Archean and Proterozoic time scales are currently defined chronometrically, with subdivisions into eras and periods being defined and allocated boundaries in terms of a round number of millions of years before present. Isotope stratigraphy is increasingly used to identify tectonic, chemical, and biological changes. The Neoproterozoic Era is characterized by at least two, and possibly four, severe and extensive glaciogenic events; for this era, chronostratigraphic subdivisions following established Phanerozoic practices are possible.
The “Precambrian” is not a formal stratigraphic term and simply refers to all rocks that formed prior to the beginning of the Cambrian Period. The task of establishing a rigorously defined and globally acceptable time scale for the Precambrian is an exceedingly difficult, and often frustrating, exercise. The reason for this is related to the fact that studying the Earth becomes increasingly difficult and uncertain the further one goes back in geological time.
The lack of a diverse and well-preserved fossil record, the generally decreasing volume of supracrustal rocks, and increasing degree of metamorphism and tectonic disturbance, as well as the uncertainties in the configuration and assembly of the continents, all contribute to making the establishment of a chronostratigraphic time scale beyond the Phanerozoic Eon problematical.
The Phanerozoic Eon broadly coincides with the most recent supercontinent cycle – a relatively well-understood sequence of geological events during which Pangea was assembled and dispersed.