Precambrian Earthand Life HistoryThe Hadean and the Archean Eon

OUTLlNE

Introduction

What Happened Durlng the Hadean?PERSPECTIVEThe Faint Young Sun Paradox-An Unresolved Controversy

Archean Earth HistoryShields, Platforms, and CratonsArchean RocksGreenstone BeltsEvolution of Greenstone BeltsArchean Plate Tectonics and the Origln ofCratons

The Atmosphere and HydrosphereHow Did the Atmosphere Form and Evolve?The Hydrosphere-Earth's Surface Waters

Life-Its Origin and Early HistoryThe Origin of LifeSubmarine Hydrothermal Vents and theOrigin of LifeEarth's Oldest Known Organisms

Archean Mineral Resources

Summary

Archean rocks in the Teton Rangeof Grand Teton National Park inWyoming. The rocks visible in thisimage are gneiss, schist, and gran-ite that date from 2.8 to 2.5 billionyears old. The Teton Range, how-ever, formed only about 10 millionyears ago, The peaks in this imageare known as the Cathedral Group.Sue Momoe

149

CHAPTER OBJECTIVES

~ Greenstone belts likelyform in several tectonic settings,but many probably evolved in back-arc basins and in riftswithin continents.

~ During the Archean, Earth possessed more radiogenic heatand primordial heat so that plates moved more rapidly andígneous activity was much more widespread that it is now.

~ Gases released by volcanoes were responsible for theorigln of the hydrosphere and atrnosphere, but the atmo-sphere had little free oxygen.

~ The oldest known fossils are of síngte-ceüed bacteria andchemical traces of bacteria-like organisms. Bacteria com-monly called blue-green algae produced irregular mats andmoundlike structures known as stromatolites.

At the end of this chapter, you will have learned that

~ Precambrian time, which accounts for most geologlc time,is divided into three intervals: the Hadean (an informalterm), the Archean Eon, and the younger Proterozoic Eon.

~ No rocks are known from the Hadean, but geologists cannevertheless make some reasonable inferences aboutevents that occurred then.

~ The Archean geologic record is difficult to interpret becausemany of the rocks are metamorphic, deformed, deeply bur-ied, and contain few fossils.

~ Each continent has at least one area of exposed Precam-brian rocks called a shield and a buried extension of theshield known as a platform. A shield and its platform makeup a eraton, an ancient stable nucleus of a continent.

~ The most common Archean rocks are granite-gneiss com-plexes with subordinate greenstone belts made up mostlyof igneous rocks and some sedimentary rocks.

~ Archean mineral resources include gold, platinum, copper,zinc, and iron.

IntroductionYou know that the concept of time is used to specify theduration of events and the intervals between events, andyou are also familiar with time from the human perspec-tive-that is, hours, days, and years-but you probablyhave no frame of reference for geologic time (see Chapter4). Indeed, geologists commonly use the phrase deep timeto emphasize the magnitude of geologic time. Earth hasexisted for 4,600,000,000 years, more conveniently statedas 4.6 billion years. So, let's suppose that one second equalsone year, and you want to count out Earths history. Shouldyou take on this task, you and your descendants would stillbe counting nearly 146 years from now.

In this and the next chapter, we are concerned onlywith that part of geologic time designated Precambrian,4.6 billion to 542 million years ago. If all geologic timewere represented by a 24-hour clock, the Precambrianalone would be more than 21 hours long and constitute88 percent of all geologic time (j-Pigure 8.1). And yet wediscuss this incredibly long interval in only two chapters.We devote 10 chapters to the more familiar PhanerozoicEon, made up of the Paleozoic, Mesozoic, and Cenozoiceras, but this seemingly disproportionate treatment isjustified when you consider that so much more is knownabout that more recent part of geologic time.

