Asked by: Paul Lowe, Manchester
Every time a volcano erupts, it’s a reminder we live on a seething cauldron of natural radioactive elements. The principal source of geothermal heat is the radioactive decay of isotopes of uranium, thorium and potassium, all of which have been present in the Earth since its formation around 4.5 billion years ago. The reason they’re still so potent is that their atoms are disintegrating at a slow rate.
This is measured by their so-called half-life: the time needed for their activity to fall by 50 per cent. All three of the main sources of radioactivity in the Earth – U-238, Th-232 and K-40 – have half-lives similar to the age of our planet, and so are still going strong.
Subscribe to BBC Focus magazine for fascinating new Q&As every month and follow @sciencefocusQA on Twitter for your daily dose of fun facts.