内不太可能再次出现。

The August 7th 2019 eruption of Volcano 0403-091, a shallow submarine volcano near Tonga, produced a 200 km2 raft of pumice - a highly porous, buoyant volcanic rock that can float in water for many weeks, during which time it is dispersed around the oceans as it drifts in the direction of winds and currents. This floating pumice raft has already reached Fiji and is expected to reach Vanuatu, New Caledonia and ultimately Australia in the coming months. Pumice rafts provide a home for numerous marine species that begin to colonise the drifting pumice, and this raft has sparked global interest because of its potential to transport species, including corals, as far as the Australian Great Barrier Reef. Pumice rafts are also of interest to scientists and society because of their potential to damage ships: past rafts have demonstrated that they can abrade hulls, jam or damage propellers and rudders, and block water intakes (causing engine failure) - even for large cargo ships. A future pumice raft that enters a major shipping lane could therefore cause severe economic disruption, just like the 2010 European aviation ash crisis. To prepare hazard mitigation strategies for such an event, we need to better understand what kinds of eruptions (hence which volcanoes) can create floating pumice, and improve our ability to forecast where rafts will travel to via ocean currents. This eruption has provided us with a unique opportunity to gain crucial data that will help us to understand how pumice rafts are formed, how their physical characteristics change as they drift across the ocean, and how pumice raft-producing eruptions impact ecosystems and biogeochemical processes both at the eruption site and in distant regions reached by the raft. We propose to visit the volcano and perform a seafloor survey of the eruptive vent and shallow volcano summit using a Remotely Operated Vehicle (ROV) that records video and collects samples, as well as additional sample collection from deeper locations using a dredge system with attached camera. This will enable us to map the new morphology and seafloor deposits of the volcano, and document the species living on the volcano and how they have been affected by the eruption and hydrothermal activity. Samples of seafloor eruptive products from near the vent (which never floated) and up to 6 km away (which floated some way before sinking) will be compared with samples of floating raft pumice collected by sailors within one week of the eruption. By analysing the chemistry (magma composition, gas contents) and comparing the physical characteristics (clast size, bubble connectivity) of these different samples we can investigate the eruption processes that control whether a volcano can produce a floating pumice raft. We also intend to sample raft pumice when it washes up on nearby coastlines (e.g. Fiji, New Caledonia) after several weeks of floating. This will reveal how pumice characteristics (e.g. size, shape, buoyancy) change during dispersal, which will help us improve models of pumice raft dispersal and hazard assessment. It will also reveal the temporal change in the number and type of marine species that colonise the raft, allowing us to assess which species may be transported to the Australian Great Barrier Reef. It is important to undertake this survey and sampling as soon as possible after the eruption, before the shallow vent can be altered by wave/cyclonic action (which can erode and redistribute eruption products) and while the seafloor ecosystem is still in a state of responding to the eruption and hydrothermal activity. This eruption and its raft have been unusually well documented e.g. we have clear satellite images of raft dispersal, whereas past rafts have been hidden by clouds. It is therefore an extremely rare and valuable opportunity to obtain both seafloor and floating samples with excellent constraints; a quality of opportunity that is unlikely to occur again in coming decades.