Mantle Circulation Constrained (MC2): A multidisciplinary 4D Earth framework for understanding mantle upwellings

The theory of plate tectonics revolutionised the Earth sciences and had impacts across society, by providing a framework to understand the motion of Earth's surface. However, plate tectonic theory does not tell us about the processes deeper in the Earth that drive plate motions, nor does it explain some of the most dramatic events in Earth history: the breakup of plates and outpouring of huge volumes of lava. The next required breakthrough is to make this leap, from a 2D description of plates to understanding the truly 4D nature of Earth's interior processes.

Motion of the Earth's interior, its circulation, involves both upwelling and downwelling. The upwelling flow in the Earth remains enigmatic, occurring in the present-day as both hot focused plumes, which are only just observable through modern seismic imaging techniques, and a hypothesised diffuse flow, which has evaded detection entirely. A third mode of mantle upwelling is currently dormant, making its mantle flow signature unknown. However, this dormant mode of flow drives massive outpourings of lava, and has been associated with continental breakup and mass extinction events.

Our project's overall goal is to constrain how mantle upwellings operate within the Earth. We will investigate how plate tectonics is linked to mantle circulation, by combining the history of plate movements across Earth's surface with observations drawn from across the geosciences, and use these to constrain state-of-the-art 4D computational models of mantle flow.

These advances are made possible by recent progress in disciplines from across the Earth sciences, expertise we bring together here in geodynamics, seismology, geomagnetism, geochemistry, petrology, and thermodynamics. We will constrain present mantle flow by gathering new seismic imaging data of the Earth's deep interior. We will constrain past mantle flow using newly collected data on the mantle's composition, past magnetic field, and the history of Earth's surface uplift. We will use these multidisciplinary approaches to generate the most spatially and temporally complete set of observational constraints on mantle circulation yet assembled.

These observations will be used to constrain and improve models that calculate mantle circulation in an Earth-like 3D geometry, driven by plate motion histories (mantle circulation models, MCMs). This is a timely development capitalising on the only recently available record of plate motion over 1 billion years of Earth History.

The MCMs predict the mantle's temperature, density, and velocity through time, providing a 4D model of the Earth. Uncertain inputs in these models such as mantle viscosity and composition will be investigated within the bounds provided by the project's geochemical and thermodynamic work packages that will develop new models of Earth's high pressure mineralogy and physical properties. We will test the present-day predictions of the MCMs by converting model outputs to predict density and material properties within the Earth, using our developments on mineral physics modelling. With these inputs and constraints, we will create the first accurate computational models of mantle circulation over the last 1 billion years, which will provide dynamical insight into what drives the diversity of upwellings in the Earth.

This project is focussed on fundamental research that will transform our understanding of the evolution of Earth's interior and its manifestation at Earth's surface. The combined geophysical, geochemical and geological expertise across complimentary work packages means that we will generate benefits spanning a range of topics. We identify five key arenas of societal and economic impact.

Short-term beneficiaries (1-5 years and beyond):

[1] School children, teachers, anyone with an interest in science: Geosciences (including Earth surface topics and plate tectonics) is taught as part of UK school curricula from early years to school leavers (Key Stages 1-3). Earth's interior and related surface phenomena are also popular topics during outreach events.

[2] Exploration industries: The exploration industry (e.g. hydrocarbon, minerals) makes use of geological histories and seismology to understand and predict distribution of natural resources (see Letters of Support, LoS).

[3] Infrastructure and sub-surface imaging: Academia and industry have made recent advances in seismic imaging (e.g. full waveform tomography) and in using derivatives to understand dynamical processes (see LoS).

Longer-term beneficiaries:

[4] Sea level and climate change: The development of reliable glacio-eustatic and climatic baselines requires information about how the Earth's interior has evolved on a range of timescales.

[5] Natural Hazards: Most natural hazards (e.g. volcanoes, earthquakes) are ultimately linked to deep-Earth processes.

How might they benefit from this research?

[1] Enhancing the quality of geosciences in school curricula can be achieved by introducing new materials and activities to spark interest of school children and teachers. A focus will be on developing and disseminating 3D printed globes depicting Earth's interior. Project partners, including UCL's GeoBus, reach hundreds of school children every year and will distribute these products widely. We will also build on successful outreach programmes at the universities involved in this consortium to deliver interactive workshops in outreach events.

[2] Crucial constraints for explorationists include histories of uplift, subsidence, basal heat-flow and landscapes evolution. Our project will provide significant new insights into the history of lithospheric dynamic support, which is a fundamental concern at frontier settings and in re-evaluation of exploration targets. Most leading exploration support companies employ sequence stratigraphic frameworks that contain little/no information about vertical motions generated by sub-plate processes, which we will directly address in this project.

[3] The recent explosion in the use of microseismic data and seismic imaging to map the sub-surface will benefit from quantifying uncertainties in seismic imaging, which is a cornerstone of this project, and from the transfer of knowledge and skills developed to mine and model large volumes of data.

[4] Estimates of sea-level change tend to be based on simplifying assumptions about the history of lithospheric vertical motions and the viscosity structure of the mantle. This project will provide insight into histories of dynamic support, and modern and historical estimates of mantle viscosity, which can be used to improve models of glacio-eustasy. The evolution of Earth's surface also plays a crucial role in regulating and perturbing Earth's climate. The histories of dynamic support and mantle temperatures we will generate will provide important constraints for these rapidly evolving fields.

[5] Mantle and lithospheric temperatures and stresses play crucial roles in determining the distribution of natural hazards (e.g. volcanism, seismicity). This project will generate new insights into the thermal and temporal evolution of Earth's convecting interior, which will provide insights into the evolution and origins on natural hazards.