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Data provided by the Landsat 7 Team at NASA’s Goddard Space Flight Center Aerial view of the Teton Range in Grand Teton National Park, Wyoming. The rocks in the range are Archean-age gneiss, schist, and granite...

Data provided by the Landsat 7 Team at NASA’s Goddard Space Flight Center Aerial view of the Teton Range in Grand Teton National Park, Wyoming. The rocks in the range are Archean-age gneiss, schist, and granite 8 that date from 2.8 to 2.5 billion years old. The range itself, however, formed only about 10 million years ago by uplift along normal faults that parallel the front of the range. Precambrian Earth and Life History The Hadean and the Archean Eon OUTLINE The Atmosphere and Hydrosphere Introduction How Did the Atmosphere Form and Evolve? What Happened During the Hadean? PERSPECTIVE The Terrestrial Planets and Plate Tectonics The Faint Young Sun Paradox The Hydrosphere—Earth’s Surface Waters Archean Earth History Life—Its Origin and Early History Shields, Platforms, and Cratons The Origin of Life Archean Rocks Submarine Hydrothermal Vents and the Origin of Life Greenstone Belts Earth’s Oldest Known Organisms Evolution of Greenstone Belts Archean Mineral Resources Archean Plate Tectonics and the Origin of Cratons Summary 151 Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. CHAPTER OBJECTIVES all geologic time ( Figure 8.1). And yet we discuss this ▼ At the end of this chapter, you will have learned that incredibly long interval in only two chapters. We devote 10 chapters to the Phanerozoic Eon, made up of the Paleo- Precambrian time, which accounts for most geologic time, is zoic, Mesozoic, and Cenozoic eras, but this seemingly divided into three intervals: the Hadean (an informal term), the disproportionate treatment is justified when you consider Archean Eon, and the younger Proterozoic Eon. that we know so much more about the more recent eras of No rocks are known from the Hadean, but geologists can nev- geologic time. ertheless make some reasonable inferences about events that Precambrian is a widely used term referring to both occurred then. time and rocks. As a time term, it includes all geologic time The Archean geologic record is difficult to interpret because from Earth’s origin 4.6 billion years ago to the beginning many of the rocks are metamorphic, deformed, deeply buried, of the Phanerozoic Eon 542 million years ago. The term and contain few fossils. also refers to all rocks lying beneath those of the Cambrian System. Because of the complexities of these rocks and Each continent has at least one area of exposed Precambrian rocks called a shield and a buried extension of the shield the scarcity of fossils, establishing formal subdivisions of known as a platform. A shield and its platform make up a cra- the Precambrian is difficult. In 1982, in an effort to stan- ton, the ancient stable nucleus of a continent. dardize terminology, the North American Commission on Stratigraphic Nomenclature recommended the use of The most common Archean rocks are granite–gneiss com- Archean Eon and Proterozoic Eon for most of Precambrian plexes with subordinate greenstone belts made up mostly of time and more recently suggested the informal term volcanic rocks and some sedimentary rocks. Hadean for the earliest part of the Precambrian for which Greenstone belts likely form in several tectonic settings, includ- there are no known rocks on Earth ( Figure 8.2). The sub- ▼ ing oceanic plateaus, oceanic ridges, and back-arc basins. divisions in Figure 8.2 are based on numerical ages rather During the Archean, Earth possessed more radiogenic heat and than time-stratigraphic units, which is a departure from primordial heat so that plates moved more rapidly and igneous standard practice. activity was more widespread than it is now. The geologic record we do have for the Precambrian, especially for the Archean (see chapter opening photo), is Gases released by volcanoes were responsible for the origin of the hydrosphere and atmosphere, but the atmosphere had little difficult to decipher. The Earth systems we discussed in free oxygen. Chapter 1 (see Figure 1.1) became operative during this time, although not all at the same time or necessarily in The oldest known fossils are of single-celled bacteria and their present form. Earth did not differentiate into a core, chemical traces of bacteria-like organisms. Bacteria, commonly mantle, and crust until millions of years after it formed called blue-green algae, produced irregular mats and moundlike structures known as stromatolites. Archean mineral resources include gold, platinum, copper, Mesozoic Era (4.0%) Cenozoic Era (1.4%) zinc, and