The two most prominent explanations are: 1. Heat left over from the planet's initial formation. Planetary accretion generates a tremendous temperature far higher than silicate melting temperatures. When considering the measured geothermal gradient, accounting for convection is able to push the estimate for earth's age up to 3 billion years using the same initial temperature (~3900K) as Kelvin did for his erroneous calculation. We now know accretion can generate initial temperatures of up to 10000K, although a fraction of this is lost into space. Further reading: https://www.scientificamerican.com/article/why-is-the-earths-core-so/ https://websites.pmc.ucsc.edu/~rcoe/eart206/England_Kelvin-Perry_AmSci07.pdf 2. Heat generated by the decay of radioactive elements. Estimates amount to at least 20 terawatts according to a study with neutrinos. Confidence from these 'geoneutrinos' derives from analysis of their velocity spectra. The fraction of power output is about half that of the total heat flux currently leaving the deep earth. So which one is more important? I think the radiogenic studies compare the total volumetric heat production to that merely leaving earth's deep interior, not its total heat content. Thus I am left with the impression that primordial heating is the dominant source. Just asking because I've been confused about this issue for a long time.