Many areas of old continental interiors have thick roots that extend to depths of 250 km or more, in contrast to the oceans and younger continents, whose lithospheric thickness is less than 100 km. These cratonic roots might perturb temperatures in their surrounding mantle, though the net result could be either cooling or heating; both of which may lead to small-scale convective flow around the root. We show here, using new data from a study of the seismic structure of the Canadian Shield, that the relative positions of the 410 km and 660 km seismic discontinuities are unperturbed beneath one of the deepest and broadest cratonic roots on the Earth. Differential arrival times and internal discontinuity structure are remarkably uniform and simple, varying by little more than ± 0.5 s over the root's 3500 km lateral extent. This implies that the root has no significant thermal effect on the underlying mantle (< 50 K), and any small-scale convection or cold mantle downwelling associated with the large free-air gravity anomaly beneath the shield must be confined to the upper mantle. Our observations are also consistent with phase changes solely in the olivine system ((Mg,Fe)2SiO4), with the caveat that our method is dominantly sensitive to S-wave velocity jumps alone.