"geothermal heat flow" . . "heat flow" . "geothermal heat flux" . "Earth's internal heat budget" . "how temperatures increased with depth" . "Primordial Heat" . . "may often be geologically inert" . "Earth's heat content" . . "Dissipation of heat from the mantle" . . "Earth's internal heat budget" . "Earth's internal heat" . . "source of heat" . "heat from the interior" . . "two main sources of heat" .