The Archeozoic Era: A Time of Ancient Rocks and Continental Formation

The Archeozoic Era stretches from about 3.8 billion to 2.5 billion years ago. Traditionally, the beginning of the Archean is defined to coincide with the oldest rocks discovered. As recent discoveries have pushed back the earliest dated rocks to about 4.0 billion years old, the beginning of the Archean has also been pushed back correspondingly. However, most texts still continue to date the beginning to 3.8 billion years ago. As the Late Heavy Bombardment (LHB) ended with the Hadean, the newly forming crust continued to stabilize, and eventually led to the creation of the continents.

When the continents first appeared is still under debate. The Earth in this period was moderately warm. Although the sun was about 30% cooler than it is today, the geological activity of the earth was much higher, leading to a somewhat temperate climate. Most of the earth was covered with oceans. The atmosphere contained mostly methane and little to no oxygen; therefore it is considered a reducing atmosphere. Although recent discoveries may change this view, it is generally believed that life first evolved in the Archean.

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Some of the oldest fossils of life on Earth include the Apex Chert (3.465 billion years old) and stromatolites (3.45 billion years old) from Australia, and the Swaziland microfossils from Africa (also about 3.45 billion years old). Dating the oldest life forms is difficult. Stromatolite-like structures have been shown to be as old as 3.5 billion years, but it can be debated whether they were made by living organisms, or natural forces (hydrothermal vents). The earliest conclusive radiometric markers of life (such as O-12 uptake, or the first evidence of photosynthesis, for example), date to about 2.

7 billion years old. However, it is widely believed that the first life appeared much earlier, possibly around the beginning of the Archean, around 3.8 billion years ago, or even in the Hadean. The earliest chemical markers of life are dated to about 3.8 billion years, but this is not the same as finding microfossils. [EDIT: the oldest conclusive evidence of life has been pushed back to about 3.43 billion years old, at Strelley Pool in Western Australia.] The first organisms were likely non-photosynthetic, utilizing methane, ammonia or sulfates for their energy needs.

Photosynthesis became common with the cyanobacteria, perhaps as early as 3.5 billion years ago. The oxygen produced by these bacteria went into oxidizing rocks on the Earth and the iron in the oceans, so there was no increase in atmospheric oxygen for a very long time. Atmospheric oxygen did not begin to rise significantly until billions of years after photosynthesis first began. The Archean was the period in which continent formation first began. The surface of the Earth had started to solidify in the Hadean, with the presence of liquid water as early as 100 million years after the formation of the Earth. But the early crust was unstable, and was continually eroded, recycled and re melted. During the Archean these areas of land increased in size and during the middle Archean the first continent sized expanses of land first appeared.

These proto continents no longer exist, but their remnants are sometimes found in cratons, areas of ancient rock that survive on some of the continental shields today. Cratons typically appear when the overlying rock (mostly volcanic igneous rock) is buried deep, but not deep enough to be re melted. Instead, the heat and pressure converts it into metamorphic rock. These are areas where the crust has thickened, with fresh igneous rock on top and metamorphic rock beneath (though folding of the crust can obscure this relationship). For reasons that are not well understood, there were extensive cratonization events towards the last third of the Archean, which have never been repeated in the history of the Earth. However, continents as we know them today, with continental plates and plate tectonics did not appear until the very end of the Archean.

The Earth

When the Archean began, the Earth's heat flow was nearly three times higher than it is today, and it was still twice the current level at the transition from the Archean to the Proterozoic (2,500 Ma). The extra heat was the result of a mix of remnant heat from planetary accretion, heat from the formation of the Earth's core, and heat produced by radioactive elements. Most surviving Archean rocks are metamorphic or igneous. Volcanic activity was considerably higher than today, with numerous lava eruptions, including unusual types such as komatiite. Granitic rocks predominate throughout the crystalline remnants of the surviving Archean crust. Examples include great melt sheets and voluminous plutonic masses of granite, diorite, layered intrusions, anorthosites and monzonites known as sanukitoids. The Earth of the early Archean may have supported a tectonic regime unlike that of the present. Some scientists argue that, because the Earth was much hotter, tectonic activity was more vigorous than it is today, resulting in a much faster rate of recycling of crustal material.

