This document discusses heat transfer on Earth. It notes that the main sources of heat transfer on Earth are the Sun and Earth's interior. Heat is transferred via conduction, convection and radiation. Heat flux is directly proportional to temperature gradient based on Fourier's Law. Measurements show highest heat flux at mid-ocean ridges and lowest in old ocean crust. Heat flux decreases with age of sea floor and continental crust due to cooling and decreasing radioactive elements. Conservation of energy equations relate heat flux to temperature gradient and internal heat sources.