A After a rock forms, nuclei of a radioisotope inside the rock start to decay. As they decay, the amount of the original, or parent, isotope decreases, while the amount of its stable decay product, or daughter isotope, increases. By measuring the relative amounts of parent and daughter isotopes and knowing the rate of decay, scientists can determine how long the parent isotope has been decaying. This provides an estimate of the rock’s age.
Context:
Gregory is a geologist who went on an excursion to Australia. While hiking in a cave, Gregory found two radioactive rocks containing Uranium. Gregory then brought the rocks back to his lab in Germany to conduct tests. Gregory discovered that rock A had high amounts of the original uranium isotope present, and low amounts of the daughter uranium isotope present. In rock B, however, there were low amounts of the original uranium isotope present and high amounts of the daughter uranium isotope present.
Q: Which rock, rock A or rock B, had been decaying over a longer period of time?