They are: Principle of superposition: 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.
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.
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.
Many rocks contain small amounts of unstable isotopes and the daughter isotopes into which they decay.
If the nucleus has not yet decayed, there is always that same, slight chance that it will change in the near future.
Return to top A tasty way for students to understand about half life is to give each team 100 pieces of "regular" M & M candy.