This may have prevented cratonisation and continent formation until the mantle cooled and convection slowed down. Others argue that the oceanic lithosphere was too buoyant to subduct, and that the rarity of Archean rocks is a function of erosion by subsequent tectonic events. The question of whether plate tectonic activity existed in the Archean is an active area of modern research. There are two schools of thought concerning the amount of continental crust that was present in the Archean. One school maintains that no large continents existed until late in the Archean: small protocontinents were the norm, prevented from coalescing into larger units by the high rate of geologic activity.

The other school follows the teaching of Richard Armstrong, who argued that the continents grew to their present volume in the first 500 million years of Earth history and have maintained a near-constant ever since: throughout most of Earth history, recycling of continental material crust back to the mantle in subduction or collision zones balances crustal growth. Opinion is also divided about the mechanism of continental crustal growth. Those scientists who doubt that plate tectonics operated in the Archean argue that the felsic protocontinents formed at hotspots rather than subduction zones. Through a process called "sagduction", which refers to partial melting in downward-directed diapirs, a variety of mafic magmas produce intermediate and felsic rocks.[citation needed]

Others accept that granite formation in island arcs and convergent margins was part of the plate tectonic process, which has operated since at least the start of the Archean. An explanation for the general lack of Hadean rocks (older than 3800 Ma) is the efficiency of the processes that either cycled these rocks back into the mantle or effaced any isotopic record of their antiquity. All rocks in the continental crust are subject to metamorphism, partial melting and tectonic erosion during multiple orogenic events and the chance of survival at the surface decreases with increasing age. In addition, a period of intense meteorite bombardment in the period 4.0-3.8 Ga pulverized all rocks at the Earth's surface during the period. The similar age of the oldest surviving rocks and the "late heavy bombardment" is thought to be not accidental

Palaeoenvironment

The Archean atmosphere is thought to have nearly lacked free oxygen. Astronomers think that the sun had about 70–75% of the present luminosity, yet temperatures appear to have been near modern levels even within 500 Ma of Earth's formation, which is puzzling the faint young sun paradox. The presence of liquid water is evidenced by certain highly deformed gneisses produced by metamorphism of sedimentary protoliths. The equable temperatures may reflect the presence of larger amounts of greenhouse gases than later in the Earth's history.

Alternatively, Earth's albedo may have been lower at the time, due to less land area and cloud cover. By the end of the Archaean c. 2500 Mya, plate tectonic activity may have been similar to that of the modern Earth. There are well-preserved sedimentary basins, and evidence of volcanic arcs, intracontinental rifts, continent-continent collisions and widespread globe-spanning orogenic events suggesting the assembly and destruction of one and perhaps several supercontinents. Liquid water was prevalent, and deep oceanic basins are known to have existed by the presence of banded iron formations, chert beds, chemical sediments and pillow basalts.

Geology

Although a few mineral grains are known that are Hadean, the oldest rock formations exposed on the surface of the Earth are Archean or slightly older. Archean rocks are known from Greenland, the Canadian Shield, the Baltic Shield, Scotland, India, Brazil, western Australia, and southern Africa. Although the first continents formed during this eon, rock of this age makes up only 7% of the world's current cratons; even allowing for erosion and destruction of past formations, evidence suggests that continental crust equivalent to only 5-40% of the present amount formed during the Archean. In contrast to Proterozoic rocks, Archean rocks are often heavily metamorphized deep-water sediments, such as graywackes, mudstones, volcanic sediments, and banded iron formations.

