erathem

An eratheme is the stratigraphic term for an era, which is a long span of geologic time. Eras are the second-longest divisions of geologic time, following eons. Ten eras are formally recognized: the Eoarchean, Paleoarchean, Mesoarchean, Neoarchean, Paleoproterozoic, Mesoproterozoic, Neoproterozoic, Paleozoic, Mesozoic, and Cenozoic. The eras within Precambrian time (before 541 million years ago) are classified separately due to dating difficulties. Eras are composed of one or more geologic periods. Stratigraphy, the study of rock successions, uses these time scales to interpret Earth's history. The law of superposition, stating that the oldest rock layer is at the base, is a fundamental principle in determining relative ages. Radiometric dating has allowed for numerical ages to be assigned to these geologic time scales.123