Three-dimensional structures of thermal convection are studied by numerical experiments in a rectangular domain of a plane fluid layer. The main question is the joint effect of the heating mode and the depth-dependent viscosity on the planform of the circulation. Heating mode is defined by the ratio of internal and basal heat inputs. For viscosity-depth functions, simple models of the mantle viscosity are used. If the Rayleigh number is of the order 105--106 (based on the maximum viscosity), this choice produces cellular flow with a closed network of cold descending sheets around isolated upwellings, independently of the heating mode. The central upwellings are concentrated in narrow cylindrical plumes it there is a significant basal heat input. Convection of the Earth's mantle may occur with descending currents represented by subduction and ascending currents in the form of plumes situated below hotspots. ¿ American Geophysical Union 1993