Convection clashes: Plume splitting beneath eastern Australia

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Convection of the Earth's mantle is a fundamental dynamic process that profoundly influences the surface of our planet, affecting processes as diverse as plate tectonics, long-term sea level change, and climate. Convection requires a balance between material sinking deep into the mantle and rising towards the surface. Whilst we know that downwelling material is dominated by subducting slabs that eventually sink thousands of km into the mantle, the locations, durations, and dynamics of the required deep upwelling material are much more ambiguous. The best-known surface indication of such upwelling is intraplate volcanism (volcanism located far from plate boundaries), classically associated with plumes (or 'hotspots') of hot material rising from the lower mantle. Particularly voluminous examples of intraplate volcanism occur when the broad heads of these plumes reach the surface to produce large igneous provinces (LIPs). These LIPs affect the world's atmosphere via the release of massive amounts of gases like sulphuric acid, changing the climate with damaging effects on the ocean, atmosphere, and biology (i.e., mass extinctions). The trails of hotspot volcanoes that come after the LIP have also proved a powerful tool in discovering the past motions of tectonic plates. For these reasons, understanding the origins and evolution of intraplate volcanism is an important part of Earth science.The classic example of hotspot intraplate volcanism is Hawaii on the Pacific plate: a series of volcanic islands and submerged undersea mountains ('seamounts') that stretches away to the northwest, becoming progressively older the further they are from the actively erupting island of Hawaii. However, intraplate volcanism on Earth is very diverse. Many localities do not fit the classic model of a hot plume rising from the deep mantle, but instead appear to have been caused by processes in the upper mantle or have a mix of deep and shallow characteristics.For this project, the seas off Eastern Australia are an ideal region for studying the processes involved in the formation of intraplate volcanism. This region is crossed by not one, but three sub-parallel chains of intraplate volcanoes, which erupted simultaneously between 35 and 6 million years ago. These volcanoes are up to 5 km high and 100 km across, and are almost entirely submerged beneath the ocean. The long life and exceptional age progression of the chains are strong indicators of a classic deep upwelling source, but the configuration of the three chains challenges our understanding of this fundamental driving force of our planet. Neither three closely spaced plumes (~500 km apart) nor an upwelling sheet fit well with our understanding of the underlying physics: they are either unstable or are not observed in models of Earth's mantle convection. Instead, these observations suggest a deep upwelling splitting as it nears the surface, perhaps due to obstacles in the mantle, or eddies in the mantle convection.This proposal builds on a collaboration with Australia, who has already funded a 28 day voyage (worth ~£1.8 million) to collect rock samples and carry out geophysical studies. The voyage will target the two marine chains, as well as the Louisiade Plateau (a 100,000 square km area of raised seafloor that could be a LIP) north of the Tasmantids. We will study these volcanoes using a multi-faceted approach combining chronology (to determine their ages), chemistry (to determine what type of mantle melted), and geodynamic modelling (to examine the processes in the mantle that formed the volcanoes). The geodynamic models will also be applied to the Canary and Comoros Islands (west and east of Africa, respectively) to examine the mechanisms behind intraplate volcanoes elsewhere on the planet. This project will give us significant insight into the formation of enigmatic intraplate volcanism and how material flowing from deep in the Earth's mantle interacts with obstacles as it rises.