Radioactive substances?

Question

Why are radioactive substances useful for

measuring geologic time?

Answer 1

Radioactive substances decay into stable elements or into other radioactive elements.  What is known is that each element has a fixed time period in which half of it will decay.

So a carbon-based lifeform when it dies stops takuing in carbon, and the carbon-14 half-life gives us a measuring stick.  The proportion of carbon-14 to carbon-12 in the specimen will tell us approximately when the carbon fixed in the compounds.

I should point out that items made from petroleum products, whose carbon ws fixed millions of years ago, like the heel of a plastic shoe, will likely test very old or even beyond the rnde of the test, but then the living tissue that collected the carbon which made the oil died that long ago.

Longer half-life decays can measure other things.  An isotope of rubidium, rubidium 87 hs a half-life of 48,8 billion years and decays into stable strontium 87.  The ratio of rubidium to strontium in some rocks can date them accurately with an accuracy that exceeds what we believe to be the age of the universe.