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This is the second of two companion papers that present a theoretical and experimental study of the thermal history of planetesimals in which heating by short‐lived radioactive isotopes generates an internal magma ocean and the subsequent cooling and crystallization thereof. We study the conditions required to form and preserve basal cumulates and flotation crusts, and the implications for the thermal evolution of planetary bodies. Our model predicts that planetesimals larger than 30 km can reach 1300°C and a melt fraction of 40 vol%, producing a solid‐like to liquid‐like rheological transition that triggers an internal magma ocean. In the magma ocean regime core‐mantle differentiation occurs very quickly and the mantle convects under a relic of chondritic material whose thickness is controlled by the temperature of rheological transition. We show that the magma ocean episode is associated with time …