Author Question: How many half-lives are required to yield a mineral with 2,000 atoms of parent isotope and 30,000 ... (Read 142 times)

asmith134 · **on:** Jul 16, 2018

How many half-lives are required to yield a mineral with 2,000 atoms of parent isotope and 30,000 atoms of daughter isotope? If the half-life of the parent-daughter isotope pair is 1.0 billion years, how old is the rock? Please show your work.

What will be an ideal response?

Question 2

During what geological period was the conglomerate deposited?

What will be an ideal response?

kswal303 · **Reply #1 on:** Jul 16, 2018

Answer to Question 1

The Parent+Daughter=Total, so 2000+30000 = 32000

P D HL
32000 0 0
16000 16000 1
8000 24000 2
4000 28,000 3
2,000 30,000 4
4 x 1by = 4 billion years old

The answer is 4 half-lives have gone by; the age of the rock is 4 billion years old.

Answer to Question 2

Cretaceous, because the conglomerate contains a Cretaceous fossil. Note: Many students will conclude a Triassic age, based on the age of the cobble (sample A). This is an important opportunity to link conceptual understanding of rock-dating methods to the rock cycle.