6. Some elements are unstable. Their atoms give off particles over time at specific and constant rates. When molten rock cools to form igneous rock, some radioactive atoms are trapped. By using the rate of decay
and looking at how many unstable atoms are left, scientist can estimate when that rock was formed.
That's called radiometric dating. Sedimentary rocks, where fossils are usually found, can only be dated
this way if they are younger than 50,000 years old. But imagine a fossil found between layers of volcanic
ash. Scientists could use radiometric dating for the layers above and below the fossil. Then, according to
the law of superposition, they would know that the fossil's age is somewhere within that range.
Based on the passage, when is radiometric dating possible?
A when a substance is made of unstable
B. when a substance contains a fossil
atoms
C. when the mass of a fossil is unknown
D. when relative dating will not work