THE MORE STABLE FORM IT TURNS INTO IS CALLED THE DAUGHTER ELEMENT.Unlike people, you can’t really guess the age of a rock from looking at it.RADIOACTIVE ISOTOPES ARE UNSTABLE VERSIONS OF NORMAL ELEMENTS.BECAUSE THEY ARE UNSTABLE THEY GIVE OFF RADIATION TO TRY TO BECOME NORMAL.THIS PROCESS OF GIVING OFF RADIATION TO BECOEM STABLE IS CALLED RADIOACTIVE DECAYEACH TIME AN ISOTOPE RELEASE RADIATION, IT CHANGES INTO A NEW MORE STABLE ELEMENT.THE ORIGINAL RADIOACTIVE ISOTOPE IS CALLED THE PARENT ELEMENT.The half-life of carbon 14, for example, is 5,730 years.On the other hand, the half-life of the isotope potassium 40 as it decays to argon is 1.26 billion years.
For really old dates we need to use radioactive isotopes.
This method works because some unstable (radioactive) isotopes of some elements decay at a known rate into daughter products. Half-life simply means the amount of time it takes for half of a remaining particular isotope to decay to a daughter product. Good discussion from the US Geological Survey: geochronolgists just measure the ratio of the remaining parent atom to the amount of daughter and voila, they know how long the molecule has been hanging out decaying. So to date those, geologists look for layers like volcanic ash that might be sandwiched between the sedimentary layers, and that tend to have radioactive elements.
What’s more, if the whole rock is badly weathered, it will be hard to find an intact mineral grain containing radioactive isotopes.
When the meltwater reaches a lake the heaiest sediments (sand and silt) sink to the bottom quickly and eventually formed thick layers of light colored rock.
Later in the year, by wintertime, the clay has had enough time to settle to the bottom and it forms a thin layer of dark colored rock.