All of the radioactive elements that made up the early Earth started out in the hot ash of ancient supernova explosions. This means they have been working through their half-lives for at least 5 billion years since our planet was born. How much hotter was Earth鈥檚 interior then? What would the heat from nuclear reactions mean for tectonic activity and the evolution of life in our world鈥檚 feverish youth?
鈥� The initial heat of the Earth was caused by the collisions of smaller objects that came together to make it. Gravitational attraction accelerated objects towards each other, and when they struck their kinetic energy converted to heat.
鈥淭he initial heat of the Earth was caused by the collision of smaller objects that came together to make it鈥�
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As time went on, the amount of energy contributed by objects falling onto the proto-Earth became larger as the planet and its gravity became larger. Towards the end of the process, any object hitting the Earth would contribute at least 60 kilojoules for every gram of its mass. If that heat were confined to the colliding object, it would heat it to thousands of degrees. This heat did not have time to radiate away as the planet grew, so the result was a hot Earth.
Besides this, the Earth produced heat through radioactive decay. Early on, the amount of heating per unit volume was greater than at present. Today, heating is almost all due to the decay of uranium-238, thorium, and potassium-40, in roughly equal measure. But 4.5 billion years ago, the amount of uranium-235 was close to the amount of uranium-238 we have today (instead of being a minor component) and uranium-235 produces heat much faster than uranium-238. There was also about 13 times as much potassium-40 as there is today. The presence of these nuclides helped to heat up the interior of the Earth during the first billion years of its existence.
Eric Kvaalen, Les Essarts-le-Roi, France