Small‐scale convection beneath continental lithosphere is likely to initiate viscous flow in the ductile lower crust. Positive lithospheric bending will develop above upwelling mantle flow leading to a lateral squeezing out of crustal material from elevated areas which results in significant crustal thickness variations in its final stage. This process was studied with a finite element approach for viscous material for a number of different viscosity contrasts between lower crust and lithospheric mantle. Significant Moho undulations of the order of at least 5 km within a time period of about 50 m.y. can only be reached if the lower crustal viscosity is less than 1021 Pa s or if the lithospheric mantle viscosity is less than 1024 Pa s. The maximum topography is a function of the viscosity contrast and is generally of the order of a few hundreds of meters with a relaxation time between 5 and 103 m.y., strongly dependent on the crustal viscosity. In conclusion, this process might be important to explain crustal thinning and thickening of the order of about 10 km in areas of enhanced thermal gradient.