ARCHAEAN EON                                                          3.8 - 2.5 Billion Years Ago Edit

Both life and geological processes begin in the Archaean Eon, amidst a brew of atmospheric gases which probably includes ammonia and methane - but little to no free oxygen. As the molten Earth cools, a crust forms over it. Mounded bacterial formations, called Stromatolites, appear in the shallow waters of the early oceans. These very, very gradually, produce increasing quantities of oxygen. At around the same time, samll land blocks collide with one another, forming the first continents. The second Eon of the three that make up the Precambrian 'Super-Eon'. At the beginning of the Archaean Eon, the thermal flow of the Earth was nearly three times higher than it is today, and was still twice the current level by the beginning of the Proterozoic Eon, some 2.5 Billion Years Ago. The extra heat may have been remnant heat from the planetary formation, partly heat of formation of the iron core, and partially caused by greater radiogenic heat production from short-lived radionuclides such as Uranium ions.

The majority of Archean rocks which still survive are metamorphic and igneous. activity was considerably higher than today, with numerous hot spots, rift valleys, and eruption of lavas including unusual types such as komatiite. Nevertheless, intrusive igneous rocks predominate throughout the crystalline cratonic remnants of the Archean crust which survive today. These are magmas which infiltrated into host rocks, but solidified before they could erupt at the Earth's surface. 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 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 subcontinental lithospheric mantle was 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.

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 unless they were buoyant enough or fortunate enough to avoid energetic subduction zones.

An explanation for the general lack of Hadean rocks 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 Hadean Eon The Archaean Eon The Proterozoic Eon The Phanerozoic Eon The Palaeozoic Era The Cambrian Period The Ordovician Period The Silurian Period The Devonian Period The Carboniferous Period The Permian Period The Mesozoic Era The Triassic Period The Jurassic Period The Cretaceous Period The Cenozoic Era The Tertiary Period The Palaeocene Epoch The Eocene Epoch The Oligocene Epoch The Miocene Epoch The Pliocene Epoch The Quaternary Period The Pleistocene Epoch The Holocene Epoch

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