iron. Paleozoic Era (6.3%) Eon zoic ero 24 an 23 1 Ph 22 Introduction 21 4600 2 66 MYA mya 3 251 You know that the concept of time is used to specify the Hadean MYA 20 4 54 duration of events and the intervals between events, and (13.0%) 2 M you are familiar with time from the human perspective— YA 19 5 that is, hours, days, and years—but you probably have no N frame of reference for geologic time (see Chapter 4). Indeed, B RIA geologists commonly use the term deep time to empha- 18 6 size the duration of geologic time. Earth has existed for Archean Eon P M R E C A (32.6%) 4,600,000,000 years, more conveniently stated as 4.6 billion 17 Proterozoic Eon 7 years. So let’s suppose that one second equals one year and (42.5%) 2500 you want to count out Earth’s history. If you take on this 16 8 task, you and your descendants will be counting for nearly MYA 146 years. 15 9 In this and the next chapter, we are concerned only 14 10 with that part of geologic time designated Precambrian, 13 12 11 4.6 billion–542 million years ago. If all geologic time were Figure 8.1 Geologic Time Represented by a 24-Hour Clock If 24 ▼ represented by a 24-hour clock, the Precambrian alone hours represented all geologic time, the Precambrian would be more would be more than 21 hours long and constitute 88% of than 21 hours long, thus more than 88% of the total. 152 CHAPTER 8 Precambrian Earth and Life History Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  (see Figure 1.9), but once it did, internal heat was respon- sible for moving plates and for the origin and continu- ing evolution of the continents. Earth’s early atmosphere PRECAMBRIAN evolved from one that was rich in carbon dioxide to one with free oxygen and an ozone layer, surface waters began AGE Eon Era Period AGE (Ma) (Ma) to accumulate, and organisms appeared as much as 3.5 bil- 542 lion years ago. In short, Earth was very different when it 600 Ediacaran 630 formed, but during the Precambrian, it began to evolve proterozoic 700 Cryogenian and became increasingly like it is today. Neo- 800 850 900 Tonian What Happened 1,000 1,000 1,100 Stenian During the Hadean? proterozoic 1,200 1,200 Meso- 1,300 Ectasian 1,400 1,400 All geologic time from Earth’s origin until the beginning Proterozoic 1,500 Calymmian of the Archean Eon is encompassed by the informal term 1,600 1,600 Hadean (Figure 8.2). In fact, the onset of the Archean corre- 1,700 Stratherian sponds to the age of the oldest known rocks on Earth, which 1,800 1,800 are part of the Acasta Gneiss of Canada. Recently, however, 1,900 Orosinian some geologists claim that rocks of the Nuvvuagittuq green- proterozoic 2,000 stone belt, also in Canada, are as much as 4.4 billion years Paleo- 2,050 2,100 old. Others disagree and think that these rocks are 3.8 billion 2,200 Rhyacian years old, certainly old but not record setters. In any case, the 2,300 2,300 Hadean was the time during which Earth and its neighbors 2,400 Siderian accreted from rocky bodies called planetesimals, and dif- 2,500 2,500 ferentiated into a core and mantle (see Figures 1.5 and 1.9); continental curst began forming perhaps as much as 4.4 bil- archean 2,600 Neo- 2,700 lion years ago. 2,800 2,800 As the accreting planet grew, it swept up the debris 2,900 in its vicinity, and like the other terrestrial planets, it was archean relentlessly impacted by meteorites and comets. In addi- Meso- 3,000 3,100 tion, Earth was likely hit by a Mars-sized body 4.4 to 4.6 billion years ago, causing the ejection of a huge mass of 3,200 3,200 material that coalesced to form the Moon. Then another 3,300 Archean episode of impacts called the Late Heavy Bombardment archean Paleo- 3,400 took place 3.8 to 4.1 billion years ago. Scientists have iden- 3,500 tified sites of 180 meteorite impacts on Earth, most of 3,600 3,600 which are rather young geologically speaking, but one in Eoarchean 3,700 Greenland dated at nearly 3 billion years old was recently 3,800 reported. 3,900 After it first formed, Earth retained considerable heat 4,000 from its origin, and much more heat was generated by 4,100 radioactive decay; as a result, volcanism was ubiquitous 4,200 ( Figure 8.3). Gases emitted by volcanoes formed an ▼ Hadean 4,300 atmosphere, but it was very different from the oxygen-rich 4,400 one present now, and when the planet cooled sufficiently, 4,500 surface waters began to accumulate. If we could somehow 4,600 go back and visit early Earth, we would see a rapidly rotat- ing, hot, barren, waterless planet bombarded by meteorites Figure 8.2 The Precambrian Geologic Time Scale This most recent and comets. There were no continents; cosmic radiation ▼ version of the geologic time scale was published by the International would have been intense; and of course, you would see no Commission on Stratigraphy (ICS) in 2009. See Figure 1.14 for organisms. the complete time scale. Notice the use of the prefixes eo (early, or The age of the oldest continental crust is not known dawn), paleo (old, or ancient), meso (middle), and neo (new, or re- cent). The age columns on the left and right sides of the time scale for certain, but we can be sure that at least some was pres- are in hundreds and thousands of millions of years (1,800 million ent by 4 billion years ago. In addition, detrital sedimentary years = 1.8 billion years, for example). rocks in Australia have zircons (ZrSiO4) reliably dated at What Happened During the Hadean? 