Almost half of Earth's heat comes from naturally occurring radioactive decay of underground materials, U.S. and other scientists have calculated.

Kansas State University physics Professor Glenn Horton-Smith was part of a team making some of the most precise measurements of Earth's radioactivity ever recorded, a KSU release said Tuesday.

Observations of the activity of subatomic particles -- particularly uranium, thorium and potassium -- undergoing natural fission produced close estimates of the decay's contribution to the globe's heat.

"It is a high enough precision measurement that we can make a good estimate of the total amount of heat being produced by these fissions going on in naturally occurring uranium and thorium," Horton-Smith said.

The measurements were made using a neutrino detector in Japan.

Neutrinos are neutral elementary particles that are emitted by nuclear reactions or natural radioactive decay.

By observing geoneutrinos -- neutrinos from a geological source -- they determined radioactive decay was responsible for about half the Earth's heat output of about 44 trillion watts.

The other half comes from primordial sources left over when the Earth formed, said the scientists who took part in the international research collaboration and whose findings are published in the July issue of Nature Geoscience.

Earth's internal heat is the mechanism behind plate movement, magnetic fields, volcanoes and seafloor spreading.

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