Question

Imagine two rocks. One rock contains nearly all of its original potassium-40. In a second rock, most of the potassium-40 has decayed. Which statement about the first rock is true?

A. It is a metamorphic rock.
B. It is harder than the second rock.
c. It is older than the second rock.
D. It is younger than the second rock.
Explain why you chose this answer please!

Answer 1

The correct answer is C. It is older than the second rock.

Step-by-step explanation:

Potassium-40 is a radioactive isotope with a half-life of 1.25 billion years. This means that over time, half of the potassium-40 in a rock will decay into argon-40. If one rock contains nearly all of its original potassium-40, then it must not have been subject to much radioactive decay, and thus it must be relatively young. In contrast, if most of the potassium-40 in the second rock has decayed, then it must have been around for a longer period of time, and thus it must be older than the first rock.Metamorphic rocks are formed from pre-existing rocks that have been altered by heat and pressure, so the amount of potassium-40 in a metamorphic rock would depend on the amount in the original rock. The hardness of a rock also has nothing to do with its potassium-40 content. Therefore, options A and B are not correct.Option D is also incorrect, as we have already established that the first rock must be younger than the second rock if it contains more potassium-40.