Thermal erosion by hot lavas during their laminar flow over cold ground is investigated both theoretically and experimentally. An analysis of the steady forced convective heat transfer by a laminar channel flow at large P¿clet numbers is presented, which is used to determine the final steady state erosion velocity as well as the thicknesses and timescales of the associated thermal boundary layers in both the lava and ground. The initial transient period involving the growth and remelting of a basal chill layer is also quantified. Laboratory experiments are described in which hot, molten wax flowed over and thermally eroded an underlying bed of solidified wax. The experimental observations of chill formation and thermal erosion are found to be in excellent agreement with the theoretical predictions. The theory is applied to predict rates of thermal erosion consistent with those observed in recent flows of basaltic lava in Hawaii and of carbonatite lava in Tanzania. ¿ 2001 American Geophysical Union