Scientists date igneous rocks to indirectly date nearby sedimentary rocks. Why doesn’t radiometric dating typically work on sedimentary rocks?

Question

A.
Sedimentary rocks form too slowly to be dated accurately.

B.
Sedimentary rocks contain bits of sediment, each much older than the rocks they form.

C.
Sedimentary rocks lack radioactive isotopes.

D.
Sedimentary rocks form from a mixture of minerals, not a single mineral.

Answer 1

B. Sedimentary rocks contain bits of sediment, each much older than the rocks they form.

Radiometric dating typically does not work on sedimentary rocks because they are composed of particles derived from older rocks, and thus, the ages indicated by the minerals may not represent the time of formation of the sedimentary rock itself. Instead, radiometric dating is more effective for igneous rocks, which crystallize from molten material and provide a clear formation age.