Radiometric dating and relative dating
The same rock formation also contains a type of trilobite that was known to live 415 to 425 million years ago.
Since the rock formation contains both types of fossils the ago of the rock formation must be in the overlapping date range of 415 to 420 million years.
A Danish scientist, Nicolaus Steno is accredited for the Law of Superposition.
This law basically tells us that each layer of sedimentary rock is younger than the one below it which is depicted in figure 8.3 in the text.
According to Lutgens and Tarbuck, Relative dating is the process in putting events in their proper sequence.
Relative dating is based on assumption, not able to give an accurate date but an estimated time period where the rock originated from.
Steno is also accredited for the Principle of Original Horizontality.
The majority of the time fossils are dated using relative dating techniques.We define the rate of this radioactive decay in half-lives.If a radioactive isotope is said to have a half-life of 5,000 years that means after 5,000 years exactly half of it will have decayed from the parent isotope into the daughter isotopes.Scientists can use certain types of fossils referred to as index fossils to assist in relative dating via correlation.Index fossils are fossils that are known to only occur within a very specific age range.
This uses radioactive minerals that occur in rocks and fossils almost like a geological clock.