Precambrian is a widely used term referring to bothtime and rocks. As a time term, it includes all geologic timefrom Earths origin 4.6 billion years ago to the beginningof the Phanerozoic Eon 542 million years ago. The termals o refers to all rocks lying beneath those of the Cam-brian System. Because of the complexities of these rocksand the scarcity of fossils, establishing formal subdivisions

, 150 CHAPTER8 PrecambrianEarth and LifeHistory

Mesozoic Era (4.0%) Cenozoic Era (1.4%)

Paleozoic Era (6.3%) \ \..J~

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'õ-(\e~ 23 24«'0 22-:

21

Hadean(13.0%)

5

6

ProterozoicEon(42.5%)

Archean Eon(32.6%)

7

12 11

~ Figure 8.1 Geologic Time Represented by a 24-Hour ClockIf 24 hours represented aligeologic time, the Precambrianwouldbemore than 21 hours long, thus more than 88 percent of the total.

of the Precambrian is difficult. In 1982, in an effort tostandardize terminology, the orth American Commis-sion on Stratigraphic Terminology recommended the useof Archean Eon and Proterozoic Eon for most of Precam-brian time, and more recently suggested the informal term

Hadean for the earliest part of the Precambrian for whichthere are no known rocks (•.Figure 8.2). The subdivisionsin Figure 8.2 are based mostly on absolute ages rather thantime-stratigraphic units, which is a departure from stan-dard practice.

The geologic record we do have for the Precambrian,.especially for the Archean (see chapter opening photo), isdifficult to decipher. It was during this time that the Earthsystems we discussed in Chapter 1 (see Figure 1.1) becameoperative, although not all at the same time or necessar-ily in their present formo Earth did not differentiate intoa core, mantle, and crust until millions of years after itformed (see Figure 1.6), but once it did, internal heatwasresponsible for moving plates and for the origin and con-tinuing evolution of the continents. Earth's early atmo-sphere evolved from one rich in carbon dioxide to onewith free oxygen and an ozone layer, surface waters beganto accumulate, and organisms appeared as much as 3.5 bil-lion years ago. In short, Earth was very different when itformed, but during the Precambrian it began to evolve andbecame increasingly like it is today.

PRECAMBRIAN

AGE Eon Era Period(Ma)

AGE(Ma)

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1,400 1,400

1,500

1,600

1,700

1,800

1,900

2,000

2,100

2,200

2,300

2,400

2.500

2,600

2,700

2,800

2,900

1,600

1,800

2,050

What Happened During theHadean?

2,300

2,500

All geologic time from Earths origin (4.6 billion years ago)to the beginning of the Archean Eon (4.0 billion years ago)is encompassed by the informal term Hadean (Figure 8.2).The Acasta Gneiss of Canada, at about 4.0 billion yearsold, is the oldest known rock on Earth, with the exceptionof meteorites, so we have no geologic record for the first600 million years of Earth history. Nevertheless, geologistscan make some reasonable inferences about events thattook place during the Hadean. Of course, the Solar Sys-tem formed and the terrestrial planets accreted from plan-etesimals; Earth differentiated into a core and mantle (seeFigures 1.5 and 1.6); and continental crust began to form,perhaps as much as 4.4 billion years ago.

As the accreting planet grew, it swept up the debris inthe vicinity, and just like the other terrestrial planets, Earthwas bombarded by meteorites and comets until about3.8 billion years ago. Unlike Mercury, Mars, and Earth'sMoon, however, the evidence of this period of impacts hasbeen obliterated by weathering, erosion, volcanism, platemovement, and mountain building. In addition to bom-bardment by meteorites, Earth was probably hit by a Mars-sized planetesimal 4.4 to 4.6 billion years ago, causing theejection of a huge mass of hot material that coalesced toform the Moon.

After it first formed, Earth retained considerable heatfrom its origin, and much more heat was generated byradioactive decay: as a result, volcanism was ubiquitous(•.Figure 8.3). Gases emitted by volcanoes formed an atmo-sphere, but it was very unlike the oxygen-rich one pres-ent now, and when the planet cooled sufficiently, surface

2,800

3,000

3,100

3,200 3,200

3,300

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3,900

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4,300 c»

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4,400

4,500

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•. Figure 8.2 The Precambrian Geologic Time Scale Thismost recent version or the geologlc time scale was published by theInternational Cammission on Stratigraphy (ICS) in 2009. See Figure1.11 for the complete time scale. Natice the use of the prefixes ea(early, ar dawn), palea (old, ar ancient), meso (middle), and neo (new,ar recent). The age columns on the left and right sides of the timescale are in hundreds and thousands of millions of years (1,800 mil-lion years = 1.8 billion years, for example).

