Radiometric dating of sedimentary rocks
-Guide fossil (index fossil)- fossil with a short range (lived for a short period of time). and are ~3.9 BY old -meteorites and some moon rocks, however, dated at ~4.6 BY; believed to have formed at same time as Earth; age of Earth, therefore, believed to be ~4.6 BY.-Assemblage range zones- use more than 1 fossil in rock layer; area of age overlap is age (figs. Relative geologic time scale- early geologists constructed based on correlation of rocks from all over world (mostly by fossils); only relative geologic time periods, no actual dates (fig. Absolute Dating Specific age given in number of years before present. It’s just that I kept adding and adding to make it all make more sense and before I knew it, I had 3000 words on dating fossils! So, last time, we discussed the basics of radiometric dating and ended with a quick and dirty example of how a parent:daughter isotope ratio can be used to find the age of a sample.
Yes, we’re back to relative dating principles—but this time, with numbers!So how come I started this whole thing by saying that it’s actually not possible to date fossils directly? Because although radiometric dating will almost always provide an answer, in most cases, unless you’re applying the methods to an igneous rock, it won’t answer the question you’re asking.