Zircon (ZrSiO4) forms in magma as it cools into igneous rock. It is also useful for numerical dating.
a. If you walk on a modern New Jersey beach, then you will walk on some zircon sand grains. Yet if you determine the numerical age of the zircons, it does not indicate a modern age (zero years) for the beach. Why?
b. Suggest a rule that geologists should follow when they date rocks based on the radiometric ages of crystals inside the rocks.