Radiometric dating of fossils
The older of the two dating methods, the geologic timescale, is actually a circular argument and is therefore considered by many scholars to be weak.
Nevertheless, the geologic timescale was thought to have been redeemed and refined with the advent of radiometric dating.
When paleontologist Mary Schweitzer found soft tissue in a Tyrannosaurus rex fossil, her discovery raised an obvious question -- how the tissue could have survived so long?
The bone was 68 million years old, and conventional wisdom about fossilization is that all soft tissue, from blood to brains, decomposes.
Certain fossils are unique to certain layers of rock.
Some of these fossils have been chosen to be what are called "index fossils".
How do scientists know the bones are really 68 million years old?
You can't predict when a specific unstable atom, or parent, will decay into a stable atom, or daughter.
Radiometric dating relies on the properties of isotopes.
These are chemical elements, like carbon or uranium, that are identical except for one key feature -- the number of neutrons in their nucleus.
Scientists generally agree that the answer to the riddle of the age of the earth is carefully concealed within the earth's crust.
Thus, the geologic timescale and radiometric dating have been developed in an effort to determine the age of the earth.
To read the time on this radioactive clock, scientists use a device called a mass spectrometer to measure the number of parent and daughter atoms.