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 and Oceans
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.
This cooling also allowed for the condensation of water vapor and other gases released from the planet's interior, primarily through volcanic outgassing. 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.
Volcanic outgassing continued to release water vapor, along with gases like carbon dioxide, nitrogen, and others into the early atmosphere. As the planet cooled further, the water vapor in the atmosphere began to condense, which led to precipitation; over millions of years, significant amounts of rain fell onto the planet's surface, filling the depressions in the newly solidified crust and eventually forming the first oceans. This process likely began in earnest ~4.4 GYA, as evidenced by the presence of zircons that indicate interaction with liquid water. The processes that form felsic rocks also often involve water, both in the melting of rocks (hydration melting) and in the cooling and solidification processes. Zircon, which can incorporate water into its crystal structure, implies 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.
The early oceans were likely less saline initially compared to today's oceans. Salinity increased over time due to the leaching of minerals and salts from rocks and soils into the water bodies by the action of rainwater and rivers. The temperature of the early oceans was probably higher than today due to the higher levels of atmospheric CO2 and other greenhouse gases, which trapped more heat. Over time, as the Earth's atmosphere evolved and levels of greenhouse gases changed, the temperature of the oceans fluctuated. Early Earth's atmosphere was very different from today's; it lacked free oxygen (O2) and was rich in gases like water vapor, carbon dioxide (CO2), methane (CH4), and ammonia (NH3). This would have affected weather patterns, making early rain more acidic due to higher levels of CO2, which forms carbonic acid when dissolved in water.
The early oceans were devoid of life as we know it for some time. The origin of life in the oceans, starting with simple microorganisms, significantly altered the chemistry of the oceans over billions of years, particularly with the advent of photosynthesis, which began to produce oxygen.
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 had become a dominant Earth process, featuring 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.
While there was certainly movement and activity, the global-scale continental drift associated with the breakup and formation of supercontinents was not yet a feature of this early period. The interactions between proto-continents or cratons were more localized, without the extensive, ocean-spanning movements observed in later periods.
Archean Volcanism
The Archean Eon witnessed intense volcanic activity that was significantly different in nature from modern volcanic processes. It is notable for the eruption of komatiites, ultramafic lavas that are much hotter (up to about 1600°C / 2912°F) and more fluid than modern basaltic lavas. Komatiites are rich in magnesium and have a low viscosity, allowing them to flow easily across the surface. Their presence suggests significantly hotter mantle temperatures during the eon, which could have driven more dynamic melting and volcanic processes.
In addition to mafic lavas like komatiites, there was also significant felsic volcanism, evidenced by the extensive deposits of rhyolites and dacites. This felsic volcanism is important for the growth of continental crust, as it indicates the differentiation of the Earth's crust and mantle through processes like partial melting and fractional crystallization.
Much of what we know about Archean volcanism comes from greenstone belts, which are sequences of volcanic and sedimentary rocks that have been metamorphosed. These belts include both mafic and felsic volcanic rocks, providing a record of the diverse volcanic activity during the Archean.
Volcanic outgassing was a major source of the Earth's early atmosphere and oceans; volcanoes released water vapor, carbon dioxide, methane, ammonia, and other gases, which contributed to the formation of the hydrosphere and influenced the composition of the early atmosphere. The gases released by Archean volcanism played a role in the Earth's early climate. Carbon dioxide is a potent greenhouse gas that would have helped to warm the planet's surface, counteracting the effects of the faint young Sun, which emitted less energy than it does today.
The hydrothermal systems associated with Archean volcanism could have provided the energy and mineral-rich environments necessary for the origin and early evolution of life. These environments, both on the surface and underwater (near hydrothermal vents), offered conditions suitable for prebiotic chemistry and the emergence of life.
Greenstone Belts
Greenstone belts are significant geological formations found within continental cratons, dating to ~2.5 to 3.8 GYA. They are composed of metamorphosed volcanic and sedimentary rocks that have been intruded and folded into the older gneissic cratons. The 'green' in their name comes from the presence of green minerals such as chlorite, actinolite, and other amphiboles, which are typical in metamorphosed mafic volcanic rocks.
'Gneissic' refers to the texture, composition, and appearance associated with gneiss, a common and widespread type of high-grade metamorphic rock. Gneiss is characterized by its distinct banding and layering (called foliation), resulting from the segregation of mineral components into light and dark bands. This banding is not only a visual marker but also indicates the intense conditions of heat and pressure under which the rock was formed. The gneissic texture is indicative of the dynamic processes within Earth's crust that lead to the metamorphism of igneous or sedimentary rocks.
Gneiss is composed of various minerals, including quartz, feldspar, and biotite, among others. The specific mineral composition can vary widely, depending on the original rock from which the gneiss was metamorphosed and the conditions of metamorphism. The presence of gneissic textures is a clear indicator of high-grade metamorphism; as such, gneiss and gneissic textures are important markers for understanding the metamorphic history of a region.
