Editing Archean Eon (section)

== 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 Eon|Hadean]].  Isotopic age refers to the determination of the age of a rock, mineral, or other material based on the rate of decay of '''radioactive isotopes''' contained within it. Each radioactive isotope has a characteristic half-life, the time it takes for half of the original amount of the isotope to decay into its daughter product(s). By measuring the ratio of parent isotopes to daughter products in a sample, scientists can calculate the time elapsed since the rock or mineral formed. 

Various isotopic systems are used for dating, each with its own range of applicable ages and materials. Some of the most commonly used systems include uranium-lead (U-Pb), potassium-argon (K-Ar), rubidium-strontium (Rb-Sr), and carbon-14 (C-14), though the latter is mainly used for dating relatively recent materials (up to about 50 kiloyears old). 

~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 <u>magnesium</u> and <u>iron</u> (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 <u>silica</u> (SiO2) and <u>aluminum</u> (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.

=== Iron in Anoxic Conditions ===
In the absence of oxygen, iron was primarily present in its reduced form, '''ferrous iron''' (Fe²⁺), which is highly '''soluble''' in water. This meant that the Archean oceans could have held much higher concentrations of dissolved iron than modern oceans. The primary source of iron to the Archean oceans was likely '''hydrothermal vent''' systems, which emitted large quantities of ferrous iron. Volcanic activity, weathering of terrestrial rocks, and underwater volcanic eruptions also contributed to the iron content in the seawater.

One of the most significant geological impacts of iron in Archean ocean chemistry is the formation of Banded Iron Formations (BIFs). These are sedimentary rocks composed of alternating layers of iron-rich minerals (like hematite Fe₂O₃ or magnetite Fe₃O₄) and silica (SiO₂). The high concentrations of dissolved ferrous iron in the Archean oceans could '''precipitate out''' of solution when oxidized. Given the anoxic conditions, the oxidation agents were likely not free oxygen but rather other oxidants such as UV light or photochemically produced '''free radicals'''.

The deposition of iron in BIFs was likely periodic and linked to changes in the availability of these oxidizing conditions or in the concentration of dissolved iron. This periodicity is reflected in the characteristic banding of BIFs. The formation of BIFs might also have played a role in regulating the levels of atmospheric oxygen, as the process of iron oxidation and precipitation could have acted as a sink for oxygen produced by photosynthetic organisms, delaying the rise of oxygen levels in the atmosphere.

Dissolved iron in the Archean oceans would have also served as an essential '''nutrient''' for early marine life, including the iron-requiring enzymes of photosynthetic organisms. Iron's ability to exist in both reduced (Fe²⁺) and oxidized (Fe³⁺) states under different conditions made it a crucial element for early biological electron transfer processes, influencing the evolution of metabolic pathways.