Radiometric dates calculated from minerals in sedimentary rocks produce ages that __.
a. are just as accurate as when the technique is applied to igneous rocks
b. are too young (postdate sedimentary deposition)
c. are too old (predate sedimentary deposition)
d. represent the time the sediments became lithified
The Correct Answer and Explanation is:
The correct answer is c. are too old (predate sedimentary deposition).
Radiometric dating is a method used to determine the age of rocks and minerals based on the decay of radioactive isotopes. While this technique is highly accurate when applied to igneous rocks, it presents significant challenges when applied to sedimentary rocks.
Sedimentary rocks form from the accumulation and compaction of sediments, such as sand, silt, and clay, which often originate from the erosion of pre-existing rocks. Since sedimentary rocks are composed of materials that have been transported and deposited over time, the individual grains or minerals within these rocks can be much older than the time when the sediment was actually deposited. This is because the minerals in sedimentary rocks are often derived from older rocks that have eroded over time. When radiometric dating is applied to these minerals, the resulting dates reflect the age of the source rocks, rather than the time when the sediments were deposited and lithified into sedimentary rock.
For example, if a mineral grain within a sedimentary rock was eroded from an ancient igneous rock and then redeposited in a sedimentary environment, radiometric dating of that grain would yield the age of the original igneous rock. This age would be older than the time when the sedimentary rock was formed, resulting in an age that predates sedimentary deposition.
In contrast, radiometric dating of igneous rocks provides an accurate age of the rock because igneous rocks form from the cooling and solidification of magma or lava, a process that sets the radioactive “clock” at the time of solidification.
Thus, radiometric dating of minerals in sedimentary rocks tends to produce ages that are too old, representing the time when the source rock formed rather than when the sedimentary rock was deposited and lithified.