Archean Eon

The Archean is the first formally recognized geological eon (though it follows the informal Hadean Eon that saw the formation of the Sun and its system); it is the period where life first formed on Earth. The atmosphere was vastly different in composition from today's: it was a reducing atmosphere (where oxidation is prevented) rich in methane. The Earth's surface was mostly water: there was continental crust, but most of it was under an ocean deeper than today's.

The earliest known life, mostly represented by shallow-water microbial mats called stromatolites, started in the Archean and remained simple prokaryotes (archaea and bacteria) throughout the eon. The earliest photosynthetic processes, especially those by early cyanobacteria, appeared in the mid/late Archean and led to a permanent chemical change in the ocean and the atmosphere thereafter.

Dating

Starting 4.031 GYA (4.031 billion years ago)

Length 1.531 billion years

Ending 2.500 GYA (2.5 billion years ago)

Etymology

The word Archean is derived from the Greek word arkhē (αρχή), meaning 'beginning, origin'.

Formation of the Earth's Crust

The starting date of the Archean eon is based on the isotopic age of the earliest solid materials on Earth; accretions from before this eon do not survive. The earliest terrestrial material is not rock, but mineral (and dates from the Hadean Eon) most of which has been found in Western Australia. These are grains of relict zircon (a silicate mineral, zirconium silicate, ZrSiO₄, the principal source of zirconium), with an isotopic age of 4.2-4.4 GY. Evidence of rocks from the preceding Hadean Eon are restricted to non-rock and non-terrestrial sources, such as individual mineral grains and lunar samples.

The oldest rock formations exposed on the surface of the Earth are Archean; they have been found in Greenland, Siberia, the Canadian Shield, Montana, Wyoming (exposed parts of the Wyoming Craton), Minnesota (Minnesota River Valley), the Baltic Shield, the Rhodope Massif, Scotland, India, Brazil, western Australia, and southern Africa. Granitic rocks predominate throughout the crystalline remnants of the surviving Archean crust.

These include great melt sheets and voluminous plutonic masses of granite, diorite, layered intrusions, anorthosites and monzonites known as sanukitoids. Archean rocks are often heavily metamorphized deep-water sediments, such as graywackes, mudstones, volcanic sediments, and banded iron formations. Volcanic activity was considerably higher than today, with numerous lava eruptions, including unusual types such as komatiite. Carbonate rocks are rare, indicating that the oceans were more acidic, due to dissolved carbon dioxide. Greenstone belts are typical Archean formations, consisting of alternating units of metamorphosed mafic igneous and sedimentary rocks, including Archean felsic volcanic rocks. The metamorphosed igneous rocks were derived from volcanic island arcs, while the metamorphosed sediments represent deep-sea sediments eroded from the neighboring island arcs and deposited in a forearc basin. Greenstone belts, which include both types of metamorphosed rock, represent sutures between the protocontinents.

Plate tectonics likely started vigorously in the Hadean but slowed down in the Archean, probably due to an increase in the viscosity of the mantle due to outgassing of its water. Plate tectonics likely produced large amounts of continental crust, but the deep oceans of the Archean probably covered the continents entirely. Only at the end of the Archean did the continents likely emerge from the ocean, initiating continental weathering that left its mark on the oxygen isotope record by enriching seawater with isotopically light oxygen.

When the Archean began, Earth's internal heat budget (the flow of heat from the planet's interior to its surface) was nearly three times as high as it is today (estimated at 47±2 terawatts).

Initial Crust Formation
Crustal Differentiation and Recycling
Growth of Continental Crust

Geological Processes and Features

Plate Tectonics and Early Continental Drift
Formation of the First Supercontinents
Greenstone Belts and Their Significance
Archean Volcanism
Impact Cratering

Atmosphere and Hydrosphere Development

Evolution of the Early Atmosphere
Formation of the Oceans
Atmospheric Composition and Greenhouse Gases
Water Cycle and Climate

Origin and Evolution of Life

Emergence of the First Life Forms
Microbial Mats and Stromatolites
Evolution of Photosynthesis
Archean Ecosystems and Biodiversity

Geochemical Cycles

Carbon Cycle and Its Impact on Climate
Sulfur and Nitrogen Cycles
Role of Iron in Ocean Chemistry

Mineral Deposits and Economic Geology

Banded Iron Formations (BIFs)
Archean Gold and Nickel Deposits
Greenstone-Hosted Mineral Resources

Tectonic and Magmatic Activity

Archean Tectonic Regimes
Cratonization and Craton Stability
Komatiites and Their Petrogenesis

Precambrian Life and the Fossil Record

Evidence of Early Life and Dating Techniques
Fossilized Microorganisms and Biochemical Markers
The Great Oxygenation Event (GOE)

Archean Paleoenvironment and Climate

Proxy Records and Climate Indicators
Archean Ice Ages: Evidence and Controversies
Solar Evolution and Its Impact on Climate

Scientific Exploration and Techniques

Zircon Crystals and U-Pb Dating
Isotope Geochemistry in Archean Studies
Deep Time and the Geological Time Scale

Challenges in Archean Studies

Preservation Bias and the Rock Record
Interpreting the Early Geological Record
Debates on the Timing and Mechanisms of Key Events