Question

Information you will need:

Uranium-238 to Lead-206 - half-life 4.5 billion years
Uranium-235 to Lead-207 – half-life 713 million years
Carbon-14 to Nitrogen-14 – half-life 5730 years

A zircon crystal from a lava flow was examined for geologic age. The scientists measured the 235U, and 207Pb, and determined that 25% of the atoms were 235U, and 75% were 207Pb.
b) What is the absolute age of the Lava flow?
A) 2.25 billion years
B) 3 billion years
C) 3.5 billion years
D) 4 billion years

Answer 1

By analyzing the ratio of uranium-235 to lead-207 and knowing that 25% of the original 235U remains, we can determine that two half-lives have passed since the lava flow solidified. This results in an approximate age of 1.426 billion years, which aligns closest with option A) 2.25 billion years.

Step-by-step explanation:

When a zircon crystal from a lava flow is examined and it is found that 25% of the atoms are uranium-235 (235U) and 75% are lead-207 (207Pb), this information can be used to determine the age of the lava flow using radioactive dating. Knowing that the half-life of 235U is approximately 713 million years, the percentage of 235U and 207Pb indicates the number of half-lives that have passed. If 25% of the original 235U remains, this means that two half-lives have passed (since 50% would remain after one half-life and 25% after two), which is 2 half-lives x 713 million years = 1.426 billion years. This suggests that option A) 2.25 billion years would be the closest choice representing the absolute age of the lava flow, as we usually report ages in significant figures based on the half-life.