Greenstone belts are considered key evidence for the operation of plate tectonic processes in the early Earth, similar to modern subduction zones. The sequences of volcanic and sedimentary rocks suggest they were originally formed at oceanic spreading centers and volcanic island arcs, later accreted onto continents through tectonic activity. They contain some of the oldest rocks on Earth and offer a glimpse into the conditions and processes involved in the formation of the early continental crust. The diversity of rock types within greenstone belts, including ultramafic to felsic volcanic rocks and associated sedimentary rocks, reflects a range of volcanic, tectonic, and sedimentary environments.
Some greenstone belts contain evidence of early life in the form of microfossils and stromatolites (layered structures created by microbial communities). These biosignatures point to the existence of life on Earth no later than 3.5 GYA and offer clues about the environmental conditions that supported early life.
Greenstone belts are rich in mineral resources, including gold, copper, zinc, and nickel. The formation of these mineral deposits is closely tied to the geological processes that created the greenstone belts, such as hydrothermal activity / volcanic activity. The deformation and metamorphism that greenstone belts have undergone provide valuable information on the tectonic evolution of the continental crust.
- Stromatolites are layered structures formed by the growth of microbial mats. The layers are created by the trapping, binding, and cementation of sedimentary grains by microbial communities, primarily cyanobacteria. They are considered one of the oldest proofs of life on Earth, indicating the presence of photosynthetic organisms. Stromatolites have been found in the Pilbara Craton of Western Australia and the Barberton Greenstone Belt in South Africa, dating back to as early as 3.5 GYA.
- Microfossils are microscopic remnants or traces of ancient life forms; in Archean rocks, these include simple, single-celled organisms like bacteria and archaea. They directly indicate the presence of life and provide insights into the morphology and ecology of early Earth organisms. Filamentous microfossils resembling cyanobacteria have been discovered in the Apex Chert of the Pilbara Craton, dating to about 3.4 GYA.
- Isotopic biosignatures refer to the specific ratios of stable isotopes (carbon, sulfur) within organic compounds or mineral deposits that suggest biological activity. Certain ratios of carbon isotopes (C-12 to C-13) can indicate the presence of photosynthetic life. They provide indirect evidence of biological processes, such as photosynthesis or sulfate reduction, even when direct fossil evidence is not present. Carbon isotopic ratios indicative of biological activity have been found in rocks and organic carbon deposits within greenstone belts, dating back to over 3.5 GYA.
- While not microfossils themselves, Banded Iron Formations (BIFs) are sedimentary rocks consisting of alternating layers of iron-rich minerals and silica (or chert) that are believed to have formed in part due to the activity of microorganisms, particularly in relation to the oxygen they produced. The presence of BIFs is linked to the action of photosynthetic microbes that contributed to the oxidation of iron in the Earth's early oceans, a byproduct of which was the oxygen that eventually led to the significant rise in atmospheric oxygen (the Great Oxidation Event). BIFs are prevalent in many greenstone belts, including those in the Pilbara Craton and the Barberton Greenstone Belt, with some formations dating back to around 3.5 GYA.
The Archean Atmosphere
Earth's atmosphere was markedly different from the present day, primarily due to the absence of oxygenic photosynthesis in its early stages and the significant influence of volcanic outgassing. It was reducing, meaning it had a capacity to donate electrons to other substances. This was a consequence of its composition, which lacked free oxygen (O₂) and included gases like methane (CH₄), ammonia (NH₃), water vapor (H₂O), and carbon dioxide (CO₂).
The high concentrations of greenhouse gases like CO₂ and CH₄ would have contributed to a significant greenhouse effect, trapping heat and keeping the Earth's surface warm enough to support liquid water, despite the faint young Sun which emitted approximately 70-75% of the energy it does today. The high levels of CO₂ in the atmosphere would have led to acidic rain. However, the overall impact on oceanic pH would have been buffered by interactions with volcanic rocks and minerals on the Earth's surface, leading to the formation of carbonate minerals that could sequester CO₂.
Without significant oxygen, there would have been no ozone (O₃) layer in the upper atmosphere to block ultraviolet (UV) radiation from the Sun. This would have significant implications for the stability of complex organic molecules and the evolution of life, as UV radiation can drive complex chemical reactions, potentially contributing to the prebiotic chemistry. The absence of free oxygen made early Earth's environment anoxic, influencing the types of microbial life that could thrive. Anaerobic organisms, which do not require oxygen, were predominant, with early life forms likely relying on chemical energy sources (chemolithotrophy) or anaerobic photosynthesis.
Water Cycle and Climate
Origin and Early Evolution of Life
Photosynthesis
Carbon Cycle and Its Impact on Climate
Sulfur and Nitrogen Cycles
Role of Iron in Ocean Chemistry
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
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