What Happened During the Hadean? 151

partially melted and differentiated into a core and mantle.Rather than the Earth being a fiery orb for more than a halfbillion years, as was formerly accepted, some geologists nowthink that it had cooled enough for surface water to accumu-late by 4.4 billion years ago (see Perspective). They base thisconclusion on oxygen 18to oxygen 16ratios in tiny inclusionsin zircon crystals that indicate reactions with surfacewaters. ~

When Earth differentiated durinz the Hadean, the coreand mantle formed, but we have made little mention so far

Ir of the crust. Remember that we have defined two types ofi crust, oceanic and continental, which differ in composi-.~ tion, density, and thickness. The first crust probably was~ ultramafic, but upwelling mantle currents of mafic magma@ disrupted the crust, subduction zones formed, and the first

island ares developed (. Figure 8.4a). Weathering of theseisland arcs yielded sediments richer in silica, and partialmelting of mafic rocks yielded magma richer in silica. Col-lisions between island ares formed a few continental nucleias silica-rich materials were metamorphosed and intrudedby magma (Figure 8.4b). As these larger island ares col-lided, the first protocontinents formed and continued togrow by accretion along their margins (Figure 8.4c).

~ Figure 8.3 Earth As It May Have Appeared 500n After ItFormed No roeks are known from this earliest time in Earth history,but geologists can make reasonable inferences about the nature ofthe newly formed planet.

waters began to accumulate. If we could somehow go backand visit early Earth, we would see a rapidly rotating, hot,barren, waterless planet bombarded by meteorites andcomets. There were no continents,cosmic radiation would have beenintense, and, of course, you wouldsee no organisms.

The age of the oldest continen-tal crust is uncertain, but we can besure that at least some was presentby 3.8 billion years ago, and detri-tal sedimentary rocks in Australiahave zircons (ZrSiO 4) 4.4 billionyears old, indicating that sourcerocks that old must have existed. Infact, 3.8-billion-year-old rocks areknown from severa] areas, some ofwhich are metamorphic, so theirparent rock must be even older.

The friction caused by theMoon on the oceans as well as thecontinents causes the rate of Earthsrotation to slow very slightly everyyear. When Earth formed, it mayhave rotated in as little as 10hours,so there were many more days ina year; there is no evidence indi-cating that Earth's orbital periodaround the Sun has decreased.Another effect of the Earth-Moontidal interaction is the recession g>

of the Moon from Earth at a few .~rs=2:it:~=:::~===::~~centimeters per year. Thus, during '*t"the Hadean, the view of the Moon ~L ---------..would have been spectacular! :::l

@Geologists agree that whenEarth forrned, it was exceedinglyhot, at least hot enough that it

~ Figure 8.4 Origin of Granitic Continental Crust

Island are Sea levei Island are

(a) An andesitie island are forms by subduetian of oeeanie lithosphere and partial melting of basal-tic oceanic crust. Partial melting of andesite yields granitie magrna.

Continental nueleus

(b) The island are in (a) collides with a previously formed island are, thereby forming a continentalcore.

(e) The proeess oeeurs agaín when the island are in (b) collides with the evolving continent,thereby forming a craton, the nucleus of a eontinent.

152 CHAPTER8 Precambrian Earth and Life History

The Faint Young Sun Paradox-An Unresolved ControversyIt should not be surprising that olderevents in written human history aremore poorly understood than morerecent ones, because older records areincomplete, fragmentary, and difficultto decipher. A similar situation holdsfor Earth history, especially for theArchean Eon for reasons already noted;alteration of the rocks, deep burial, andfew fossils. Nevertheless, on someaspects of early Earth history we canbe certain: Archean oceans existed;rocks that probably represent continen-tal crust were present, and organismsappeared by at least 3.0 billion yearsago, and possibly as much as 3.5 bil-lion years ago.

