Archean Eon

The Archean eon 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.

Artist's impression of the landscape during the early Archean. There was a profusion of volcanoes, the sky was orange (lots of methane), the sea was green (iron), shorelines were marked with stromatolites. (from a painting by Peter Sawyer, © The Smithsonian Institute)

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. 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 first organisms were likely non-photosynthetic, utilizing methane and ammonia 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.

Eoarchean

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.

Mineral:  A mineral is some substance (element or compound) formed by natural geological processes, which usually has a crystalline structure. Examples of minerals are quartz, mica, hematite, olivine, etc.

Rock: A rock is an aggregate of two or more minerals. Therefore, it does not have a fixed chemical composition.  Some rocks contain predominantly a single mineral, such as limestone, which is made primarily of the mineral calcite. Other rocks can be aggregates of many different minerals.

Felsic: These are lighter rocks, with relatively high amounts of silicon (felsic is a combo-word made up of feldspar and silica).  These rocks have a specific gravity less than 3. The commonest felsic rock is granite.  Felsic rocks have a high content of felsic minerals such as quartz, orthoclase and pagioclase.

Mafic:  These rocks are darker colored than felsic rocks, and have specific gravities greater than 3. Basalt is a common mafic rock.  Mafic rocks contain about 75% by weight of mafic minerals, such as olivine, pyroxene, amphibole, etc.

Ultramafic:  Ultramafic rocks have 90% or more mafic minerals. They are usually dark, heavy and high in iron and magnesium. The earth's mantle is made of ultramafic rocks. Most ultramafic rocks on the earth's surface came from the archean, or very early proterozoic.  Volcanoes today rarely produce ultramafic rocks.

Craton: A craton is a very old and stable part of the continental crust. Cratons are at least 500 million years old, though some are much older. Many cratons are the remnants of the earliest proto continents formed during the archean. Cratons are unusually thick, sometimes twice the thickness of the normal crust (200 km instead of 100 km). They are usually found in the central parts of the present day continents. The centers of cratons are made of extremely old rock, known as "shields". Some of the oldest shields include the Australian, Canadian, Siberian, Indian and Scandinavian shields.

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

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.

Banded iron formation, Australia.

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.

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.

 

Read on about the Proterozoic Eon, or return to the index.