Isotopic dating of rock containing fossils yields the rock's age, which is typically related to the fossil's age in which way?

• A. Rock age is usually younger than the fossil
• B. Rock age is usually the same as the fossil
• C. Rock age is usually older than the fossil
• D. Rock age is unrelated to the fossil age

Answer: (C)

When we use isotopic dating on a rock that contains fossils, the date we get is the age of the minerals that make up the rock, not the exact time the fossil’s organism lived. In sedimentary rocks, many of those minerals are grains that formed long before they were pressed together into the sediment. These detrital grains carry their own older crystallization ages, so dating the rock can yield an age that predates the deposition of the sediment and the organisms fossilized within it. In other words, the rock’s numeric age is often older than the fossil’s age. To narrow down the fossil’s actual time, geologists use other clues, like dating interbedded volcanic material or using fossil relative dating within the sedimentary sequence.