However, standard models for theevolution of stars hold that Earth's earlySun was only about 70 percent to75 percent as luminous as it is now. Ifthis is correct, Earth's surface shouldhave been cold enough for ali water tohave frozen-but the evidence for liquidwater during the Archean is convincing,hence the Faint Young Sun Paradox. First,why has solar luminosity increased, andsecond, what is the evidence for andagainst the Faint Young Sun Paradox?

When stars such as the Sun formand evolve, their initial low luminos-ity increases as a function of complexchanges, including nuclear reactions,taking place in their core (hydrogen

fusing to form helium) and changesin chemical composition. Calculationsindicate that early Earth's temperatureshould have been as much as 25°Clower than at present, but the evidenceindicates liquid water was present andsurface temperatures were not notice-ably different than they are now.

The Faint Young Sun Paradox wasproposed in 1972 by astronomer CarlSagan and his colleague George Mul-len. Since then, scientists have pro-posed several solutions to the paradox,but so far none of these proposedsolutions are without critics, One ofthe most obvious solutions is that anearly atmosphere with appreciableamounts of greenhouse gases suchas carbon dioxide and methane keptEarth's surface warm even though theSun was fainter (Figure 1). Most agreethat these gases were present in theArchean atmosphere, but not ali agreethat they were abundant enough toaccount for significantly higher surfacetemperatures, so surface waters shouldhave frozen.

Another suggestion is that earlyEarth had a lower albedo; that ls, itreflected lesssolar radia-tion back intospace than itdoes now and

Figure 1 Some scientists think that a greenhouseeffect onearly Earth accounts for the FaintYoungSun Paradoxo

Weaker solar radiation

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.. ·1'n atmoS" ..°'0", •••••

(a) EarlyEarth and a stronggreenhouse effect.

(b) Earth today and aweakergreenhouseeffect.

accordingly was warm enough for waterto remain liquid. We know that ArcheanEarth had vast oceans, but there islittle evidence for extensive continents,which have a higher albedo than water.So, according to this hypothesis, oce-anic waters absorbed more heat andkept Earth's surface temperature highenough to inhibit freezing (Figure 2).Also, a proposed thinner cloud coverwould have allowed more solar energy toreach the surface.

Estimates of the rate at whichcontinental growth occurs vary widely.Nevertheless, given that more residualheat and radiogenic heat were availableduring the Precambrian, most geolo-gísts think that continents grew morerapidly by accretion back then. In fact,some estimate that the continentshad reached about 75 percent of theirpresent volume by the beginning of thePhanerozoic Eon. And, of course, atthe same time the Sun had increasedits luminosity by about 30 percent. Inthe final analysis, geologísts think thatEarth's surface temperatures haveremained fairly constant through mostof geologic time.

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Figure 2 Earth'salbedovaries dependingon the surface materiais sun-llght encounters.Oneproposalfor the FaintYoungSun Paradoxis thatearly Earthhadvast oceansthat had a low albedoand thus absorbedheat,therebyoffsetting the weakerenergyfrom the Sun.

The FaintYoungSun Paradox-An UnresolvedControversy 153

1

180 140 100 60

PacificOcean

20 60 100 140 180

20

o~--------------~20

20

IndianOcean

20

40 • Exposed Precambrian rocks (shields)

D Covered Precambrian rocks (platforms)

,. Figure 8.5 The Distribution of Precambrian Rocks Areas of exposed Precambrian rocks constitutethe shields, whereas the platforms consist of buried Precambrian rocks. A shield and its adjoining platformare 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 percent of ali geologlc time.

accretion as they evolved to their present sizes and shapes.In North America, for example, the Superior, Hearne,Rae, and Slave cratons, all within the Canadian shield,amalgamated along deformation belts to form a largercratonic unit during the Proterozoic Eon (see Chapter 9).Both Archean- and Proterozic-age rocks are found in era-tons, many of which indicate several episodes of defor-mation accompanied by igneous activity, metamorphism,and mountain building. However, most of the cratonshave experienced remarkably little deformation since thePrecambrian.

