Relative Dating
Relative dating determines the order in which rocks or events occurred, older or younger, without giving an exact age in years.
Definition
Relative dating is figuring out which rocks or fossils are older and which are younger without knowing their exact age in years; it puts things in order, like sorting photos by "before" and "after." It uses a set of principles to sequence geologic events: superposition (lower layers are older), original horizontality (layers form flat), cross-cutting relationships (a fault or intrusion is younger than the rock it cuts), inclusions (fragments are older than the rock containing them), and faunal succession (fossil assemblages appear in a definite order), while unconformities mark gaps of missing time. These stratigraphic principles combine with biostratigraphic correlation using index fossils to build regional and global sequences. Unconformities (angular, disconformity, nonconformity) record erosion or nondeposition and divide the record into sequences, and cross-cutting relationships extend to metamorphic fabrics and deformation, so a foliation that is folded records at least two deformation events. Relative dating is the framework into which radiometric ages are inserted; the two methods are complementary, not competing.
Example
In a pile of laundry, the shirt at the bottom was dropped first, and in rock layers the bottom layer formed first. In a road cut on I-40 through the Blue Ridge, a granite dike slicing across layered gneiss must be younger than the gneiss, and a lava flow sitting on top of a soil layer is younger than the soil. Hutton's unconformity at Siccar Point, Scotland, shows near-vertical Silurian strata overlain by gently dipping Devonian red sandstone, requiring deposition, tilting, erosion, and renewed deposition, a sequence that demanded vast time. Magnetostratigraphy adds another relative tool by matching magnetic reversal patterns.
Key Insight
Relative dating answers "which came first?" and never answers "how many years ago?" It was used for a century before anyone could measure an absolute age, and the geologic time scale was built entirely from relative order first. Relative dating is powerful precisely because it needs no equipment. A single outcrop can yield a sequence of five or six events that any trained observer can verify with their eyes.