| Thorium is one of the five abundant, long-lived, naturally-occurring radioactive elements in the Earth’s crust. The others are potassium, radon, radium, and uranium. There are several other naturally-occurring radioactive elements but they are rare and/or have short half-lifes.
Potassium, thorium, and uranium are the important internal fuels that cause the Earth’s interior to be hot, magmas and volcanoes to exist, the crust to float on the mantle, tectonic plates to move, the outer iron core to be liquid, and the inner iron core to be solid..............................................Full Article: Source
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