In North America, the exposed part of the craton is theCanadian shield, which occupies most of northeastern Can-ada, a large part of Greenland, the Adirondack MountainsofNew York, and parts of the Lake Superior region of Min-nesota, Wisconsin, and Michigan (Figure 8.5). Much of theCanadian shield is an area of subdued topography, numer-ous lakes, exposed Archean and Proterozoic rocks thinlycovered in places by Pleistocene glacial deposits. The rocksare volcanic, plutonic, and sedimentary, many of whichhave been altered to varying degrees by metamorphism.

Drilling and geophysical evidence indicate that Pre-cambrian rocks, that is, rocks of the platform, underliemuch of North America as well as other continents, butbeyond the shields they are seen only in areas of erosionand uplift. For instance, Archean and Proterozoic rocksare present in the deeper parts of the Grand Canyon and inmany ranges of the Rocky Mountains and the AppalachianMountains (see chapter opening photo andj- Figure 8.6).

60

180 140 100

Archean Earth HistoryWe have already mentioned that the Precambrian is longe rthan we can imagine, but just the Archean alone inc1udes32.6 percent of all geologic time (Figure 8.1). Archean-agerocks are known from several areas, but they are difficult tointerpret because (1) many are metamorphic and complexlydeformed; (2) most are deeply buried beneath youngerrocks; and (3) they contain few fossils, and those that areexposed are oflittle use in time-stratigraphic correlation.

Shields, Platforms, and CratonsContinents are not simply parts of Earths crust above sealevel. It is true that the continents are made up of ali groups ofrocks, but they have an overall composition similar to gran-ite, whereas the ocean basins are underlain by crust madeup of gabbro and basalt. Furthermore, continental crustis thicker and less dense than oceanic crust. The transitionfrom continental to oceanic crust occurs beneath the conti-nental slope, so the margins of continents are below sea level.All continents also have a vast area of exposed ancient rocksealled a Preeambrian shield, and extending outward fromshields are broad platforms ofburied Precambrian rocks. Ashield and its adjacent platform made up a craton, which weean think of as a eontinent's ancient nucleus (~Figure 8.5).

The cratons are the foundations of the continents, andalong their margins more continental crust was added by

154 CHAPTER8 Precambrian Earth and Life History

• Figure 8.6 Archean Rocks in North America Archean rocks in North America are exposed mostly in the Canadian shield and elsewherein areas of uplift and deep erosion.

(a) Outerop of the Aeasta Gneiss in the Northwest Territories of Can-ada. At about 4.0 billion years old, these are the oldest known rockson Earth except for meteorites.

Archean RocksOnly 22 percent of Earths exposed Precambrian crust isArchean, with the largest exposures in Africa and NorthAmerica. Archean crust is made up of many kinds of rocks,but most of them are greenstone belts and granite-gneisscomplexes, the latter being by far the most common. Sev-eral types of rocks are found in these granite-gneiss com-plexes, but granitic gneiss and granitic rocks predominate,both of which were probably derived from plutons that wereemplaced in volcanic island ares (Figure 8.4). Nevertheless,there are other rocks, ranging from peridotite to sedimen-tary rocks, all of which have been metamorphosed. Green-stone belts are subordinate, accounting for only 10 percentof Archean rocks, and yet they are important in unravelingorne of the complexities of Archean tectonic events.

Greenstone BeltsA greenstone beIt has three main rock associations; itslower and middle parts are mostly volcanic, whereas theupper rocks are mostly sedimentary (s-Figure S.7a). Green-tone belts typically have a synclinal structure, measure

anywhere from 40 to 250 km wide and 120 to 800 kmlong, and have been intruded by granitic magma and cutby thrust faults. Many of the igneous rocks are green-ish because they contain green minerals such as chlorite,actinolite, and epidote that formed during low-grademetamorphism.

