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

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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.