Carbonate rocks are rare, indicating that the oceans were more acidic due to dissolved carbon dioxide than during the Proterozoic. Greenstone belts are typical Archean formations, consisting of alternating units of metamorphosed mafic igneous and sedimentary rocks. The meta-igneous rocks were derived from volcanic island arcs, while the metasediments represent deep-sea sediments eroded from the neighboring island arcs and deposited in a forearc basin. Greenstone belts represent sutures between protocontinents

Life during the Era

Fossils of cyanobacterial mats (stromatolites, which were instrumental in creating the free oxygen in the atmosphere ) are found throughout the Archean, becoming especially common late in the eon, while a few probable bacterial fossils are known from chert beds. In addition to the domain Bacteria (once known as Eubacteria), microfossils of the domain Archaea have also been identified. Life was probably present throughout the Archean, but may have been limited to simple non-nucleated single-celled organisms, called Prokaryota There are no known eukaryotic fossils, though they might have evolved during the Archean without leaving any fossils.No fossil evidence has been discovered for ultramicroscopic intracellular replicators such as viruses.

Eoarchean Period

The earliest part of the Archean eon is known as the Eoarchean. We've defined it chronometrically as a 200 million year period from 3.8 to 3.6 billion years, although the earlier boundary (3.8 billion) is not universally recognized. Since the Archean begins roughly with the earliest known rocks, the beginning of the Eoarchean will vary, based on estimates of the ages of the oldest rocks currently known. The Eoarchean is best known through the Isua Greenstone Belt, which is the oldest known rock formation (3.8 - 3.7 billion years old). This area, located in southwestern Greenland, contains metamorphosed volcanic (mafic) and sedimentary rocks. Much of the belt is derived from basaltic and high-magnesium basaltic pillow lavas. During the Eoarchean, crust formation (which began in the Hadean) continued.

Due to the cessation of LHB, some of this crust survived and became incorporated into continents, which formed much later. The earth was mostly covered with water, with volcanoes and volcanic islands emerging here and there. The oceans were green and acidic from dissolved iron compounds. They sky was orange from high concentrations of methane, ammonia and carbon dioxide. The climate was probably temperate. Earth produced about 3 times as much heat internally as it does today, which compensated for the dimmer sun, and made the earth intensely geoactive. Life first emerged during this period, if not earlier. The earliest life was probably based on methane or some similar chemistry.

Paleoarchean Period

The Paleoarchean is a 400 million year long period within the archean eon, dating from 3.6 to 3.2 billion years ago. There are no specific rocks layers that separate this level - it has been defined chronometrically. This era is very significant for the history of life on earth. Both archaea and eubacteria evolved during the paleoarchean, implying that the last universal common ancestor (LUCA) of all life of earth existed during this era. The oldest stromatolites date back to about 3.5 billion years, within the Paleoarchean. These were colonies of cyanobacteria, which are the only class of bacteria that produce oxygen as a by-product of photosynthesis. They might not have been the oldest photosynthetic bacteria (some reports suggest that purple bacteria or rhodobacter developed photosynthesis first), but vast numbers of cyanobacteria were instrumental in changing the geology of earth and the evolution of life through the production of oxygen.

Although cyanobacteria first started producing oxygen in this era, it is important to remember that no significant amounts of oxygen existed in the atmosphere at this time, because of vast quantities of oxidizable materials in the earth's crust and the iron in the oceans, which absorbed any oxygen that was produced. Continent formation continued, with increasingly larger land masses emerging from the oceans. It has been proposed that the first super continent, Vaalbara, came into existence in this era, around 3.3 billion years ago (may have been as early as 3.6 billion years ago). This is based on the similarity in sedimentary sequences on the South African Kaapvaal craton and the West Australian Pilbara craton (hence the name vaal-bara). This theory is controversial, and if Vaalbara did exist, it had started to break up by about 2.8 billion years ago, shown by the diverging paleomagnetic history of these two cratons from that time on.

Mesoarchean

The Mesoarchean is another era that has been defined chronometrically, rather than geologically. This era covers the middle of the archean, from 3.2 to 2.8 billion years ago. The Mesoarchean continued the trends from the previous Paleoarchean era. Continent formation continued. Plate tectonics forced the separation of the Kaapvaal and Pilbara cratons, and the separation of these ancient parts of South Africa and Australia was complete by the end of the Mesoarchean, around 2.8 billion years ago. Another super continent that may have originated during the mesoarchean was Ur. This consisted of the South African Kaapvaal and West Australian Pilbara cratons (which were originally together in Vaalbara, but no longer contiguous now), plus the Indian Bhandara and Singhbhum cratons, and some regions of what is now the east Antarctica.

