Archaikum

Archaikum, the German form for the Archaean Eon, is a major interval in Earth’s geological history that spans roughly from 4.0 to 2.5 billion years ago. It follows the Hadean Eon and precedes the Proterozoic. The term Archaikum itself comes from the Greek archaikos, meaning ancient. The eon is subdivided into Eoarchean, Paleoarchean, Mesoarchean, and Neoarchean, reflecting progressively older to younger portions of the era.

During the Archaikum, the Earth's crust began to stabilize and continental crust formed, though the rock record from this time is scarce and often heavily metamorphosed. Some of the oldest preserved crustal materials date to around 4.0 Ga, and detrital zircons up to about 4.4 Ga have been identified. The existence and style of early plate tectonics remain topics of research and debate; evidence suggests crust formation, recycling, and possibly early subduction, but modern plate tectonics may have operated differently than today.

Atmosphere and climate in the Archaikum were likely reducing, with low levels of free oxygen and a Sun that was somewhat dimmer than in the present. Despite these conditions, volcanic outgassing and greenhouse effects kept the surface habitable. Photosynthetic life began to emerge in the later part of the eon, with cyanobacteria contributing to biological activity and setting the stage for rising atmospheric oxygen.

Evidence for life from the Archaikum includes isotopic signatures and possible microfossil-like structures, dated to around 3.5–3.8 Ga, though interpretations are debated. The late Archaikum leads into the Proterozoic as oxygenic photosynthesis increases and atmospheric oxygen begins to accumulate, shaping subsequent evolutionary and geological developments.