Absolute age dating definition
The half-lives of several radioactive isotopes are known and are used often to figure out the age of newly found fossils.Different isotopes have different half-lives and sometimes more than one present isotope can be used to get an even more specific age of a fossil.
After you prepare your sample and put it into the machine, your readout says you have approximately 75% Nitrogen-14 and 25% Carbon-14.After two half-lives, another half of your leftover Carbon-14 would have decayed into Nitrogen-14.Half of 50% is 25%, so you would have 25% Carbon-14 and 75% Nitrogen-14.Below is a chart of commonly used radiometric isotopes, their half-lives, and the daughter isotopes they decay into.Let's say you found a fossil you think to be a human skeleton.Now it is time to put those math skills to good use.
At one half-life, you would have approximately 50% Carbon-14 and 50% Nitrogen-14.
If any of the fossils are unique to one of the geologic time periods, then the rock was formed during that particular time period. But these two methods only give the relative age of rocks--which are younger and which are older. Or how do we know how long ago a particular group of fossilized creatures lived?
The age of a rock in years is called its absolute age.
The ages in years of the different geological time periods are found by measuring the absolute ages of many rocks from all of the different periods.
The absolute ages of some of the different geologic time periods are shown along the right side of the Staircase of Time.
This is what your readout said, so your fossil has undergone two half-lives.