Thick accumulations of pillow lava are common ingreenstone belts, indicating that much of the volcanismwas subaqueous (Figure 8.7b). Pyroclastic materials, incontrast, almost certainly formed by subaerial eruptionswhere large volcanic centers built above sea level, The most

(b) Shell Creek in the Big Horn Mountains of Wyoming has eut a deepgorge into this 2.9-billion-year-old (Mesoarchean) granite.

interesting igneous rocks in greenstone belts are komatiitesthat cooled from ultramafic lava flows, which are rare inrocks younger than Archean, and none occur now.

To erupt, ultramafic magma (magma with less than45 percent silica) requires near-surface magma tempera-tures of more than 1,600°C; the highest recorded surfacetemperature for recent lava flows is 1,350°C.During its earlyhistory, however, Earth possessed more radiogenic heat,and the mande was as much as 300°C hotter than it is now.Given these conditions, ultramafic magma could reach thesurface, but as Earth's radiogenic heat production decreased,the mande cooled and ultramafic flows no longer occurred.

Sedimentary rocks are found throughout greenstonebelts, but they predominate in the upper unit (Figure 8.7a).Many of these rocks are successions of graywacke (sand-stone with abundant day and rock fragments) and argillite(slightly metamorphosed mudrocks). Small-scale cross-bedding and graded bedding indicate these rocks repre-sent turbidity current deposition (see Figure 6.3).

Archean Earth History 155

1~ Figure 8.7 Greenstone Belts and Granite-Gneiss Complexes

_ Granitic intrusives

D Upper sedimentary unit: sandstones and shalesmost 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 than2.8 billion years old-have an ultramafic unit overlain by a basalticunit. In younger belts, the succession is from a basaltic lower unit toan andesite-rhyolite unit. In both cases, sedimentary rocks are foundmostly in the uppermost unit.

Other sedimentary rocks are also present, includingsandstone, conglomerate, chert, and carbonates, althoughnone arevery abundant. lran-rich racks known as bandediron jormations are also found, but they aremore typical of Proterozoic deposits, so wewill discuss them in Chapter 9.

The oldest large,well-preserved greenstonebelts are in South Africa and date fram about3.6 billion years ago. The 3.7- to 3.8-billion-year-old Isua greenstone belt of Greenland,consisting of metamorphosed lava flows,schists, quartzites, and banded iron formations(~Figure 8.8), has some of the oldest knownrocks on Earth. And given that it containsaltered sedimentary rocks, it is safe to assumethat even older source rocks were present.

ln North America, most greenstone beltsare found in the Superior and Slave cratons 1!

l'of the Canadian shield (~Figure 8.9), but they ~are also found in Michigan, Minnesota, and ~

CJ

Wyoming. Most formed between 2.7 and <9

2.5 billion years ago. The Abitibi greenstonebelt of Ontario and Quebec, Canada, is espe-cially well known for its extensive resourcesincluding gold, copper, and zinco

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0~Vi.8~",0e 0Q):J~ '"G

(b) Pillow lava of the Ishpeming greenstone belt in Michigan.

156 CHAPTER8 Precambrian Earth and Life History

(c) Gneiss from a granite-gneiss complex in Ontario, Canada.

~ Figure 8.8 The Isua Greenstone Belt, Greenland Felsic dikes cross-cuttingthe central gneiss, which is exposed between arms of the Isua Greenstone 8elt. At3.7 to 3.8 billion years old, the rocks in the Isua Greenstone 8elt are some of theoldest known on Earth.

V~SlaveCraton

SuperiorCraton

g>

'"~"C>~C>

"UM

oN~--~~--------------~~--------~~------~§

Figure 8.9 Greenstone Belts in North America Areheangreenstone belts (shown in dark green) of the Canadian shield aremostly in the Superior and Slave eratons.

