The theory of plate tectonics established fifty years ago has formed a robust framework for understanding how the Earth works across a range of scales. While the kinematics of plate tectonics is well established, the dynamics remain a challenge. Key to this is the mechanics of the lithosphere, and hence an understanding of the nature of the oceanic lithosphere–asthenosphere system. The oceanic lithosphere forms at mid-ocean ridges (MORs) and comprises the majority of Earth's surface. The classic view suggests that MORs are places of passive mantle upwelling driven by plate divergence, and that the oceanic lithosphere forms by conductive cooling away from the ridge. However, recent geophysical observations provide clear evidence that in addition to temperature, melt is clearly implicated in the formation of oceanic lithosphere.
Here I will present dynamic, two-phase flow numerical models of MORs that can reconcile theory and observations by incorporating buoyancy-driven flow associated with temperature, composition and porosity, and complex rheology of the lithosphere. In the first part, I show that melting-induced buoyancy effects may provide an explanation for both the asymmetric distribution of melt beneath the axis and the inferred short-wavelength variations in the lithosphere-asthenosphere boundary (LAB). Melting and crystallization of enriched material leads to a dynamic LAB closer to the ridge axis. Models of older oceanic LAB are also susceptible to the influence of thermal instabilities, which can erode the lithosphere and limit the base of the ocean lithosphere from cooling. In the second part, I will show some preliminary results using a consistent continuum theory for partially-molten rocks using visco-elasto-plastic rheology. This theory allows for formation and evolution of fluid-driven fractures, such as dikes and sills, in tectonic-scale models. I am currently using this newly developed theory to investigate how magma interacts with the solid-rock closer to the MOR axis, and whether magma is transported primarily in dikes and/or along fault planes.