How does potassium 40 dating work
If you have 100 grams of strontium-90, after 28 years it will be halved to 50 grams. Then it takes another 28 years for half of what was left to decay.
So, after 56 years, there will still be a quarter of the original strontium-90 left: 25 grams.
When this happens, the crystal structure of the rock retains details of the internal structure of the animal.
That's how fossils are formed, but how do we know how old they are?
Plus lots of other isotopes with a great range of half-lives.Physicists understand why this happens, and they know the rate at which each isotope decays.So radioactive isotopes act as miniature clocks, thousands of tiny clocks embedded through the substance of a rock.Unfortunately, fossils are not found in volcanic rock - molten lava is not conducive to preserving an animal's shape!Fossils are normally found only in sedimentary rocks - hardened mud, silt or sand, in which the corpse can lie peacefully for long enough to permit the fossilisation process to get going. The particles of mud and sand that settle to make the sediments were earlier ground down from volcanic rocks, and they were zeroed at some point in history.
Very occasionally, however, rocks take up some kind of permanent imprint of the body and preserve it for us to see even after millions of years. Often a fossil retains only the external shape of the body.