The mechanics of thermal plume formation and intrusion into the lithosphere are investigated using a combination of laboratory experiments and numerical simulations. In the experiments a deep layer of strongly temperature-dependent viscous sucrose solution is heated from below and cooled from above to produce high-viscosity surface and low-viscosity basal thermal boundary layers. Plumes develop from instabilities in the hot boundary layer, ascend, and partially intrude into the cold layer (lid). A set of two-dimensional finite element calculations are made using the experimental geometry, fluid properties, and boundary and initial conditions. Satisfactory agreement is obtained for the number and shape of plumes and the time variation of surface heat flux, basal temperature, and, to a lesser extent, lid thickness. The major discrepancy is the onset time for instability, which is typically 30% late in the calculations. Numerical simulations of mantle plume-lithosphere interaction with subsolidus creep rheology show that thermal plumes do not effectively intrude and erode the lithosphere if upper mantle viscosity is a function of temperature only, unless the activation energy is anomalously low, 50 kJ mol-1 or less. However, plumes can significantly erode the lithosphere on time scales of 10--20 m.y. if a low-viscosity asthenospheric channel, due to pressure dependence of viscosity, is present. ¿ American Geophysical Union 1988