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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.
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.
In other words, during 704 million years, half the U-235 atoms that existed at the beginning of that time will decay to Pb-207. 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.
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.
This first time of shaking represents one half life, and all those pieces of candy that have the printed M facing up represent a change to the daughter isotope.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.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.