Mantle convection and all its consequences, including plate tectonics, are governed by density contrasts - but those contrasts are not fully explained and that is a first order problem in understanding mantle behaviour. Thermal, chemical and/or mineralogical variations play roles. Global seismology reveals depths at which density varies over rather short distances, the nominal 410 and 660 km discontinuities. These relate to mineralogical changes induced primarily by pressure, which is due to the weight of rock above, and hence explains a recognisable though nominal depth. In subduction zones, temperature modifies the mineral reaction pressures and hence depths. Today we can constrain mantle discontinuity depths to (smaller than) 10 km and many anomalies have been recognised, not explained by temperature variations but currently by a variety of tentative ideas.
There are two new ideas to explain discontinuity geometries I will present here, both of which will be tested in my NERC Large Grant. First, we propose that “dynamic” pressures due to flow are big enough to affect the mineral transformations. Secondly, we propose that we must look at the direct effects of stress on mineral transformation. Stress comprises different forces per unit area in different directions, so what is the net effect? This is an important but controversial question. Moving our understanding of this forwards will illuminate questions about crustal metamorphism as well as mantle convection.
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