The boundary between the upper and lower mantles of Earth corresponds to breakdown of (Mg,Fe)2SiO4, spinel (ringwoodite), into (Mg,Fe)SiO3perovskite (pv) + (Mg,Fe)O, magnesiowüstite (mw). The rheology of these materials is important for understanding deeply subducted slabs, the termination of deep earthquakes, mantle convection, post-glacial rebound, etc. It has been proposed that decomposition of ringwoodite as subducting slabs enter the lower mantle leads to very fine-grained material that is inherently weak because it flows by grain-boundary sliding. Such abrupt and great weakening would have important geophysical implications. Here we test whether products from such decomposition are weak, using a realistic analogue high-pressure system (Co2TiO4). Our results show that spinel breakdown products are complicated intergrowths (symplectites) that flow by dislocation creep, rather than fine-grained domains that flow by diffusion creep as is commonly assumed from very fine phase domains seen in two dimensions. Application to Earth strongly suggests that ringwoodite breakdown is likely to strengthen the slab, reflecting the inherently greater viscosity of the uppermost lower mantle revealed by geophysical measurements.