Coevality

Coevality is the condition of being of the same age. An object, event, or unit is described as coeval with another if they originated or occurred during the same time interval, within the precision of the dating methods used. The term is commonly used in geology and astronomy, but appears in other disciplines as well.

In geology and stratigraphy, coevality refers to rock units or events that formed contemporaneously, possibly separated by space or by later deformation or erosion. Correlation techniques—radiometric dating, biostratigraphy using index fossils, magnetostratigraphy, and chemostratigraphy—help establish coeval relationships between sequences in different regions. Coeval layers may be separated by gaps yet still be considered coeval if they represent the same time slice.

In paleontology and archaeology, coevality describes fossil assemblages or cultural layers that originated in the same time interval, aiding the reconstruction of environments or occupations.

In astronomy, coeval phrases describe star clusters or stellar populations that formed in roughly the same epoch, enabling studies of star formation histories and galactic evolution. Age estimates for stars are uncertain, so coevality is often stated within stated confidence intervals.

Limitations include dating precision, the potential for diachronous deposition (layers accumulating at different times), and reinterpretation as new data emerge. Coevality remains a useful concept for linking events across space and disciplines through the time dimension.