Thermal convection in rapidly rotating systems is investigated numerically with self-gravitating spherical shells, within Boussinesq approximation, with the radius of the inner sphere η = 0.4 relative to that of the outer sphere. Marginal convection experiences a series of metamorphoses with decreasing Prandtl number: from the columnar mode to the spiralling mode, and then to the wall-attached mode. The physical processes of the metamorphses are clarified by diagnosing both the vorticity and the thermodynamic balances.