153 Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  Figure 8.3 Earth as It May Have Appeared Shortly After Forming No rocks are known from this earliest ▼ time in Earth’s history, but geologists can make some reasonable inferences about the nature of the newly formed planet. Copyright and photograph by Dr. Parvinder S. Sethi 4.4 billion years old, indicating that source rocks that old crust—oceanic and continental—which differ in composi- must have existed. ( Figure 8.4). In fact, 3.8-billion-year- tion, density, and thickness. The first crust probably was ▼ old rocks are known from several areas, most of which are ultramafic, but upwelling mantle currents of mafic magma metamorphic; so their parent rock must be even older. disrupted the crust, subduction zones formed, and the The friction caused by the Moon on the oceans as well first island arcs developed ( Figure 8.5a). Weathering of ▼ as the continents causes the rate of Earth’s rotation to slow these island arcs yielded sediments richer in silica, and very slightly every year. When Earth formed, it may have partial melting of mafic rocks yielded magma richer in rotated in as little as 10 hours; so there were many more silica. Collisions between island arcs formed a few conti- days in a year. No evidence indicates that Earth’s orbital nental nuclei as silica-rich materials were metamorphosed period around the Sun has decreased. Another effect of and intruded by magma (Figure 8.5b). As these larger the Earth–Moon tidal interaction is the recession of the island arcs collided, the first protocontinents formed Moon from Earth at a few centimeters per year. Thus, dur- and continued to grow by accretion along their margins ing the Hadean, the view of the Moon would have been (Figure 8.5c). spectacular! Oceans were almost certainly present by the latter part of the Hadean, but we do not know how extensive they were. The Earth–Sun distance has remained the same through time, but as already noted, the Moon was much closer to Earth when it formed and has since receded and continues to do so. The gravitational attraction of the Moon, and to a lesser extent the Sun, causes the rhythmic rise and fall of ocean tides; so when the Moon was closer to Earth, tidal fluctuations must have been much greater than they are now. Geologists agree that early Earth was exceedingly hot, at least hot enough that it partially melted and differentiated Photo courtesy of John Valley into a core and mantle. Rather than Earth being a fiery orb for more than a half billion years, as was formerly accepted, some geologists now think that it had cooled enough for surface water to accumulate by 4.4 billion years ago. They base this conclusion on oxygen 18 to oxygen 16 isotope ratios in tiny inclusions in zircon crystals that indicate reactions with surface waters. Figure 8.4 Hadean-Age Zircon from Australia No rocks of Hadean ▼ age are known on Earth, but there are rocks with the mineral zircon When Earth differentiated during the Hadean, the (ZrSiO4) that has a numerical age of 4.4 billion years. The tiny partial core and mantle formed, but we have made little mention crystal shown here is blue because it has been bombarded with elec- of the crust. Remember that we have defined two types of trons; normally, it is red. 154 CHAPTER 8 Precambrian Earth and Life History Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  Figure 8.5 Origin of Granitic Continental Crust ▼ (a) An andesitic island arc forms by sub- Island arc Sea level Island arc duction of oceanic lithosphere and partial melting of basaltic oceanic crust. Partial melting of andesite yields granitic magma. Oceanic lithosphere Granitic magma Asthenosphere (b) The island arc in (a) collides with Continental nucleus a previously formed island arc, thereby forming a continental core. (c) The process occurs again when the island arc in (b) collides with the evolving continent, thereby forming a craton, the nucleus of a continent. The Faint Young Sun Paradox deformed, (2) most