Definition radiometric dating
No geologists were present when most rocks formed, so they cannot test whether the original rocks already contained daughter isotopes alongside their parent radioisotopes.
For example, with regard to the volcanic lavas that erupted, flowed, and cooled to form rocks in the unobserved past, evolutionary geologists simply assume that none of the daughter argon-40 atoms was in the lava rocks.
They also measure the sand grains in the bottom bowl (the daughter isotope, such as lead-206 or argon-40, respectively).
By dating these surrounding layers, they can figure out the youngest and oldest that the fossil might be; this is known as "bracketing" the age of the sedimentary layer in which the fossils occur.
Teach your students about absolute dating: Determining age of rocks and fossils, a classroom activity for grades 9-12.
So in order to date most older fossils, scientists look for layers of igneous rock or volcanic ash above and below the fossil.
Scientists date igneous rock using elements that are slow to decay, such as uranium and potassium.
Obviously, these eruptions took place very recently, after the Canyon’s layers were deposited ().