Archean

Archean is a division of the geologic timescale that spans roughly from 4.0 to 2.5 billion years ago. The name derives from the Greek arche, meaning ancient or first. It follows the Hadean eon and precedes the Proterozoic.

During the Archean, Earth’s crust cooled and thickened, enabling the formation of the first stable continental masses. Greenstone belts and early cratons record crustal growth through accretion and partial melting. The existence and style of early plate tectonics are debated; evidence suggests subduction-related processes and horizontal movement in places, but the tempo and mechanism of early tectonics may have differed from modern plate tectonics.

Biological activity appears in the Archean’s rock record. The earliest widely accepted signs of life are microbial in nature, including stromatolites and microfossils dating to around 3.5 billion years ago. Isotopic data also point to biological influence on metabolism. Life at this time was primarily anaerobic, and the rise of oxygenic photosynthesis—and the attendant accumulation of atmospheric oxygen—likely occurred toward the late Archean or in the subsequent Proterozoic.

The Archean climate and atmosphere were distinct from today’s. The atmosphere contained little free oxygen, and surface temperatures were influenced by a higher mantle heat flux and greenhouse gases that helped compensate for the faint young Sun. The deeper oceans were, in many places, reducing environments favorable to early microbial life.

Subdivisions commonly used for Archean time are the 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). The Archean ends with transitions to the Proterozoic eon, marked by further continental growth and evolving biospheric conditions.