Evolution of Greenstone BeltsMost greenstone belts are found in Archean and Protero-zoic terrains in multiple, parallel belts, each separated fromthe next by granite-gneiss complexes (Figure 8.7a, c). Mostgeologists would probably agree that greenstone belts origi-nate in several tectonic settings that might include oceanicplateaus, rifted continental margins, rifts within continents,as well as in back-arc basins that subsequently dose. Exact1yhow they form and evolveis not fully resolved, but the back-are basin model-involving a basin lying between a volcanicisland are and a continent much like the present-day Sea ofJapan-is appealing for the origin of some greenstone belts.

According to this model (s-Figure 8.10), there is anearly stage of extension as the back-arc basin forms,accompanied by volcanism, emplacement of plutons, andsedimentation, followed by an episode of compressionwhen the basin doses. During this latter stage, the evolv-ing greenstone belt rocks are deformed, metamorphosed,and intruded by granitic magma. Proponents of this modelsuggest that multiple episodes of the opening and closingofback-arc basins account for the parallel arrangement ofgreenstone belts.

The back-arc basin model for the origin of greenstonebelts is accepted by many geologists, but others think thatsome of these belts form in intracontinental rifts aboverising mantle plumes (.Figure 8.11). As the plume risesbeneath sialic (silica- and aluminum-rich) crust, it spreadsand generates tensional forces that cause rifting. The man-tle plume is the source of the lower and middle volcanicunits of the greenstone belt, and the uppersedimentaryunit results from erosion of the volcanic rocks along theflanks of the rift. And finally, there is an episode of closureof the rift, deforrnation, low-grade metarnorphism, andemplacement of plutons (Figure 8.11).

~ Figure 8.:1.0 Origin of a Greenstone Belt in a Back-ArcBasin

Voleanieare

(a) Rifting on the eontinent side of a voleanie are forms a baek-arebasin. Partial rnelting of subdueted oeeanie lithosphere suppliesandesite and diorite magmas to the island are.

Back-arc basin

Continental Sedimentsediment lavas

~ ----

(b) Basalt lavas and sediment derived from the island are and eonti-nent fill the baek-are basin.

(e) Closure of the baek-are basin, eompression, and deformation. Asyncline-like strueture forms, whieh is intruded by granitic rnagma.

Archean Earth History :1.57

=c"~"=co=c'-'M

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~ Figure 8.1.1. Origin of a Greenstone Belt in an Intracontinental Rift

RiftingLithosphere

Continental crust

(a) An ascending mantle plume causes rifting and volcanism.

Continental crust

(b) As the plume subsides, erosion of the rift flanks yields sediments.

Graniticintrusion

Archean Plate Tectonics andthe Orlgin of CratonsCertainly, the present plate tectonic regimeof opening and closing ocean basins has beena primary agent in Earth evolution since thePaleoproterozoic. Most geologists are con-vinced that some kind of plate tectonic actív-íty took place during the Archean as well, butit differed in detail from what is going on now.With more residual heat from Earth'sorigin andmore radiogenic heat, plates must have movedfaster and magma was generated more rapidly.As a result, continents no doubt grew more rap-idly along their margins, a process called conti-nental accretion, as plates collided with islandarcs and other plates. Also, ultramafic lavaflows (komatiites) were more common.

There were, however, marked differencesbetween the Archean world and the one thatfollowed. We have little evidence of Archeanrocks deposited on broad, passive continentalmargins, but associations of passivecontinentalmargin sediments were widespread by Protero-zoic time. Deformation belts between collidingcratons indicate that Archean plate tecton-ics was active, but the ophiolites so typical ofyounger convergent plate boundaries are rare.

Nevertheless, geologists have reportedprobable Neoarchean ophiolites from sev-eral areas. One in Russia is about 2.8 billionyears old, whereas the Dongwanzi complexof China is 2.5 billion years old. Not all agreethat this one in China is actually an ophio-lite sequence; the rocks have been complexlydeformed and intruded by magma possibly asyoung as Mesozoic. Another probable ophio-lite in the Wind River Mountains of Wyo-ming is also highly deformed and difficult tointerpret. Whether these rocks truly representophiolites may be an open question, but thereis no doubt that ophiolite sequences were pres-ent by the Paleoproterozoic (see Chapter 9).