are deeply buried beneath younger rocks, and (3) they contain few fossils, and those that are Standard models for the evolution of stars holds that known are of little use in time-stratigraphic correlation. Earth’s early Sun was 70–75% as luminous as it is now. If correct, Earth’s surface temperature should have been 25°C lower and all water should have frozen—and yet there is Shields, Platforms, and Cratons convincing evidence of liquid water on Earth as much as Continents are more than parts of Earth’s crust above sea 4.4 billion years ago, and surface temperatures were not level. They consist of all rock types, although their overall noticeably different than they are now. Scientists have composition is similar to granite; they average about 35 km offered two explanations for this so-called faint young thick; and their ages vary from essentially zero to 4 billion sun paradox. One appeals to a greenhouse effect in which years. In marked contrast, the oceanic crust is made up of atmospheric carbon dioxide (CO2), methane (CH4), and basalt and gabbro, it is only 5–10 km thick, and nowhere is it water vapor (H2O) trapped solar radiation, thus keeping more than 180 million years old. The transition from conti- the planet warmer. The second explanation is that Earth nental crust to oceanic crust occurs beneath the continental had a lower albedo; that is, it reflected less sunlight back slope, so the margins of the continents are below sea level. In into space. Oceans must have covered a great deal more of fact, some other parts of continents also lie beneath the seas, Earth’s surface than they do now, and because water absorbs such as the Hudson Bay region of North America and the more sunlight rather than reflecting it as continents do, the area between mainland Europe and Great Britain. temperature was high enough to inhibit freezing. All continents have a vast area of exposed ancient rocks called a Precambrian shield, and extending outward from shields are broad platforms of buried Precambrian rocks. A Archean Earth History shield and its adjacent platform made up a craton, which we can think of as a continent’s ancient nucleus ( Figure 8.6). ▼ We already mentioned that the Precambrian lasted longer The cratons are the foundations of the continents, and than we can imagine, but just the Archean alone includes along their margins, more continental crust was added by 32.6% of all geologic time (Figure 8.1). Archean-age rocks accretion as they evolved to their present sizes and shapes. are known from several areas, but they are difficult to inter- In North America, for example, the Superior, Hearne, Rae, pret because (1) many are metamorphic and complexly and Slave cratons amalgamated along deformation belts Archean Earth History 155 Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  180 140 100 60 20 20 60 100 140 180 Arctic Ocean 60 60 Canadian 40 Shield 40 Atlantic 20 Pacific Ocean 20 Ocean 0 0 20 Indian 20 Ocean 40 Exposed Precambrian rocks (shields) 40 Covered Precambrian rocks (platforms) 60 60 180 140 100 60 20 20 60 100 140 180 Figure 8.6 The Distribution of Precambrian Rocks Areas of exposed Precambrian rocks constitute the ▼ shields, whereas the platforms consist of buried Precambrian rocks. A shield and its adjoining platform are a craton. Notice that large parts of the continents existed by the end of the Precambrian—but remember, the Precambrian makes up more than 88% of all geologic time. to form a larger cratonic unit during the Proterozoic Eon of them are greenstone belts and granite–gneiss complexes, (see Chapter 9). Both Archean- and Proterozoic-age rocks the latter being the most common. Several types of rocks are found in cratons, many of which indicate several epi- are found in these granite–gneiss complexes, but granitic sodes of deformation accompanied by igneous activity, gneiss and granitic rocks predominate, both of which were metamorphism, and mountain building. However, most probably derived from plutons emplaced in volcanic island of the cratons have experienced remarkably little deforma- arcs (Figure 8.5). Nevertheless, there are other rocks, rang- tion since the Precambrian. ing from peridotite to sedimentary rocks, all of which have In North America, the exposed part of the craton is the been metamorphosed. Greenstone belts are subordinate, Canadian Shield, which occupies most of northeastern Can- accounting for only 10% of Archean rocks, and yet they ada, a large part of Greenland, the Adirondack Mountains are important in unraveling some of the complexities of of New York, and parts of the Lake Superior region of Min- Archean tectonic events. In addition, some greenstone belts nesota, Wisconsin, and Michigan (Figure 8.6). Much of the contain important deposits of copper, zinc, and silver. Canadian Shield is an area of subdued topography, numer- ous lakes, and exposed Archean and Proterozoic rocks thinly Greenstone Belts covered in places by Pleistocene glacial deposits. The rocks are volcanic, plutonic, and sedimentary, many of which have A greenstone belt has three main rock associations; its lower been altered to varying degrees by metamorphism. and middle parts are mostly volcanic, whereas the upper rocks are mostly sedimentary ( Figure 8.8a). Greenstone ▼ Drilling and geophysical evidence indicate that Pre- cambrian rocks of the platform underlie much of North belts typically have a synclinal structure, measure anywhere America as well as other continents, but beyond the from 40 to 250 km wide and 120 to 800 km long, and have shields, they are seen only in areas of erosion and uplift. been intruded by granitic magma and cut by thrust faults. For instance, Archean and Proterozoic rocks are present Many of the igneous rocks are greenish because they contain in many ranges of the Rocky Mountains and the Appala- green minerals such as chlorite, actinolite, and epidote that chian Mountains ( Figure 8.7). formed during low-grade metamorphism. ▼ Thick accumulations of pillow lava are common in greenstone belts, indicating that much of the volcanism Archean Rocks was subaqueous (Figure 8.8b). Pyroclastic materials, in Only 22% of Earth’s exposed Precambrian crust is Archean, contrast, almost certainly formed by subaerial eruptions with the largest exposures in Africa and North America. where large volcanic centers were built above sea level. Archean crust is made up of many kinds of rocks, but most The most interesting igneous rocks in greenstone belts 156 CHAPTER 8 Precambrian Earth and Life History Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  Figure 8.7 Archean Rocks in North America Rocks of Archean age are exposed mostly in the Canadian Shield and elsewhere in areas of uplift ▼ or deep erosion. James S. Monroe James S. Monroe (b) Archean granite (2.9 billion years old) overlain by the Cambrian- age Flathead Sandstone in the Bighorn Mountains of Wyoming. (a) Outcrop of the Acasta Gneiss in the Northwest Territories of Canada. At about 4 billion years old, these are the oldest known rocks on Earth except for meteorites. Figure 8.8 Greenstone Belts and Granite–Gneiss Complexes ▼ James S. Monroe (b) Pillow lava of the Ispheming greenstone belt in Michigan. How did the pillow lava form? Granitic intrusives Upper sedimentary unit: sandstones and shales Greenstone belt succession most common Middle volcanic unit: mainly basalt Lower volcanic unit: mainly peridotite and basalt Granite–gneiss complex (a) Two adjacent greenstone belts. Older belts—those more than 2.8 billion years old—have an ultramafic unit overlain by a basaltic unit. In younger belts, the succession is from a basaltic lower unit to an andesite-rhyolite unit. In both cases, the upper unit is made up mostly of sedimentary rocks. R.V. Dietrich (c) Gneiss from a granite–gneiss complex in Ontario, Canada. Archean Earth History 157 Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. are komatiites that cooled from ultramafic lava flows, (see Figure 2.10), which are rare in rocks younger than Archean; none occur now. To erupt, ultramafic magma (magma with less than 45% silica) requires near-surface magma temperatures of more than 1,600°C; the highest recorded surface tempera- Slave ture for recent lava flows is 1,400°C. During its early his- craton tory, though, Earth’s mantle was hotter because the planet retained heat from its origin and differentiation and heat generated by radioactive decay was greater than it is now. As a result, the mantle was as much as 300°C hotter than it is at present. Given these conditions, ultramafic magma Superior craton craton could reach the surface, but as Earth cooled and radio- genic heat production decreased, the mantle cooled and ultramafic flows no longer occurred. Sedimentary rocks are found throughout greenstone belts, but they predominate in the upper unit (Figure 8.8a). Many of these rocks are successions of graywacke (sand- Figure 8.9 Greenstone Belts in North America Archean greenstone ▼ stone with abundant clay and rock fragments) and argillite belts (shown in dark green) of the Canadian Shield are mostly in the (slightly metamorphosed mudrocks). Small-scale cross- Superior and Slave cratons. bedding and graded bedding indicate that these rocks rep- resent turbidity current deposition (see Figure 6.3). Other sedimentary rocks are also present, including multiple openings and closings of back-arc basins account sandstone, conglomerate, chert, and