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LECTURE 2: THE EARLIEST EARTHLY EON

The Hadean (pronounced /ˈheɪdiən/) is the geologic eon before the Archean. It started at Earth's formation about 4.6 billion years ago (4600 Ma), and ended roughly 3.8 billion years ago, though the latter date varies according to different sources. The name "Hadean" derives from Hades, Greek for "unseen" or "Hell" and suggesting the underworld or referring to the conditions on Earth at the time. The geologist Preston Cloud coined the term in 1972, originally to label the period before the earliest-known rocks. W. B. Harland later coined an almost synonymous term: the "Priscoan period". Other older texts simply refer to the eon as the Pre-Archean, while during much of the 19th and 20th centuries, the term Azoic (meaning without life) .

Since few geological traces of this period remain on Earth there are no official subdivisions. However, several major divisions of the lunar geologic timescale occurred during the Hadean, and so these are sometimes used unofficially to refer to the same periods of time on Earth.

In the last decades of the 20th century geologists identified a few Hadean rocks from Western Greenland, Northwestern Canada and Western Australia. The oldest known rock formations (the Isua greenstone belt) comprise sediments from Greenland dated around 3.8 billion years ago somewhat altered by a volcanic dike that penetrated the rocks after they were deposited. Individual zircon crystals redeposited in sediments in Western Canada and the Jack Hills region of Western Australia are much older. The oldest dated zircons date from about 4400 Ma^[1] - very close to the hypothesized time of the Earth's formation.

The Greenland sediments include banded iron beds. They contain possibly organic carbon and imply some possibility that photosynthetic life had already emerged at that time. The oldest known fossils (from Australia) date from a few hundred million years later.

The late heavy bombardment happened during Hadean times and affected the Earth and the Moon.

A sizeable quantity of water would have been in the material which formed the Earth.^[2] Water molecules would have escaped Earth's gravity until the planet attained a radius of about 40% of its current size; after that point, water (and other volatile substances) would have been retained.^[3] Hydrogen and helium are expected to continually leak from the atmosphere, but the lack of denser noble gases in the modern atmosphere suggests that something disastrous happened to the early atmosphere.

Part of the young planet is theorized to have been disrupted by the impact which created the Moon, which should have caused melting of one or two large areas. Present composition does not match complete melting and it is hard to completely melt and mix huge rock masses.^[4] However, a fair fraction of material should have been vaporized by this impact, creating a rock vapor atmosphere around the young planet. The rock vapor would have condensed within two thousand years, leaving behind hot volatiles which probably resulted in a heavy carbon dioxide atmosphere with hydrogen and water vapor. Liquid water oceans existed despite the surface temperature of 230°C because of the atmospheric pressure of the heavy CO₂ atmosphere. As cooling continued, subduction and dissolving in ocean water removed most CO₂ from the atmosphere but levels oscillated wildly as new surface and mantle cycles appeared.^[5]

Study of zircons has found that liquid water must have existed as long ago as 4400 Ma, very soon after the formation of the Earth.^[6][7][8] This requires the presence of an atmosphere.

The oldest rock or rocks on Earth, as an aggregate of minerals, are from the Archean Eon and are only partially exposed on the surface.^[1]

There is some controversy about the oldest rocks based on the oldest dated mineral zircon. Some of the oldest surface rock can be found in the Canadian Shield, Australia, Africa and in other more specific places around the world. The ages of these felsic rocks are generally between 2.5 and 3.8 billion Earth years. The approximate ages have a large margin of error, of plus or minus thousands, if not millions of Earth years.

The oldest material of terrestrial origin that has been dated is a zircon mineral of 4,404 +/- 8 Ma from a sedimentary gneiss in the Jack Hills of the Narryer Gneiss Terrane of Australia. ^[2] This zircon is part of a population of zircons within the gneiss of greater than 3,900 Ma; the gneiss is considered to be no older than 3,800 Ma, which is the age of the youngest zircon in the rock.

The oldest rock formation on Earth is, depending on the latest research, either part of the Isua Greenstone Belt, Narryer Gneiss Terrane or the Acasta Gneiss. The difficulty in assigning the title to one particular block of gneiss is that the gneisses are all extremely deformed, and the oldest rock may be represented by only one streak of minerals in a mylonite, representing a layer of sediment or an old dyke. This may be difficult to find or map, and hence, the oldest dates yet resolved are as much generated by luck in sampling as by understanding the rocks themselves.

It is thus premature to claim any of these rocks, or indeed that other formations of early Archaean gneisses, are the oldest formations or rocks on Earth; doubtless new analyses will continue to change our conceptions of the structure and nature of these ancient continental fragments.

Nevertheless, the oldest cratons on Earth include the Kaapvaal craton, the Western Gneiss Terrane of the Yilgarn craton (~2.9 - >3.2 Ga), the Pilbara Craton (~3.4 Ga), and portions of the Canadian Shield (~2.4 - >3.6 Ga). Parts of the poorly studied Dharwar craton in India are greater than 3.0 Ga.

