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.

a) How many half-lives have passed since the zircon formed?
b) What is the absolute age of the Lava flow?
c) What is the age of the rocks BELOW the lava flow?
d) What is the age of the rocks ABOVE the lava flow?

Answer 1

To determine the age of the zircon crystal, scientists measure the ratio of Uranium-235 (235U) to Lead-207 (207Pb), with two half-lives having passed. This indicates the lava flow is approximately 1.426 billion years old. Rocks below the flow are older, and rocks above are younger than the flow itself.

Step-by-step explanation:

Radioactive dating is a technique used to determine the age of materials such as rocks or artifacts. The process is based on the known half-lives of radioactive isotopes present in the materials. In the case of the zircon crystal from a lava flow that was examined for geologic age, scientists used the ratio of Uranium-235 (235U) to Lead-207 (207Pb).

To calculate the number of half-lives that have passed since the zircon crystal formed:

If the sample currently has 25% 235U remaining, this means 75% has decayed into 207Pb. When 50% of the original 235U remains, one half-life has passed. With 25% remaining, two half-lives have occurred.

The absolute age of the Lava flow can be found by multiplying the number of half-lives by the half-life of 235U:

Two half-lives are equivalent to 2 * 713 million years = 1.426 billion years.

The age of the rocks below the lava flow must be older than the lava flow, as the lava would have solidified on top of the existing rock layers. Similarly, the age of the rocks above the lava flow must be younger than the lava flow since these layers would have been deposited or formed after the lava solidified.

Answer 2

a. The zircon crystal from the lava flow has gone through two half-lives of Uranium-235 decay.

b. The absolute age of the lava flow is estimated to be around 1.426 billion years.

c. The age of rocks below the lava flow must be older than 1.426 billion year.

d. The age of rocks above the lava flow must be younger than 1.426 billion years.

Step-by-step explanation:

The student is asking how to determine the geological age of a zircon crystal from a lava flow using radioactive dating. They have identified the proportional amounts of Uranium-235 (235U) and Lead-207 (207Pb) within the crystal. To answer their questions:

1. To determine how many half-lives have passed since the zircon formed, we use the fact that 25% of Uranium-235 remains and 75% has decayed into Lead-207. This implies that two half-lives have elapsed, as with each half-life, the amount of radioactive isotope decreases by half (from 100% to 50%, then from 50% to 25%).
2. The absolute age of the lava flow can be calculated by multiplying the number of half-lives by the half-life duration of Uranium-235. Since the half-life of Uranium-235 is 713 million years, and two half-lives have passed, the zircon and the lava flow are approximately 1.426 billion years old.
3. The age of rocks below the lava flow must be older than 1.426 billion years, as they would have been present before the lava flow solidified and deposited the zircon crystal.
4. The age of rocks above the lava flow must be younger than 1.426 billion years, as they would have formed after the lava flow occurred.