The rapid rise of the central Andean plateau between ∼10 and 6.8 Ma implies that mantle lithosphere, including eclogitized lower crust, was removed from beneath the region in that time interval; we infer from that removal that the average viscosity coefficient of mantle lithosphere was quite low when removal occurred. Using scaling laws for the growth of perturbations to the thickness of a dense layer over an inviscid substratum (Rayleigh-Taylor instability), we place bounds on the average viscosity coefficient for central Andean lithosphere. When compared with laboratory measurements of flow laws for olivine and eclogite, the allowed range of viscosity coefficients yields bounds on the temperature of ∼500–800°C at the Moho beneath this region and suggests that mean stresses across mantle lithosphere during continental deformation are less than ∼50 MPa. This range of temperature is comparable with, if a slightly lower than, we might expect for lithosphere approximately doubled in thickness and not yet equilibrated with the doubled crustal radioactivity. The mean deviatoric stress is comparable to that associated with stresses that drive plates and hence shows that lithospheric material is not too strong to prevent removal of its mantle part.