Does the pull of a subducting slab drive plate motions?  Or is it the upwellings of convection cells in the mantle?  We now have a new way to shed light on this question.  It's called seismic anisotropy, which is the spreading out of seismic waves according to their direction of polarization.  This happens when the mantle through which the waves travel has crystals which are preferentially aligned, and that occurs when there is deformation or flow going on.  So we can work backwards to use the observed dispersion of seismic wave arrival times to infer flow patterns in the mantle.  In the podcast, Ana Ferreira explains how we do this, and describes our first views of an interaction between a mantle plume pushing up into and flowing around a subducting plate in the southwest Pacific.  But, despite the vigor of such plumes, she concludes that it is probably the pull of the sinking slabs that is still the primary driver of plate motions.
Ana Ferreira is Professor of Seismology at University College, London.   She collects and analyzes seismic data from around the world, focusing particularly on seismic anisotropy.
See her maps of mantle flow at geologybites.com.  
If you're enjoying the series, I'd be grateful if you could rate the podcast and leave a review.