# Where does the Earth's heat come from?

Gold Member
Hello.

I have an 'Earth and Space Science' topic test tomorrow.
Heavily revising, I come across a section that will be in the test: geothermal energy.
It works by cold water being pumped down in pipes, underground and then back up, hot, in form of steam to spin turbines and generate electricity.
Now to the question. Where does Earth's heat originally come from? It forms in nuclear reactions like in the Sun or the Sun itself?
Thank you.

Homework Helper
Gold Member
originally come from?
"Accretion" is more than a little likely the expected answer within the/a geothermal context.

Homework Helper
MHB
lekh2003, davenn and ISamson
Gold Member
Hi ISamson!

See https://en.m.wikipedia.org/wiki/Earth's_internal_heat_budget :
The flow of heat from Earth's interior to the surface is estimated at 47 terawatts (TW)[1]and comes from two main sources in roughly equal amounts: the radiogenic heat produced by the radioactive decay of isotopes in the mantle and crust, and the primordial heat left over from the formation of the Earth.[2]

Thank you! Just what I needed.
So the radioactive decay powers the Earth's heat along with the heat left from the formation of the Earth. So the Earth inside is radioactive!?

Homework Helper
MHB
Thank you! Just what I needed.
So the radioactive decay powers the Earth's heat along with the heat left from the formation of the Earth. So the Earth inside is radioactive!?
Yup.

From the same wiki article:

The radioactive decay of elements in the Earth's mantle and crust results in production of daughter isotopes and release of particles and heat energy, or radiogenic heat. Four radioactive isotopes are responsible for the majority of radiogenic heat, uranium-238 (238U), uranium-235 (235U), thorium-232 (232Th), and potassium-40 (40K).[14] Due to a lack of rock samples from below 200 km depth, it is not possible to do a simple radiogenic heat estimate of known radioactive isotope concentrations in rock throughout the whole mantle.[14] For the Earth's core, geochemical studies indicate that it unlikely to be a significant source of radiogenic heat due to an expected low concentration of radioactive elements.[2]

ISamson