carbonates, although for the parallel arrangement of greenstone belts. none are very abundant. Iron-rich rocks known as banded Another model for greenstone belt evolution pro- iron formations are also found, but they are more typical of poses a succession of island arcs colliding with a craton ( Figure 8.11). This model shows the proposed Archean ▼ Proterozoic deposits; so we will discuss them in Chapter 9. The oldest large, well-preserved greenstone belts are crustal evolution of the southern Superior craton of Can- in South Africa and date from about 3.6 billion years ada as a result of the evolution of greenstone belts, pluto- ago. The 3.7- to 3.8-billion-year-old Isua greenstone belt nism, and deformation resulting from collisions. of Greenland, consisting of metamorphosed lava flows, Greenstone belts may also form in intracontinen- schists, quartzites, and banded iron formations, has some tal rifts above rising mantle plumes. As the plume rises of the oldest known rocks on Earth. And given that it con- beneath silica-rich continental crust, it spreads, generat- tains altered sedimentary rocks, it is safe to assume that ing tensional forces that cause rifting. Furthermore, the even older source rocks were present. plume is the source for the greenstone belt’s volcanic units, In North America, most greenstone belts are found whereas erosion of the rift’s flanks provides sediments for in the Superior and Slave cratons of the Canadian Shield the upper unit. Finally, there is an episode of rift closure, ( Figure 8.9), but they are also found in Michigan, Min- deformation, and emplacement of plutons. ▼ nesota, and Wyoming. Most formed between 2.7 and 2.5 billion years ago. The Abitibi greenstone belt of Ontario Archean Plate Tectonics and the and Quebec, Canada, is especially well known for its extensive resources that include gold, copper, and zinc. Origin of Cratons Among the terrestrial planets, only Earth has active plate tectonics. It is doubtful that plate movements ever took Evolution of Greenstone Belts place on Mercury and Earth’s Moon, but planetary geolo- Greenstone belts are found in Archean terrains in multiple gists concede that it is possible that plate tectonics occurred parallel belts, each separated from the next by granite–gneiss on Venus and Mars during their very earliest histories (see complexes (Figures 8.8a and 8.8c). The tectonic settings in Perspective). During the Archean, Earth’s lithosphere was which greenstone belts form include oceanic ridges, island probably thinner, hotter, and thus more buoyant; so subduc- arcs, back-arc basins, and oceanic plateaus and perhaps even tion of moving plates would have been difficult. Neverthe- at rifted continental margins. If this is correct, greenstone less, most geologists are convinced that some kind of plate belts, or at least their precursors, must be forming in simi- tectonic activity took place at this time, but it must have dif- lar tectonic settings today where sequences of volcanic and fered in detail from what is going on now. sedimentary rocks accumulate. Exactly how they form and With more residual heat from Earth’s origin and more evolve is not fully resolved, but according to one model, they radiogenic heat, plates must have moved faster and magma form in back-arc basins that initially open and subsequently was generated more rapidly accounting for a great deal more close ( Figure 8.10). Proponents of this model suggest that Archean volcanism. As a result, continents no doubt grew ▼ 158 CHAPTER 8 Precambrian Earth and Life History Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  Figure 8.10 Origin of a Greenstone Belt in a Back-Arc Basin Figure 8.11 Origin of Greenstone Belts and Granite–Gneiss ▼ ▼ Complexes This model shows a plate tectonic origin of greenstone belts. Volcanic Deformed arc Wabigoon belt Uchi belt sediments Trench Berens River craton Extension Quetico belt English River belt Lava flows Granitic intrusion Mantle Berens River craton Continental Wawa belt Wabigoon belt Uchi belt crust Quetico belt English River belt (a) This plate tectonic model shows the origin of greenstone belts in the southern Superior craton of Canada. The figure is a north-south (a) Rifting on the continent side of a volcanic arc forms a back-arc cross section of the area in (b), and the upper diagram shows an basin. Partial melting of subducted oceanic lithosphere supplies earlier stage of development than the lower one. andesite and diorite magmas to the island arc. Back-arc basin Continental Sediments and Hudson Bay sediment lava Pha n ero zoi cs Amis ed k im Windigo en ts a atic Lake Berens Op Winnipeg Uchi h R iver tico Englis n Que wa igoo Wa Mantle CANADA Wab USA