Rheological control of the dynamics of Earth's transition zone

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Mountain belts and ocean basins on the Earth's surface are a consequence of convection of the interior. Heat, generated and stored in the core and mantle, drives a gigantic convective engine which moves material throughout the interior and across the surface. Geography, geology and even life on Earth are the outcome of this process. On the largest scale we can identify two major boundaries in the Earth that act as barriers to convection. Very little material moves across the boundary between rocky crust and mantle, and the fluid oceans and atmosphere. There is a similarly trivial exchange of matter between the fluid outer core and solid mantle 2891 km beneath the surface. Heat crosses these two boundaries by conduction, not convection. A third barrier within the mantle is more enigmatic. In the transition zone at depths between 410 and 660 km a series of structural rearrangements in mantle minerals leads to dramatic changes in the properties of the mantle. Below this boundary the lower mantle appears to deform, and thus convect, with much greater difficulty than the upper mantle above it. Some have argued that this acts as another barrier to convection in the Earth. However, the latest evidence shows that a significant quantity of material penetrates the transition zone. This project is designed to explore how convection operates in the Earth on a long time-scale and in response to the presence of the transition zone and answer questions like: How much material crosses the boundary and what are the consequences of a partial barrier to convection for Earth's evolution?A key challenge in this project is to accurately describe the process that allows the mantle to deform. It is these deformation processes that make mantle convection interesting to study and challenging to model: mantle deformation is multi-scale (with interactions between atoms, between crystal imperfections and between adjacent mineral grains all being important) and multi-physics (involving processes best described by theories as diverse as those of flowing fluids and quantum mechanics of electrons). A second fascinating aspect of mantle convection are the many feedback processes acting between the surface environment and the dynamics of the mantle. For example, adding water from the oceans to the mantle causes dramatic weakening. In large-scale simulations the mantle is often treated like extraordinarily sticky treacle slowly flowing on a million-year timescale. However, the mantle does not actually work like this: it is a rock. The mantle in fact 'flows' by the motion of atomic imperfections in the structure of its crystals and this process can be described using atomic scale computer simulations. The difference between rock and treacle is most important because of the ability of rocks to store the history of their previous deformation. As they deform rocks change their properties by, for example, changing the grain size or the concentration of atomic imperfections in their crystals. Unlike treacle this means that how the mantle deforms now depends on how it has deformed in the past. The project will, for the first time, incorporate these processes directly into simulations of mantle convection. Without this ability, one has to seriously question our understanding of how the Earth's heat engine operates and, indeed, the inferences we make from simulations of convection that ignore a key aspect of the behaviour of rocks.