era, a very long span of geologic time; in formal usage, the second longest portions of geological time (eons are the longest). Ten eras are recognized by the International Union of Geological Sciences: the Eoarchean Era (4.0 billion to 3.6 billion years ago), the Paleoarchean Era (3.6 billion to 3.2 billion years ago), the Mesoarchean Era (3.2 billion to 2.8 billion years ago), the Neoarchean Era (2.8 billion to 2.5 billion years ago), the Paleoproterozoic Era (2.5 billion to 1.6 billion years ago), the Mesoproterozoic Era (1.6 billion to 1.0 billion years ago), the Neoproterozoic Era (1.0 billion to 541 million years ago), the Paleozoic Era (541 million to about 252 million years ago), the Mesozoic Era (roughly 252 million to 66 million years ago), and the Cenozoic Era (66 million years ago to the present). Because of the difficulties involved in establishing accurate chronologies, the eras contained within Precambrian time (approximately 4.6 billion to 541 million years ago) are classified independently. An era is composed of one or more geological periods. The stratigraphic, or rock, term that corresponds to “era” is “erathem.”