Geothermal gradient
Heat sources
Temperature within the Earth increases with depth. Highly viscous or partially molten rock at temperatures between 650 to 1,200 °C (1,200 to 2,200 °F) are found at the margins of tectonic plates, increasing the geothermal gradient in the vicinity, but only the outer core is postulated to exist in a molten or fluid state, and the temperature at the Earth's inner core/outer core boundary, around 3,500 kilometres (2,200 mi) deep, is estimated to be 5650 ± 600 Kelvin.

Geothermal gradient

Section Title: Heat sources
Table Section Text: Temperature within the Earth increases with depth. Highly viscous or partially molten rock at temperatures between 650 to 1,200 °C (1,200 to 2,200 °F) are found at the margins of tectonic plates, increasing the geothermal gradient in the vicinity, but only the outer core is postulated to exist in a molten or fluid state, and the temperature at the Earth's inner core/outer core boundary, around 3,500 kilometres (2,200 mi) deep, is estimated to be 5650 ± 600 Kelvin.

Isotope	Heat release [W/kg isotope]	Half-life [years]	Mean mantle concentration [kg isotope/kg mantle]	Heat release [W/kg mantle]

238U	9.46 × 10−5	4.47 × 109	30.8 × 10−9	2.91 × 10−12

235U	5.69 × 10−4	7.04 × 108	0.22 × 10−9	1.25 × 10−13

232Th	2.64 × 10−5	1.40 × 1010	124 × 10−9	3.27 × 10−12

40K	2.92 × 10−5	1.25 × 109	36.9 × 10−9	1.08 × 10−12

Sentence(s)

Geothermal gradient isotopes in the Earth are potassium-40, uranium-238, uranium-235, and thorium-232.

final_sentence: Geothermal gradient isotopes in the Earth are potassium-40, uranium-238, uranium-235, and thorium-232.