We have assessed the influences of viscous and adiabatic heatings on the transient development of plumes by employing the time‐dependent convection equations in the extended‐Boussinesq limit. Both temperature‐dependent and temperature‐ and pressure‐dependent viscosities are employed. For constant thermal expansion of coefficient we find the vigor of plume flows is diminished with larger magnitudes of the thermal expansivity. This effect is caused by the local domination of adiabatic decompression over viscous heating inside the developing plume for both the cartesian and axisymmetric domains. For deep mantle flows a thermal expansivity decreasing with depth is required for thin plume structures to exist. Values of the thermal expansivity contrast, which can promote plume growth, are in accord with estimates from recent studies in seismic equation of state.