Archean
The Archean is the eon from about 4.0 to 2.5 billion years ago, when Earth developed its first stable continents and widespread microbial life.
Geologic time scale: Earth’s history divided into nested units such as eons, eras, periods, and epochs. The Archean is one eon in this framework, between the Hadean and Proterozoic.
Craton: An ancient, stable part of continental lithosphere that has resisted major deformation for billions of years. Archean crust survives most clearly within these long-lived continental cores.
Acasta Gneiss: A roughly four-billion-year-old gneiss complex in Canada, among the oldest known intact rock bodies on Earth. It provides direct evidence of ancient continental crust near the Archean’s beginning.
Great Oxidation Event: A major rise in atmospheric oxygen beginning about 2.4 billion years ago. It followed the Archean and marks a profound change in Earth’s surface chemistry.
Origin of life: The set of scientific questions about how living systems first arose from nonliving chemistry. The earliest evidence places life in or before the Archean, but its beginnings remain unknown.
Hadean: The earliest eon of Earth’s history, preceding the Archean and ending about four billion years ago. It marks the turbulent interval before the Archean’s preserved rocks and stable crust.
Greenstone belt: A sequence of metamorphosed volcanic and sedimentary rocks commonly preserved in ancient continental terranes. These belts record Archean volcanism, sediments, and interactions between crust and oceans.
Isua Greenstone Belt: A geological belt in southwest Greenland containing some of Earth’s oldest widely studied rocks. Its rocks preserve clues to early oceans, crust, and possible biological activity.
Banded iron formation: A chemical sedimentary rock with alternating iron-rich and silica-rich layers, widespread in Precambrian deposits. Its formation records interactions between dissolved oceanic iron and changing oxygen availability.
LUCA: The last universal common ancestor of all life alive today, inferred from shared biological features. Its age and environment may connect surviving lineages to Archean ecosystems.