Final answer:
Earth's internal heat, primarily caused by the decay of radioactive isotopes in its interior, is the reason for melting rock within the Earth, not the Sun's energy.
Step-by-step explanation:
The cause of melting rock within the Earth is not primarily due to the Sun's energy; instead, it is the result of Earth's internal heat. The chilling effect of a clear breezy night is a result of radiative heat transfer to cold outer space, which is quite different from the geological processes at work within the Earth.The Earth's age being approximately 4.5 billion years old suggests that, if the Sun were the source of the internal heat, the Earth's center should have cooled a long time ago. It was the discovery of radioactivity that provided the answer: the decay of radioactive isotopes in the Earth's interior releases heat, keeping the Earth's core molten despite heat loss through conduction, convection, and radiation.
This internal heat, derived from radioactivity, is significant: geologists estimate that about half of the Earth's current internal heat budget comes from this decay of radioactive isotopes. The fact that elements such as uranium and thorium have long half-lives means that they have been heating the Earth from the inside since its formation.