The Scientific Context
Earth’s core has generated a magnetic field for billions of years. However, the nature of that field has varied considerably through time for reasons that are still poorly understood. Both the average strength of the field and the average frequency of geomagnetic reversals have changed over timescales of 100s of millions to billions of years. These changes relate to fundamental questions about the evolution of Earth’s deep interior, such as the influence of the mantle on core convection and the age of the inner core.
The timescales over which the paleomagnetic field characteristics change are much longer than any timescales associated with the internal dynamics of the convecting liquid iron outer core. Therefore, the observed paleomagnetic changes are likely related to factors external to the core that arise from the overall thermochemical evolution of the planet. One possible explanation for these very long timescale changes is the initiation and ongoing crystallisation of the inner core. Growth of the inner core changes both the geometry of the system (full sphere vs spherical shell) and the distribution of buoyancy that drives convection (e.g., by releasing latent heat and light elements at the inner core boundary). Another possibility is changes in core-mantle interaction. Just as mantle convection alters the distribution of continents over geologically long timescales, so too does it alter the pattern and amplitude of heat flow extracted across the core-mantle boundary.
The figure below shows example out put from one of our recent simulations of the geodynamo process that generates Earth’s magnetic field, with the view centred over the Pacific. The strength of the radial magnetic field at the top of the core is show by the green and magenta colours, with strong patches at high latitudes. The arrows show the average horizontal flow of the liquid iron east-west flows near the equator are connected to large gyres at higher latitudes. The contours show the influence of the mantle on temperature at the top of the core, the central Pacific is particularly warm.
In this project you will investigate how the long-term evolution of the core (i.e., changes in the size and growth rate of the inner core and the heat extracted from the core by the mantle) are expected to influence Earth’s magnetic field. We are now well placed to investigate these questions due to a combination of increased computing power, enabling simulation of more Earth-like conditions, and better theoretical understanding, allowing more robust extrapolations of those simulations.
You will run new simulations that describe the detailed process of magnetic field generation on short (decadal to million-year) timescales. You will compare the simulated fields to improved theoretical predictions of this behaviour to make robust extrapolations to planetary core conditions and couple these results to models of Earth’s thermal history to predict the long-term evolution of the paleomagnetic field. You will compare these model predictions to paleomagnetic observations, building on established collaborations between Leeds and colleagues at the University of Liverpool and the Scripps Institute of Oceanography.
Overall, through this project you will shed new light on fundamental questions in deep Earth geophysics concerning the influence of the mantle on core convection, the age of the inner core, how Earth’s magnetic field has been powered over billions of years, and why its fundamental characteristics change through time.
Biggin, A. J. et al. Palaeomagnetic field intensity variations suggest Mesoproterozoic inner-core nucleation. Nature 526, 245–248 (2015).
Davies, C. J. et al. Dynamo constraints on the long-term evolution of Earth’s magnetic field strength. Geophys J Int 228, ggab342- (2021).
Greenwood, S., Davies, C. J. & Mound, J. E. On the evolution of thermally stratified layers at the top of Earth’s core. Phys Earth Planet In 318, 106763 (2021).
Meduri, D. G. et al. Numerical Dynamo Simulations Reproduce Paleomagnetic Field Behavior. Geophys Res Lett 48, (2021).
Pétrélis, F., Besse, J. & Valet, J. P. Plate tectonics may control geomagnetic reversal frequency. Geophysical Research Letters 38, L19303 (2011).