Final answer:
Radioactive dating can be used to estimate the age of a pegmatite rock layer based on the abundance of U-235 isotope. The age calculation involves determining the number of U-235 half-lives that have passed since the rock formed.
Step-by-step explanation:
The age of a pegmatite rock layer can be estimated by using radioactive dating. In this case, the rock layer was tested for the abundance of U-235 isotope, which is a radioactive isotope of uranium. By comparing the amount of U-235 to its decay product, an estimate of the rock's age can be determined.
The decay of U-235 follows a half-life of 704 million years. This means that if a rock sample is found to contain 75% U-235, it can be deduced that the rock is approximately 0.42 half-lives old. Multiplying the half-life by the number of half-lives gives an estimate of the rock's age.
Using this information, the approximate age of the pegmatite rock layer can be calculated as follows: Age = 704 million years/half-life * 0.42 half-lives = 296.88 million years.