Arquean

Arquean is a less common variant spelling of the Archaean eon, used in some historical writings and translations to denote the same interval in Earth’s early geologic history. In contemporary geology, Archaean is the standard English term, while Arquean appears mainly in older texts or non-English sources.

The Archaean eon spans roughly from 4.0 to 2.5 billion years ago, beginning after the Hadean eon and ending where the Proterozoic eon begins. It is divided into four major subunits: Eoarchean (about 4.0–3.6 Ga), Paleoarchean (3.6–3.2 Ga), Mesoarchean (3.2–2.8 Ga), and Neoarchean (2.8–2.5 Ga). During this time, Earth’s crust stabilized and early continents began to form, though tectonic activity and crustal dynamics differed from modern plate tectonics in several respects.

Biologically, the Archaean hosts some of the earliest evidence for life. Microbial ecosystems and stromatolites appear in the fossil record around 3.5 billion years ago, indicating the presence of photosynthetic and other microbial processes. The era saw notable, albeit gradual, development of metabolic pathways, with atmospheric oxygen remaining at very low levels until the subsequent Proterozoic.

Etymology and usage notes that Arquean derives from the same root as Archaean, from Greek archaios meaning ancient. Today, Archaean is the preferred spelling in English, with Arquean appearing chiefly in older or non-English literature. See also Archaean eon.