The teacher should tell the students that there are two basic principles used by geologists to determine the sequence of ages of rocks.
They are: Principle of superposition: Younger sedimentary rocks are deposited on top of older sedimentary rocks.
Students not only want to know how old a fossil is, but they want to know how that age was determined.
Some elements have forms (called isotopes) with unstable atomic nuclei that have a tendency to change, or decay.U-235 is the parent isotope of Pb-207, which is the daughter isotope.Many rocks contain small amounts of unstable isotopes and the daughter isotopes into which they decay.4) To demonstrate how the rate of radioactive decay and the buildup of the resulting decay product is used in radiometric dating of rocks. (A single watch or clock for the entire class will do.) 6) Piece of paper marked TIME and indicating either 2, 4, 6, 8, or 10 minutes.5) To use radiometric dating and the principles of determining relative age to show how ages of rocks and fossils can be narrowed even if they cannot be dated radiometrically. 2) Large cup or other container in which M & M's can be shaken. 7) 128 small cards or buttons that may be cut from cardboard or construction paper, preferably with a different color on opposite sides, each marked with "U-235" all on one colored side and "Pb-207" on the opposite side that has some contrasting color.For example, U-235 is an unstable isotope of uranium that has 92 protons and 143 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 125 neutrons.This is a stable condition, and there are no more changes in the atomic nucleus.A nucleus with that number of protons is called lead (chemical symbol Pb). This particular form (isotope) of lead is called Pb-207.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.