Australian Museum



Dating Fossils – How Are Fossils Dated?

There are two types of age determinations. Geologists in the late 18th and early 19th century studied rock layers and the fossils in them to determine relative fosssils. William Smith was one rocls the most important scientists from this time who helped to develop knowledge of the succession of different fossils by studying their distribution through the sequence of sedimentary rocks in southern England. It wasn't until well into the 20th century that enough information had accumulated about the rate of radioactive decay that the age of rocks and fossils in number of years could be determined through radiometric age dating.

This activity on determining age of rocks and fossils is intended for 8th or 9th grade students. It is estimated to require four hours of tso time, including approximately one hour total of occasional instruction and explanation from the teacher and two hours of group team and individual activities by the students, plus one hour of discussion among students within the working groups. Explore this link for additional information datnig the topics covered in this lesson: MCKINNEY THE AGE of fossils aer almost everyone.

Students not only want to know how old a fossil is, but they want to know how that age was determined. Some very straightforward principles are used to determine the age of fossils. Students should be able dahing understand the principles and have that as a background so that age determinations by paleontologists and geologists don't seem like fosails magic.

This activity consists of several parts. Objectives of this activity are: Return to top MATERIALS REQUIRED FOR EACH GROUP 1 Block diagram Figure 1. A single watch or clock for whay entire class will do. Return to top PART 1: After students have decided how to establish the relative age of each rock unit, they should list them under the block, from most recent at the top of the list to oldest at the bottom.

The teacher should tell the students that there are two basic principles used by geologists to determine the sequence of ages of rocks. Younger sedimentary rocks are deposited on top of older sedimentary rocks. Principle of cross-cutting relations: Any geologic feature is younger than anything else that it cuts across. For example, U is an unstable isotope whqt uranium that has 92 protons and neutrons in the nucl eus of each atom.

Through a series of changes within the nucleus, it emits several particles, ending up with 82 protons and neutrons. This is a stable condition, and there are no more changes in the atomic nucleus. A nucleus with that number of protons is ths lead chemical symbol Pb. What are the two methods of dating rocks and fossils protons 82 and neutrons total This particular form fwo of lead is called Pb U is the parent isotope of Pb, which is the daughter isotope.

Many rocks contain small amounts of unstable isotopes and the daughter isotopes into which they decay. Where the amounts of parent and daughter isotopes can be accurately measured, the ratio can be used to determine how old the rock is, as shown in the following activities. That chance of decay is very small, but it is always present and what are the two methods of dating rocks and fossils never changes. In other words, the nuclei do not "wear out" or get "tired".

If the nucleus has not yet decayed, there is always that same, slight chance that it will change in the near future. Atomic nuclei are held meghods by an attraction between the large nuclear particles protons and neutrons that is known as the "strong nuclear force", which must exceed the electrostatic repulsion between the protons within the nucleus. In general, with the exception of the single proton that constitutes the twoo of the most abundant isotope of hydrogen, the number of neutrons must at least equal the number of protons in an atomic nucleus, because electrostatic repulsion prohibits denser packing what are the two methods of dating rocks and fossils protons.

But if there are too many neutrons, the nucleus is potentially unstable and decay may be triggered. This happens at any time when addition of the fleeting "weak nuclear force" to the ever-present electrostatic repulsion exceeds methoss binding energy required to hold the nucleus together. Very careful measurements in laboratories, made on VERY LARGE numbers of Fossilz atoms, have shown that each of the atoms has a In other words, during million years, half the U atoms that existed at the beginning of that time will decay to Pb This is known as the half life of U- Many elements have some isotopes that are unstable, essentially because they have too many neutrons to be balanced by the number of protons in the nucleus.

Each of these unstable isotopes has its own characteristic half life. Some half lives are several billion years long, and others are as short as a ten-thousandth of a second. On a piece of notebook paper, each whta should be placed with the printed M facing down. Whta represents the parent isotope. The candy should be poured into a container large enough for them to bounce around freely, it should be shaken thoroughly, then poured back onto the paper so that it is spread out instead of making a pile.


Relative/Absolute Dating