Superior Partial melting w a L. of upper mantle Wa (b) Basalt lavas and sediment derived from the island arc and conti- (b) Geologic map showing greenstone belts (dark green) and granite– nent fill the back-arc basin. gneiss complexes (light green). Basin closes more rapidly along their margins, a process called conti- nental accretion, as plates collided with island arcs, oceanic plateaus, and other plates. Continental accretion accounts for the increased area of continents by additions at their margins, but continents also grow in volume by underplating, when Mantle rising magma from a subducted plate or a mantle plume accumulates beneath a continent or within continental crust. Geologists use several criteria to estimate when plate tectonics began on Earth, but one of the most important is the first occurrence of ophiolites (see Figure 3.17). These successions of rocks representing upper mantle and oce- anic crust were not common until about 1.0 billion years ago, although several are known from the Archean, includ- ing the 3.8-billion-year-old Isua Ophiolite of Greenland (c) Closure of the back-arc basin, compression, and deformation. (Figure 8.9). In any case, ophiolites were present during the A syncline-like structure forms, which is intruded by granitic magma. Archean, and they were widespread by the Paleoproterozoic. Archean Earth History 159 Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  Almost all geologists who study Precambrian Earth his- now. Both have also played an important role in the develop- tory agree that some kind of plate tectonic activity took place ment of the biosphere. during the Archean, at least 3 billion years ago and likely ear- lier. However, at that time, it may have been episodic rather How Did the Atmosphere Form and than continuous, but even so, it was likely widespread by 2.7 billion years ago. Nevertheless, there were marked differ- Evolve? ences between the Archean world and the one that followed. Today, Earth’s atmosphere is quite unlike the noxious one For instance, we have little evidence of Archean sedimen- described earlier. Now it is composed of 78% nitrogen (N2) tary rocks deposited on broad, passive continental margins, and 21% free oxygen (O2), meaning oxygen not combined although associations of these rocks were widespread by the with other elements as in carbon dioxide (CO2) and water Proterozoic. In addition, ultramafic lava flows (komatiites) vapor (H2O). It also has small but important amounts of were more common during the Archean. Recall that higher other gases such as ozone (O3), which, fortunately for us, is mantle temperatures are necessary for komatiite lavas to common enough in the upper atmosphere to block most of erupt and that none are forming now. the Sun’s ultraviolet radiation. Several small cratons that were present during the Earth’s earliest atmosphere was probably composed of Archean Eon grew by accretion along their margins and by hydrogen and helium, the most abundant gases in the uni- underplating, thus forming the nuclei around which more verse. If so, they would have quickly been lost into space for accretion would take place to form the continents as they two reasons. First, Earth’s gravitational attraction is insuf- exist now. The model shown in Figure 8.11 shows how cra- ficient to retain gases with such low molecular weights. tons evolved as greenstone belts and plutons formed at their Second, before Earth differentiated, it had no core or margins. By the end of the Archean Eon, perhaps 30–40% magnetic field. Accordingly, it lacked a magnetosphere, the of the present volume of continental crust had formed. area around the planet within which the magnetic field is These Archean cratons would become amalgamated into confined. So a strong solar wind, an outflow of ions from larger units during the Proterozoic (see Chapter 9). the Sun, would have swept away any atmospheric gases. But what of supercontinents such as Pangaea, with Once Earth had differentiated and a magnetosphere was which you are already familiar (see Chapter 3). Geologists present, though, an atmosphere began accumulating as a are certain that supercontinents were present by Protero- result of outgassing involving the release of gases from zoic time (so we discuss them in more detail in Chapter 9), Earth’s interior during volcanism ( Figure 8.12). ▼ but many are convinced that they also were present during the Mesoarchean and the Neoarchean. The first of these is known as Vaalbara, which may have existed from 3.1 to Escapes 2.8 billion years ago; the second is Kenorland, which may have assembled by 2.7 billion years ago. Hydrogen Events such as those in Figure 8.11 are part of a more H2 extensive