Subduction zones are a fundamental aspect of plate tectonics, yet we still don't really understand how subduction initiates.  It's a tough problem because as oceanic plates move away from a mid-ocean spreading center and cool, they get stiffer and should become more and more resistant to bending and sinking down into the mantle.  But recent work suggests that the clue to this puzzle lies in the physics of grains at the microscale.   David Bercovici is one of the geologists who has pioneered the approach that looks at what happens in the rocks of the oceanic lithosphere on the scale of a few microns.  It turns out that stresses in the plate can generate bands of vertically aligned weakness that could make the lithosphere much weaker in precisely the direction in which it sinks into the mantle.  David Bercovici is a professor of Earth and Planetary Science at Yale University.  In the podcast, he explains how the behavior of stressed mineral grains at the microscale can radically affect the strength of oceanic lithosphere on the plate scale.  The model is consistent with lab experiments and the contrasting seismic observations of the Pacific and Atlantic plates.