Archean

The Archean is the period in the earth's history from about 3.8 to 2.5 billion years ago (Ga). The term was derived from the Latin word for first because the beginning of the Archean is defined as the age of the oldest rocks identified on Earth. As the study of these rocks continues and older rocks are discovered, some scientists now expand the Archean back to 4 billion years to include recently dated rocks. The Archean is part of the Precambrian Era, the entire time span between the formation of the earth 4.56 billion years ago and the beginning of the Cambrian Era, 544 million years ago. The Archean is preceded by the Hadean Eon, a little used term for the period from which no rocks are preserved (4.56–3.8 Ga), and is followed by the Proterozoic (2.5–0.54 Ga).

Archean aged rocks are found mostly in the interior of continents. They provide evidence that Earth during the Archean was a very active geologic environment. Most of the rocks from the early Archean are highly regional metamorphic rocks. These granitic-gneiss versions of either sedimentary or igneous parent rocks suggest a high degree of lithospheric recycling. Later in the Archean, vast lava flows were erupted from undersea rift zones as pillow basalts. Subsequent metamorphism altered the basalts into greenstones. Some sedimentary rocks are preserved from the Archean and are largely coarse and poorly sorted sandstones and conglomerates. These observations suggest that the Archean Earth was very active tectonically with volcanic activity and movement along plate boundaries occurring at a much higher rate than today. The Archean mantle was much hotter than the modern Earth's interior, resulting in heavy mantle convection and crustal turbulence.

The active tectonics of the Archean produced numerous, relatively small continental landmasses that were very mobile as they floated on the turbulent mantle. Toward the end of the Archean, however, these minicontinents had begun to coalesce. By about 2.5 billion years ago when the Archean eon came to an end a more tectonically stable supercontinent had formed from the accreted landmasses. About 70% of modern continents are Archean in age and were derived from this single large landmass. This supercontinent had a much thicker crust than the earlier, smaller crusts and heat flow from the mantle had begun to subside. As a result volcanic and tectonic activity within and along the margins of the supercontinent, were reduced significantly by the start of the Proterozoic.

The first fossilized signs of life appeared in the Archean. Although life probably developed 3.8–3.6 billion years ago as non-photosynthetic bacteria, the oldest evidence of life on Earth are 3.5 Ga old stromatolite fossils from Australia . Stromatolites are finely layered, mound-shaped accumulations of mud trapped by growing mats of blue-green algae. Other early Archean fossils include 3.5 Ga microscopic filamentous structures resembling modern blue-green algae from Australia and cells apparently in various stages of division from South Africa in rocks that are 3.0 Ga old. The Archean atmosphere in which the primordial organisms developed was likely a reducing atmosphere of methane and ammonia. As the Archean progressed and photosynthetic organisms spread, the atmosphere became more oxygen rich.

See also Craton; Greenstone belt