<page_title> Geothermal gradient </page_title> <section_title> Heat sources </section_title> <table> <cell> 238U <col_header> Isotope </col_header> </cell> <cell> 235U <col_header> Isotope </col_header> </cell> <cell> 232Th <col_header> Isotope </col_header> </cell> <cell> 40K <col_header> Isotope </col_header> </cell> </table>
Geothermal gradient isotopes in the Earth are potassium-40, uranium-238, uranium-235, and thorium-232.