If all ra­dioac­tive el­e­ments de­cay, why are there still ra­dioac­tive el­e­ments on Earth af­ter bil­lions of years?

Focus-Science and Technology - - Q&A - 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 so­called 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.

Mount Etna erupts on Si­cily, Italy

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