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0403-091火山喷发，产生了200平方公里的浮石筏——一种高度多孔、浮力强的火山岩，可以在水中漂浮数周，在此期间，它随着风和洋流的方向漂移，分散在海洋周围。这艘浮石筏已经到达斐济，预计在未来几个月将到达瓦努阿图、新喀里多尼亚，最终到达澳大利亚。浮石筏为许多海洋物种提供了一个家园，这些物种开始在漂浮的浮石上定居，这种浮石筏引起了全球的兴趣，因为它有可能运输包括珊瑚在内的物种，远至澳大利亚大堡礁。浮石木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：过去的研究表明，浮石木筏会磨损船体，堵塞或损坏螺旋桨和方向舵，阻塞进水口（导致发动机故障）——即使是大型货船。因此，未来浮石筏进入主要航道可能会造成严重的经济混乱，就像2010年欧洲航空火山灰危机一样。为了准备应对此类事件的减灾策略，我们需要更好地了解哪种类型的喷发（因此哪些火山）会产生漂浮的浮石，并提高我们预测木筏将通过洋流到达何处的能力。这次喷发为我们提供了一个获得关键数据的独特机会，这些数据将帮助我们了解浮石筏是如何形成的，它们在漂洋过海洋时的物理特性是如何变化的，以及产生浮石筏的喷发是如何影响喷发地点和浮石筏到达的遥远地区的生态系统和生物地球化学过程的。我们建议参观火山，并使用远程操作车辆（ROV）对喷发口和浅层火山山顶进行海底调查，该车辆可以记录视频并收集样本，并使用带有附加摄像头的疏浚系统从更深的位置收集额外的样本。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喷发和热液活动的影响。来自火山口附近（从未漂浮过）和6公里外（下沉前漂浮过一段时间）的海底喷发产物样本将与水手在火山喷发后一周内收集的浮筏浮石样本进行比较。通过分析这些不同样品的化学成分（岩浆成分、气体含量）和物理特征（碎屑大小、气泡连通性），我们可以研究控制火山是否能产生浮石筏的喷发过程。我们还打算在浮石漂浮几周后，在附近的海岸线（如斐济、新喀里多尼亚）被冲上来时对浮石进行取样。这将揭示浮石的特征（如大小、形状、浮力）在扩散过程中是如何变化的，这将有助于我们改进浮石筏扩散模型和危害评估。它还将揭示在筏子上定居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。重要的是在火山喷发后，在浅层喷口被波浪/气旋作用改变（可以侵蚀和重新分配喷发产物）之前，在海底生态系统仍处于对火山喷发和热液活动做出反应的状态时，尽快进行这项调查和采样。这次喷发和它的木筏被非常好的记录下来，例如，我们有清晰的木筏散布的卫星图像，而过去的木筏被云层掩盖了。因此，获得具有良好约束条件的海底和浮动样本是一个极其难得和宝贵的机会；这种机遇的品质在未来几十年里不太可能再次出现。

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.