The Acasta Gneiss in the Canadian Shield in the Northwest Territories, Canada is composed of the Archaean igneous and gneissic cores of ancient mountain chains that have been exposed in a glacial peneplain. Analyses of zircons from a felsic orthogneiss with presumed granitic protolith returned an age of 4.03 Ga, which is the current oldest known terrestrial rock.

A potentially older rock was found in the Jack Hills metaconglomerate of Western Australia, but there is some controversy surrounding its actual age (see below). A zircon crystal dating to 4.4 Ga was found in the Jack Hills that may be the current oldest known terrestrial material. ^{[3] [4]}

The Archean (pronounced /ɑrˈkiːən/, also spelled Archaean, formerly called the Archaeozoic also spelled Archeozoic or Archæozoic) is a geologic eon before the Proterozoic and Paleoproterozoic, ending 2.5 Ga (billion years ago). Instead of being based on stratigraphy, this date is defined chronometrically. The lower boundary (starting point) has not been officially recognized by the International Commission on Stratigraphy, but it is usually set to 3.8 Ga, at the end of the Hadean eon.

At the beginning of the Archean, the Earth's heat flow was nearly three times higher than it is today, and was still twice the current level by the beginning of the Proterozoic. The extra heat may have been remnant heat from the planetary accretion, partly heat of formation of the iron core, and partially caused by greater radiogenic heat production from short-lived radionuclides such as uranium-235.

The majority of Archean rocks which exist are metamorphic and igneous rocks, the bulk of the latter being intrusive.^[clarify] Volcanic activity was considerably higher than today, with numerous hot spots, and rift valleys, and eruption of unusual lavas such as komatiite. Intrusive igneous rocks such as great melt sheets and voluminous plutonic masses of granite, diorite, ultramafic to mafic layered intrusions, anorthosites and monzonites known as sanukitoids predominate throughout the crystalline cratonic remnants of the Archean crust which exist today.

The Earth of the early Archean may have had a different tectonic style. Some scientists think that, because the Earth was hotter, plate tectonic activity was more vigorous than it is today, resulting in a much greater 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 sub continental lithospheric mantle is too buoyant to subduct and that the lack of Archean rocks is a function of erosion by subsequent tectonic events. The question of whether or not plate tectonic activity existed in the Archean is an active area of modern geoscientific research. ^[1]

There were no large continents until late in the Archean; small protocontinents were the norm, prevented from coalescing into larger units by the high rate of geologic activity. These felsic protocontinents probably formed at hotspots rather than subduction zones, from a variety of sources: igneous differentiation of mafic rocks to produce intermediate and felsic rocks, mafic magma melting more felsic rocks and forcing granitization of intermediate rocks, partial melting of mafic rock, and from the metamorphic alteration of felsic sedimentary rocks. Such continental fragments may not have been preserved if they were not buoyant enough or fortunate enough to avoid energetic subduction zones.^[2]

Another explanation for a general lack of early Archean rocks greater than 3800 Ma is the amount of extrasolar debris present within the early solar system. Even after planetary formation, considerable volumes of large asteroids and meteorites still existed, and bombarded the early Earth until approximately 3800 Ma. A barrage of particularly large impactors known as the late heavy bombardment may have prevented any large crustal fragments from forming by literally shattering the early protocontinents.

The Archean atmosphere apparently lacked free oxygen. Temperatures appear to have been near modern levels even within 500 Ma of Earth's formation, with liquid water present, due to the presence of sedimentary rocks within certain highly deformed gneisses. Astronomers think that the sun was about one-third dimmer, which may have contributed to lower global temperatures than otherwise expected. This is thought to reflect larger amounts of greenhouse gases than later in the Earth's history.

By the end of the Archaean c. 2600 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 older, 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, 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 only 5-40% of the present continental crust formed during the Archean.^[3]

In contrast to the Proterozoic, 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.^[4] Greenstone belts are typical Archean formations, consisting of alternating high and low-grade metamorphic rocks. The high-grade rocks were derived from volcanic island arcs, while the low-grade metamorphic rocks represent deep-sea sediments eroded from the neighboring island arcs and deposited in a forearc basin. In short, greenstone belts represent sutured protocontinents.^[5]

Fossils of cyanobacterial mats (stromatolites) are found throughout the Archean—becoming especially common late in the eon—while a few probable bacterial fossils are known from chert beds.^[6] In addition to the domain Bacteria (once known as Eubacteria), microfossils of the extremophilic 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 (and formerly known as Monera); there are no known eukaryotic fossils, though they might have evolved during the Archean and simply not left any fossils.^[7] However, no fossil evidence yet exists for ultramicroscopic intracellular replicators such as viruses.

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     Prof. Torgersen