Return to top Each team of 3 to 5 students should discuss together how to determine the relative age of each of the rock units in the block diagram (Figure 1).

name describe two principles used relative dating rocks-14

That chance of decay is very small, but it is always present and it never changes.

In other words, the nuclei do not "wear out" or get "tired".

This happens at any time when addition of the fleeting "weak nuclear force" to the ever-present electrostatic repulsion exceeds the binding energy required to hold the nucleus together.

Very careful measurements in laboratories, made on VERY LARGE numbers of U-235 atoms, have shown that each of the atoms has a chance of decaying during about 704,000,000 years.

Students should be able to understand the principles and have that as a background so that age determinations by paleontologists and geologists don't seem like black magic. Geologists in the late 18th and early 19th century studied rock layers and the fossils in them to determine relative age.

William Smith was one of 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.

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.

Part 2a Activity — At any moment there is a small chance that each of the nuclei of U-235 will suddenly decay.

Principle of cross-cutting relations: Any geologic feature is younger than anything else that it cuts across.

Some elements have forms (called isotopes) with unstable atomic nuclei that have a tendency to change, or decay.

Students not only want to know how old a fossil is, but they want to know how that age was determined.