The Archean Eon
The Archean Eon has given us where we lives now, land.
Land Appears
The boundary between the Archean (from the Greek word for beginning) Eon and the Hadean Eon occurs at about 3.85 Ga. Effectively, this date marks the time at which substantial quantities of crustal rocks, including rocks that originated as marine sediments, formed. With the advent of the Archean, crust was locally cool and stable enough for rocks to survive and for isotopic clocks to start ticking. Geologists still debate about whether plate tectonics in the form that occurs today operated in the early part of the Archean Eon. Most researchers picture an early Archean Earth with rapidly moving small plates, numerous volcanic island arcs, and abundant hot-spot volcanoes. Others propose that early Archean lithosphere was too warm and buoyant to subduct, and that plate tectonics could not have operated until the later part of the Archean or later; these authors argue that plume-related volcanism or some other process was the main source of new crust until the late Archean. Regardless of which model ultimately proves more accurate, it is clear that the Archean was a time during which significant volumes of new continental crust were generated. What processes produced continental crust? According to one model, early crust formed from mafic igneous rocks that originally extruded or intruded at convergent plate boundaries and/or hot-spot volcanoes. Once formed, these rocks were too buoyant to be subducted, so when the arcs and plateaus collided with one another, they sutured together to form larger blocks that remained at the Earth’s surface. The development of convergent plate boundaries along the margins of these blocks, and of rifts and hot spots within the blocks, led to production of flood basalts. Partial melting of basaltic crust yielded felsic and intermediate rocks. As collisions continued, the blocks coalesced into still larger proto-continents (figure above a, b), which slowly cooled and became stronger. As a result of these processes, the first long-lived blocks of durable continental crust came into existence between 3.2 and 2.7 Ga, and by the end of the Archean Eon, about 80% of the Earth’s continental area had formed (figure above c). A clear stratigraphic record of marine sediment deposition appears in remnants of Archean crust, indicating that oceans filled in the Archean and have existed ever since. Presumably, permanent oceans could survive only after the Earth’s surface had cooled below the boiling point of water. Prior to that time, gaseous H2O saturated the atmosphere in fact, prior to ocean formation, H2O and CO2 were the dominant gases of the atmosphere. Once the oceans formed, however, the atmosphere lost most of its H2O. And once liquid water existed, most atmospheric CO2 dissolved into it, so CO2 also went from being a major component of the atmosphere to being a trace component. Thus, the Archean saw the atmosphere change from being a foggy mixture of H2O and CO2 into being a transparent gas dominantly composed of nitrogen (N2) gas. Since nitrogen is inert (it doesn't chemically react with or dissolve in other materials), it was left behind.
The First Life
Clearly, the Archean Eon saw many firsts in Earth history. Not only did the first continents appear during the Archean, but probably also the first life. The search for the earliest evidence of life continues to make headlines in the popular media. Most geologists currently conclude that life has existed on Earth since at least 3.5 Ga, and perhaps since 3.8 Ga, for rocks of this age contain chemical signatures of organisms. The oldest undisputed fossil forms of bacteria and archaea occur in 3.2-Ga rocks (figure above a). (Shapes resembling such organisms occur in rocks as old as 3.5 Ga, but their identity remains less certain.) Archean strata at some localities contain stromatolites, distinctive layered mounds of sediment. Stromatolites that developed after about 3.2 Ga form because cyanobacteria secrete a mucuslike substance to which sediment settling from water sticks. As the mat gets buried, new cyanobacteria colonize the top of the sediment, building a mound upward (figure above b); modern examples locally occur in shallow, tropical waters. What specific environment on the Archean Earth served as the cradle of life? Laboratory experiments conducted in the 1950s led many researchers to think that life began in warm pools of surface water, beneath a methane- and ammoniarich atmosphere streaked by bolts of lightning. More recent researchers suggest instead that submarine hot-water vents, so-called black smokers, served as the hosts of the first organisms. These vents emit clouds of ion-charged solutions from which sulphide minerals precipitate and build chimneys. The earliest life in the Archean Eon may well have been thermophilic (heat-loving) bacteria or archaea that dined on pyrite at dark depths in the ocean alongside these vents. Later in the Archean, organisms evolved the ability to carry out photosynthesis, and moved into shallower, well lit water. As the Archean Eon came to a close, the first continents had formed, and life had colonized not only the depths of the sea but also the shallow marine realm. Plate tectonics had commenced, continental drift was taking place, collisional mountain belts were forming, and erosion was occurring. The atmosphere was gradually accumulating oxygen, although probably this gas still accounted for only a very small percentage of the air; Archean air was unbreathable. The stage was set for another major change in the Earth System.
Credits: Stephen Marshak (Essentials of Geology)
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