Final answer:
The basalt sample is approximately 700 million years old, as it contains a 1:1 ratio of uranium-235 to lead-207, indicating that one half-life of uranium-235 (700 million years) has passed.
Step-by-step explanation:
The process of determining the age of rocks using the decay of naturally occurring radioactive isotopes is known as radioactive dating. In the case of the basalt sample provided, we have equal amounts of uranium-235 (parent isotope) and lead-207 (daughter isotope), indicating that one half-life has elapsed.
Uranium-235 has a half-life of 700 million years, so after 700 million years, half of the original uranium-235 would have decayed into lead-207. Since the sample has reached a 1:1 ratio of uranium-235 to lead-207, it suggests that the basalt is approximately 700 million years old.