• Geologic Eon Archean, 3.8 Ga
  
• Geochemist and Geochronologist Richard Lee "Dick" Armstrong, 1937
  

Instead of being based on stratigraphy as all other geological ages are, the beginning of the Archean eon is defined chronometrically. The lower boundary (starting point) of 3.8 thousand million years has not been officially recognized by the International Commission on Stratigraphy.

The Archean customarily starts at 3.8 Ga — at the end of the Hadean Eon. In older literature, the Hadean is included as part of the Archean. The name comes from the ancient Greek Αρχή (Arkhē), meaning "beginning, origin".

The Archean is one of the four principal eons of Earth history. 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 cratonization 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. 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.

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.

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.

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 (formerly known as Monera). 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.

Richard Lee “Dick” Armstrong FRSC (August 4, 1937 – August 9, 1991) was an American / Canadian scientist who was an expert in the fields of radiogenic isotope geochemistry and geochronology, geochemical evolution of the earth, geology of the American Cordillera, and large - magnitude crustal extension. He published over 170 scientific papers.

Armstrong was born in Seattle, Washington.

In 1955, he moved to New Haven, Connecticut, to attend Yale University. He obtained his BSc in 1959 and a PhD in 1964. He stayed at Yale as assistant and associate professor in the geology department until 1973. While he was a Yale professor, he took two leaves, the first in 1963 - 1964 on a National Science Foundation Postdoctoral Fellowship at the University of Berne, and in 1968 - 1969 as a Morse and Guggenheim Fellow at the Australian National University and California Institute of Technology.

In 1973, Armstrong moved to Vancouver, Canada, to be an associate professor at the University of British Columbia. He was eventually made a full professor. In 1979, he became a Canadian citizen.

Armstrong studied the chronology of magmatism, metamorphism, and tectonics of western North America. He utilized several methodologies, including Potassium - Argon, Rubidium - Strontium, Uranium - Lead and Neodymium - Samarium to obtain isotopic data.

Armstrong’s early theories guided research for a generation. His views were controversial and contested by many prominent isotope geochemists. It took decades for other scientists to accept his ideas. Before he died, Armstrong was vindicated through the recognition he received for his model of crustal recycling at the 1990 ICOG meeting in Canberra. Most of Armstrong's effort was spent systematically building an enormous database on the American Cordillera. This database has provided a wealth of information to the scientific community.

Armstrong was an active member of the Geological Society of America and editorial boards for several journals. He participated in the peer review process of the National Science Foundation and Canada's Natural Sciences and Engineering Research Council. He also participated in Canada's Lithoprobe program a national geoscience research project.

In 1991, Armstrong died of cancer.