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, except in a few rare relict mineral formations from the Hadean.
~4.5 GYA, during the Hadean eon, the Earth was covered in a global magma ocean. As the planet cooled, the first solid crust began to form. This was primarily mafic (or ultramafic) in composition, largely minerals crystallized out of the cooling magma. These rocks are rich in magnesium and iron (ma- from magnesium, and -fic from ferrous, referring to iron, Fe). They are generally darker in hue, and denser than felsic rocks, and include minerals like olivine, pyroxene, amphibole, biotite; common mafic rocks include basalt (the most common rock type in the ocean's crust) and gabbro. By ~4.4 GYA, the magma ocean had solidified through cooling, which resulted in the Earth's initial crust.
As the crust cooled, it underwent differentiation, becoming chemically stratified. Heavier elements sank and allowed lighter elements to rise, which led to the formation of a more felsic crust. Felsic rocks are rich in silica (SiO2) and aluminum (Al), with lower quantities of magnesium and iron. Its name comes from its constituents, fel- from feldspar and -sic from silica. Common minerals include quartz, muscovite, and feldspars; zircon crystals in the Jack Hills (Western Australia) and similar relict minerals date to ~4.4 GYA and are the only lasting remnants of the Archean's earliest age. Granite is a well-known felsic rock and is prevalent in the continental crust; the dating of the rising of the felsic crust can be approximated to this period.
The processes that form felsic rocks often involve water, both in the melting of rocks (hydration melting) and in the cooling and solidification processes. The presence of zircon, which can incorporate water into its crystal structure, suggests that liquid water was present on the Earth's surface.
Until ~4 GYA, continued volcanic activity and the gradual accretion of crustal material contribute to the growth of these early crustal blocks, and the Earth's crust continued to differentiate, with more felsic material being generated through partial melting. Over time, through processes of magmatic differentiation and tectonic activity, parts of this crust became thicker and more compositionally diverse.
Plate Tectonics
The timing for the onset of plate tectonics is debated, but there is a consensus that some form of tectonic activity was occurring by at least 3.0 GYA, with some evidence suggesting it could have started as early as 4.0 GYA. Early tectonic activity would have been more akin to gradual movements or proto-plate tectonics, influenced by thermal gradients in the young Earth's mantle. Some of the earliest evidence for plate tectonics comes from greenstone belts and other rock assemblages, suggesting subduction-like processes were occurring.
No later than 3 GYA, as plate tectonics gradually became a dominant Earth process, the recycling of crustal material through subduction and the addition of new material from volcanic activity contributed to the growth of continental nuclei or cratons.
These cratons are characterized by their thick lithosphere and old, rigid rock formations that have remained largely unchanged by tectonic processes for billions of years. The process by which these early crustal blocks became cratons is known as cratonization, which involves the thickening of the lithosphere, often through tectonic processes such as the collision of crustal blocks, volcanic activity, and the accretion of sediments and other materials. Some of the rocks within are dated as early as 3.8 to 3.6 GYA.
Today, cratons can be found at the core of all modern continents, including such well-known examples as the Canadian Shield in North America, the Baltic Shield in Europe, and the Kaapvaal Craton in southern Africa.
Subduction zones not only generate new crustal material but also recycle older, denser crust back into the mantle. This recycling process is essential for the dynamic evolution of the Earth's surface. The movement of tectonic plates plays a crucial role in the formation and assembly of continents. Collisions between plates can lead to the formation of mountain ranges and the thickening of the crust, which contributes to the stability of continental regions.
Several theories attempt to explain how plate tectonics started:
- Gravitational Sliding: the cooling of the Earth's surface could have led to the formation of a rigid lithosphere that started to move over the more ductile asthenosphere beneath due to gravitational forces.
- Mantle Convection: as Earth cooled, convection currents within the mantle could have started to drag and break apart the crust into plates. The sinking of cooler, denser crust into the mantle at subduction zones and the rise of hot mantle material at mid-ocean ridges are key drivers of plate movements.
- Incremental Assembly: the process may have begun with smaller, proto-plates engaging in early subduction and accretion activities, gradually leading to the development of larger plates and more active tectonics.
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
- Preservation Bias and the Rock Record
- Interpreting the Early Geological Record
- Debates on the Timing and Mechanisms of Key Events