The half-life of a radioactive isotope is 10,000 years.This isotope and its breakdown product do not normally occur together before molten rock cools and becomes solid.You analyze a volcanic rock and find 25 percent of the original amount of radioactive isotope remains,and 75 percent has decayed to its daughter element.How long ago did the rock solidify?

Question

Quizplus · Accepted Answer

The rock solidified 20,000 years ago. This is because one half-life (10,000 years) would reduce the original amount of the isotope to 50%, and a second half-life would further reduce it to 25% of the original amount, indicating two half-lives have passed.

The Half-Life of a Radioactive Isotope Is 10,000 Years