University of Cambridge
Earth Sciences

The flow of solid rocks in Earth’s mantle is central to many fundamental problems in the earth sciences, such as the Earth’s response to melting ice sheets. A critical component of any attempt to address these fundamental problems are models of the rock viscosity at mantle conditions. These models are formulated and calibrated using laboratory experiments, which are necessarily performed at significantly higher stresses than are thought to be present in the Earth, such that the rock deforms fast enough to measure in the laboratory. Consequently, significant extrapolation is required to predict the viscosity, and therefore the rate of flow, of Earth’s mantle. Recent experiments on rocks have demonstrated a previously unappreciated phenomenon in the earth sciences. When a rock is deformed, defects in the crystal lattices organise to exert a force that resists further deformation. New theory motivated by this phenomenon is consistent with existing laboratory data, but predicts strikingly different viscosities when extrapolated to mantle conditions. This theory makes several testable predictions about the dependence of this resistive force on factors such as the force applied by the experimenter. However, there is currently insufficient data to test these predictions. For this project, I will test these predictions using a new deformation apparatus constructed at the University of Cambridge. This mechanical approach will be complemented by new electron microscopy techniques that are able to resolve the crystal defects that create this resistive force. This project will therefore transform our ability to predict the viscosity of Earth’s mantle and thereby unlock the next generation of refined geodynamic models.