In other words, half (50%) of the Carbon-14 you started with has decayed into the daughter isotope Nitrogen-14.
However, your readout from your radioactivity measuring instrument says you have only 25% Carbon-14 and 75% Nitrogen-14, so your fossil must have been through more than one half-life.
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 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.
As time passes, the "parent" radioactive elements change at a regular rate into non-radioactive "daughter" elements.
Thus, the older a rock is, the larger the number of daughter elements and the smaller the number of parent elements are found in the rock.
This technique relies on the property of half-life.
Half-life is defined as the time it takes for one-half of a radioactive element to decay into a daughter isotope. How can you tell the age of a rock or to which geologic time period it belongs?One way is to look at any fossils the rock may contain. When you find layers of rocks in a cliff or hillside, younger rocks are on top of older rocks.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.As radioactive isotopes of elements decay, they lose their radioactivity and become a brand new element known as a daughter isotope.