It is believed that Ur survived for a very long time, joining with other cratons to later form Rodinia, and even later, Pangaea. Although life evolved much earlier, the first incontrovertible fossils appear from this period. Stromatolites were prevalent in coastal waters, with their cyanobacteria continuing to pump oxygen into the atmosphere. However, atmospheric oxygen levels remained very low, as the oxygen continued to be used up in oxidizing minerals on the earth's crust and in the sea. All life from this period was consequently anaerobic. The oldest banded iron formations (BIFs) are dated to this period. BIFs are a type of sedimentary rock, consisting of layers of iron-rich minerals such as hematite and magnetite, alternating with iron-poor layers of shale and chert. It is believed that oxygen produced by the cyanobacteria precipitated out the iron (as oxides) which had previously been dissolved in the acidic oceans.

The layering indicates a pattern of cyclical activity, showing oxygen "pulses". It is unknown if these pulses corresponded to seasonal activity or some other factor. The formation of banded iron formations continued until as recently as 1.8 billion years ago, at which point it is presumed that most of the iron in the seas had already been precipitated out. There are some more recent formations, that were thought to represent events corresponding to local oxygen depletion (if oxygen is depleted, iron continues to wash into the sea through the rivers and accumulates in solution until the oxygen level rises again and it is precipitated).

However, more recent research shows that this "local" oxygen depletion may have been global -- the result of the "snowball earth" scenario where all life (including cyanobacteria) came close to extinction. Banded iron formations contain enormous amounts of oxygen, perhaps as much as 20 times the amount of oxygen present in the atmosphere today. Together with other such oxygen "sinks" they explain why it took so long for atmospheric oxygen levels to start rising after the appearance of the cyanobacteria.

Neoarchean

The last 300 million years of the Archean eon have been chronometrically classified as the Neoarchean, from about 2.8 billion years ago to 2.5 billion years ago. Many of the processes described earlier, that originated in the Mesoarchean, established themselves in the Neoarchean. Cyanobacteria started producing significant amounts of oxygen in this period. This eventually lead to the Oxygen Catastrophe during the early proterozoic, in which rising levels of oxygen poisoned much of the life that existed at the time. There is some evidence that life first colonized land during this period. There has been some evidence that microbes colonized some land masses as early as 2.75 billion years ago, but the thinking was that such colonization was very limited in scope and insignificant.

However, more recently, evidence has started to accumulate that there may have been a large scale colonization of land by microbes, which broke down rocks to release sulfur and molybdenum that eventually washed into the oceans. This was thought unlikely because at the time there was no ozone layer (which appeared hundreds of millions of years later after the oxygen catastrophe, after oxygen levels had built up sufficiently in the atmosphere), so life on land was unprotected from UV rays. However, microbes may have lived deep within the rocks.

During the Neoarchean, large continents first appeared on earth, with modern plate tectonics (with subduction zones, continental plates sliding over each other and the upwelling of lava to produce new crust where continental plates tore apart). The first large continents were formed (when we call previously existing continents such as Vaalbara or Ur "super continents" it's not because of size -- they were smaller than Australia -- but because they were the only continents around). Certainly there was recycling of crust prior to this period (perhaps all the way back to the hadean), but earlier continents formed at hotspots over mantle plumes, rather than at subduction zones.

Continents basically grow by getting lighter and tougher. Cyclic re-melting and reformation of rock through lava flows (igneous differentiation) gradually separates the lighter minerals, and allows the development of felsic rocks from mafic rocks. Lighter rocks are more buoyant, and resist recycling by floating over the liquid mantle. The archean ended about 2.5 million years ago, with the beginning of the proterozoic. This was the end of the period when mostly geological processes affected the surface of the Earth, and the beginning of the period when life started to play a significant part in what was happening on Earth.