In an unweathered sample of igneous rock,the ratio of an unstable isotope to its stable daughter isotope is 1/15.If no daughters were present at the time the rock cooled below closure temperature,and the half-life of the reaction is 50 million years,how old is the rock?
A) 200 million years
B) 400 million years
C) 750 million years
D) none of the above

Question

Quizplus · Accepted Answer

The ratio of 1/15 indicates that four half-lives have passed (since 1/15 is approximately 1/16, which is 1/2^4). Therefore, the age of the rock is 4 half-lives, or 4 x 50 million years = 200 million years.

In an Unweathered Sample of Igneous Rock,the Ratio of an Unstable