The flow of energy from the Earth's interior originates from two main sources: the radiogenic and primordial heat respectively. Radiogenic heat is produced by the radioactive decay of isotopes in the mantle, whereas primordial heat is the energy left over since the Earth's formation.
Without using any sensitive scientific equipment, this flow of energy is observable at stress points between the tectonic plates, such as volcanoes. These are ruptures in the crust of the planet through which materials from the interior of the planet are expelled.
