Scales and frequencies of Snake-River type super-eruptions of the Yellowstone hot-spot track, USA

这个项目涉及地球上最大的爆发。超级火山爆发是影响地球表面的最具灾难性的事件之一：它们会产生直接的、毁灭性的区域环境后果，并影响全球气候。为了评估它们在地壳演化、区域或全球环境变化方面的作用，我们首先需要更多地了解它们发生的频率、规模和喷发风格。要做到这一点，我们必须查阅地质记录。但是，尽管最近引起了人们的兴趣，但已知的例子相对较少，详细研究的例子更少。大多数工作都是在遥远的地方重建单个例子（例如印度尼西亚的多巴火山）。我们计划一个不同的和互补的方法：量化在一个单一的大地构造背景下，在一个延长的时期内连续的大喷发的数量、频率和体积。我们对大陆弧在相对较短的时间内的大喷发频率有所了解（例如陶波火山带；圣胡安山），但对大陆板内环境的超级喷发频率几乎没有定量研究——尽管人们越来越认识到大火成岩省内硅火山作用的重要性。众所周知，美国黄石公园最近发生了灾难性的喷发，但可能很少有人意识到，这只是一系列非常大的爆发中的最后一次，从16世纪到现在，这些爆发沿着斯内克河向东燃烧了一条从俄勒冈州到黄石公园的轨道。有些早期的火山喷发可能和黄石公园的火山喷发一样大，如果不是更大的话，但我们对它们的了解少得惊人：我们甚至不知道发生了多少次，也不知道它们的真实程度。尽管该地区是地球上最大、最容易到达、保存最完好的硅火山省之一；然而，它的大片地区仍未被研究，整体地层也没有得到解决。在一次超级喷发中喷出的岩浆被粉碎，进入巨大的环绕地面的密度流，沉积了厚厚的火山灰层，称为“火成岩”。在斯内克河平原反复发生的类似喷发产生了许多大致相似的火成岩，这些火成岩彼此之间很难区分。因此，我们将局部序列划分为“喷发单元”，然后结合详细的物理和化学分析、年代测定和古地磁工具对每个单元进行详细的指纹鉴定；例如，记录晶体和玻璃化学成分的垂直变化。初步的野外工作、矿物化学和40Ar-39Ar已经使我们能够将相邻范围内的两个单元联系起来，并使我们有信心预测，计划中的详细方法将第一次使我们能够将远距离的单个单元联系起来，从而确定喷发的数量和体积。年龄数据与每个单元的古地磁数据将提供一个新的区域时间-事件框架，以解释该省的喷发历史，并评估大型爆炸喷发的数量和频率是否随时间而变化，包括与黄石地区已知数据的比较。火山活动的地点随着时间向东移动，使我们能够考虑空间变化以及与地壳变形的可能联系。这项工作将有助于进一步了解是什么控制了如此大规模的火山喷发（例如，深度的热通量？），岩浆产生和储存的速度，以及岩浆活动与地壳之间的相互关系。该多学科研究团队在方法上有丰富的经验，并在哥伦比亚河-黄石火山省。首次制定一个长期需要的区域时间-事件框架，并将不同的研究小组聚集在一起，将成为广泛进一步研究的催化剂。

This project concerns the largest explosive eruptions on Earth. Super-eruptions are amongst the most catastrophic events that affect the Earth's surface: they have immediate and devastating regional environmental consequences and affect global climate. To assess their role with regard to crustal evolution, regional or global environmental change, we first need to need to know more about how often they occur, at what scales, and about the eruption styles. To do this we must consult the geologic record. But despite recent interest, relatively few examples are known and still fewer have been studied in detail. Most work has been on reconstructing single examples in far-flung locations (e.g. Toba volcano in Indonesia). We plan a different and complementary approach: to quantify the number, frequencies and volumes of successive large eruptions over an extended period at a single geotectonic setting. We know something about the frequencies of large eruptions during relatively short periods of time at continental arcs (e.g. Taupo Volcanic Zone; San Juan Mts) but there is virtually no quantitative work on super-eruption frequencies in the continental intraplate setting - despite a growing realisation of the importance of silicic volcanism within Large Igneous Provinces. It is well-known that Yellowstone, USA, has erupted catastrophically in recent times, but perhaps less widely appreciated is that this was just the last of a series of numerous very large explosive eruptions that burned a track along the Snake River eastwards from Oregon to Yellowstone from 16 Ma to present. Some of the earlier eruptions were probably as large as at Yellowstone, if not larger, but we know astonishingly little about them: we do not even know how many occurred, or their true extents. This is despite the region being one of the Earth's largest, most accessible, and best-preserved silicic volcanic provinces; yet large tracts of it remain unstudied and the overall stratigraphy has not been resolved. Magma erupted in a super-eruption is pulverised and enters vast ground-hugging density currents that deposit thick layers of ash, called 'ignimbrites'. Repetition of similar eruptions in the Snake River Plain has produced many broadly similar ignimbrites that are not simple to tell apart from one another. Therefore, we shall divide local successions into 'eruption-units' and then fingerprint each unit in detail using a combination of detailed physical and chemical analysis, dating, and palaeomagnetic tools; for example, recording any distinctive vertical variations in the chemistry of the crystals and glasses. Preliminary fieldwork, mineral chemistry and 40Ar-39Ar has already allowed us to correlate two units in adjacent ranges and gives us confidence to predict that the detailed approach planned will, for the first time, let us to correlate individual units across large distances, and so establish the number and volumes of the eruptions. The age data together with the palaeomagnetic data for each unit will provide a new regional time-event framework, to interpret the province's eruption history and assess whether the volumes and frequencies of large explosive eruptions have changed through time, including comparison with the known data from the Yellowstone area. The site of volcanism shifted eastwards with time, enabling us to consider spatial variations and possible links with crustal deformation. The work will add to future understanding about what controls eruptions on this large scale (e.g. heat flux at depth?), about rates of magma generation and storage, and the inter-relationships between magmatism and the crust. The multidisciplinary research team has much experience in the methods to be employed, and in the Columbia River - Yellowstone Volcanic Province. Developing, for the first time, a long-needed regional time-event framework, and drawing together the different research groups will act as a catalyst for extensive further research.