Final answer:
Numerical ages can usually only be calculated directly for igneous rocks. Sedimentary and metamorphic rocks require different methods, combining relative dating with radiometric dating of events that affect these rocks or the use of index fossils.
Step-by-step explanation:
Numerical ages can usually only be calculated directly for igneous rocks. The radiometric age of minerals in igneous rocks indicates when the mineral crystallized from magma. Radiometric dating of metamorphic rocks can also reveal when metamorphism occurred, although some isotopic systems remain unaffected by the process and still reflect the original crystallization age of the minerals in the protolith. In terms of sedimentary rocks, radiometric dating doesn't tell us when the sedimentary rock itself formed but rather when the minerals within it were originally crystallized or metamorphosed. For sedimentary rocks, establishing a numerical age typically involves combining radiometric dating of cross-cutting igneous or metamorphic events with relative dating principles. Additionally, fossiliferous rocks can be dated using index fossils which are indicative of a narrow span of geologic time.