Sedimentary rocks are rarely useful for dating because they are made up of bits of older rocks.Uranium is present in many different rocks and minerals, usually in the form of uranium-238.Recently, he appeared on the evening news to talk about a new dinosaur he just discovered. Paul says he can tell from the fossils that superus awesomus lived on Earth about 175 million years ago.Paul is super awesome, so I'm going to take him at his word.Today, with the help of isotopic dating methods, we can determine the ages of rocks nearly as well as we map the rocks themselves.For that we can thank radioactivity, discovered at the turn of the last century.For example, a geologist may examine a cutting where the rocks appear as shown in Figure 1.
His geological cross-section may look something like Figure 2.
Knowing when a dinosaur or other animal lived is important because it helps us place them on the evolutionary family tree.
Accurate dates also allow us to create sequences of evolutionary change and work out when species appeared or became extinct. These are: Where possible, several different methods are used and each method is repeated to confirm the results obtained and improve accuracy.
That only worked for sedimentary rocks, and only some of those.
Rocks of Precambrian age had only the rarest wisps of fossils.