A geological event is a temporary and spatially heterogeneous and dynamic (diachronous) happening in Earth history that contributes to the transformation of Earth system and the formation of geological strata. Event stratigraphy was first proposed as a system for the recognition, study and correlation of the effects of important physical or biological events on the broader stratigraphical record.
Geological events range in time span by orders of magnitude, from seconds to millions of years, and in spatial scale from local to regional and, ultimately, global. In contrast to chronostratigraphic or geochronological units, that define the boundaries between periods, epochs and other units of the geologic time scale, complex dynamic diachronous changes are inherent to the event-stratigraphy paradigm. The lithostratigraphic or biostratigraphic boundaries that mark the onset and termination of geological events in the stratigraphic record may be diachronous, whereas those of formal chronostratigraphic or geochronologic units have basal boundaries that are isochronous.