Original: The Archean (, also spelled Archaean, formerly called the Archaeozoic (), also spelled Archeozoic or Archæozoic) is a geologic eon before the Paleoproterozoic Era of the Proterozoic Eon, before 2.5 Ga (billion years ago, or 2,500).
Simplified: Most of the surviving Archean rocks are metamorphic or igneous.