orogenic episode that took place during the Meso- Water archean and Neoarchean. Deformation was responsible for H 2O To atmosphere the origin of some of the Archean rocks in several parts of Nitrogen N2 the Canadian Shield as well as in Wyoming, Montana, and Water the Mississippi River Valley. In the northwestern part of the Carbon H2O dioxide Canadian Shield, deformation along the Snowbird tectonic CO2 To oceans zone yielded metamorphic rocks 3.2 and 2.6 billion years old that belong to the granulite metamorphic facies, which form at very high temperatures, at least 700°C; some form at more than 1,000°C. By the time this Archean event had ended, several cratons had formed that are now found in Volcano the older parts of the Canadian Shield (see Chapter 9). Magma with gases The Atmosphere and Hydrosphere In Chapter 1, we emphasized the interactions among Earth’s Figure 8.12 Outgassing and Earth’s Early Atmosphere Erupting ▼ systems, two of which, the atmosphere and hydrosphere, have volcanoes emit mostly water vapor, carbon dioxide, and several other gases but no free oxygen—that is, oxygen not combined with had a profound impact on Earth’s surface (see Figure 1.1). other elements. In addition to the gases shown here, chemical reac- Shortly after Earth formed, its atmosphere and hydrosphere, tions in the early atmosphere probably yielded methane (CH4) and although present, were quite different from the way they are ammonia (NH3). 160 CHAPTER 8 Precambrian Earth and Life History Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  Perspective The Terrestrial Planets and Plate Tectonics Mercury, Venus, Earth, and Mars formed of volcanoes. One of the most obvious lithosphere, making them flexible enough at the same time from the same materials, features is the so-called coronae and for movement and subduction. and yet today, they differ considerably. All arachnoids that may be surface expressions But why is Venus so dry? After all, it of these terrestrial planets accreted from of rising mantle plumes (Figure 1). In addi- must have started out much like Earth, smaller rocky objects called planetesimals tion, Venus has mountains that appear to although it was probably 30% hotter in the solar nebula, and once formed, have formed by compression in its crust, because it is closer to the Sun. Its surface they differentiated into a core, mantle, but they are not aligned as they are on temperature now, however, is 462°C over and crust (see Figures 1.5 and 1.9). Fur- Earth, nor are there linear belts of volca- the entire planet because carbon dioxide thermore, during their earliest histories, noes. Some of these volcanoes may remain (CO2) has accumulated in its atmosphere, all experienced ubiquitous volcanism active as indicated by periodic increases of leading to a so-called runaway green- and were pummeled by meteorites (see sulfur dioxide in Venus’s atmosphere. house effect. So if any water was present Figure 1.8) and comets. Mercury as well Certainly, Venus is the most likely on Venus, it was lost long ago. Although as Earth’s Moon preserve the evidence of other terrestrial planet on which we would Earth’s atmosphere also contains CO2, these ancient impacts, but because Earth expect plate tectonics, but orbiting space- it does not contain very much because is such a dynamic planet, the evidence for craft indicate that its lithosphere is thicker carbon is continually recycled as plates this early episode in the evolution of the than Earth’s, thus inhibiting it from break- are consumed at subduction zones. In terrestrial planets has been obliterated. ing into separate plates. Furthermore, addition, much of Earth’s carbon is tied Tectonics is a term geologists define data from flybys and landings by the up in its biosphere and in carbonate rocks as the large-scale deformation of Earth’s former Soviet Union’s Venera spacecraft (limestone and dolostone). crust by folding, faulting, or both, and show that Venus is incredibly dry. Earth, Mars also has some tectonic features, plate tectonics causes such deformation on the other hand, has abundant liquid but once again there is no evidence of on- at or near divergent, convergent, and water, which many scientists think has going plate tectonic activity. Like Mercury, transform plate boundaries. Plate tecton- the effect of “softening” the rocks of the Mars is small and probably lost much of its ics is an important process accounting for many features on Earth, including linear Figure 1 These zones of volcanism and seismicity, the arachnoids on Venus origin of folded mountains, the evolution are circular to oval of continental crust, and the distribution structures with ra- of land and sea. Our questions here are,