In any case, several small cratons werepresent during the Archean and grew byaccretion along their margins. By the end ofthe Archean, perhaps 30 percent to 40 per-cent of the present volume of continentalcrust had formed. Remember, though, thatthese cratons amalgamated into a larger unitduring the Proterozoic (see Chapter 9). Aplate tectonic model for the Archean crustalevolution of the southern Superior cratonof Canada relies on the evolution of green-stone belts, plutonism, and deformation(e-Figure 8.12). We can take this as a modelfor Archean crustal evolution in general.

Subsidingplume

Continental crust

(c) .Closure of the rift causes compression and deformation. Granitic magma intrudesthe greenstone belt.

1.58 CHAPTER8 Precambrian Earth and Life History

~ Figure 8.12 Origin of the Southern Superior Craton

Hudson Bay

(a) Geologic map showing greenstone belts (dark green) and areasof granite-gneiss complexes (Iight green).

The events leading to origin of the southern Superiorcraton (Figure 8.12b) are part of a much more extensive oro-genic episode that took place during the Mesoarchean andNeoarchean. Deformation was responsible for the origin ofsome of the Archean rocks in several parts of the Canadianshield as well as in Wyoming, Montana, and the MississippiRiverValley.In the northwestern part of the Canadian shield,deformation along the Snowbird tectonic zone yielded meta-morphic rocks 3.2 and 2.6 billion years old that belong tothe granulite metamorphic fades, which form at very hightemperatures, at least 700°C, and some form at more than1,OOO°C.By the time this Archean event had ended, severalcratons had formed that are now found in the older parts ofthe Canadian shield (see Chapter 9).

lhe Atmosphere andHydrosphereln Chapter 1, we emphasized the interactions among sys-tems, two of which, the atmosphere and hydrosphere, havehad a profound impact on Earths surface (see Figure 1.1).Shortly after Earth formed, its atmosphere and hydro-sphere, although present, were quite different from theway they are now. They did, however, play an importantrole in the development of the biosphere.

How Did the Atmosphere Form andEvolve?Today, Earth's atmosphere is quite unlike the noxious onewe described earlier. Now it is composed of 78 percentnitrogen (N) and 21 percent free oxygen (02)' meaningoxygen notcombined with other elements as in carbon

Wabigoon belt Uchi belt

Quetico belt English River belt

Lavaflows

Berens River craton

.......•...•__ .~

.5m~ro---'c'-'M

oN

~----~.--------------~.----------~------~@(b) Plate tectonic model for the evolution of the southern Superiorcraton. The figure is a north-south cross section, and the upper dia-gram shows an earlier stage of development than the lower one.

dioxide (C02) and water vapor (H20). It also has smallbut important amounts of other gases such as ozone (03)'which, fortunately for us, is common enough in the upperatmosphere to block most of the Sun'sultraviolet radiation.

Earths earliest atmosphere was probably composed ofhydrogen and helium, the most abundant gases in the uni-verse. If so, it would have quickly been lost into space, fortwo reasons. First, Earth's gravitational attraction is insuf-ficient to retain gases with such low molecular weights.And second, before Earth differentiated, it had no core ormagnetic field. Accordingly, it lacked a magnetosphere, thearea around the planet within which the magnetic field isconfined, so a strong solar wind, an outflow of ions fromthe Sun, would have swept away any atmospheric gases.Once Earth had differentiated and a magnetosphere waspresent, though, an atmosphere began accumulating asa result of outgassing involving the release of gases fromEarths interior during volcanism (j-Pigure 8.13).

Water vapor is the most common gas emitted by vol-canoes today, but they also emit carbon dioxide, sulfurdioxide, carbon monoxide, sulfur, hydrogen, chlorine, andnitrogen. No doubt Archean volcanoes emitted the samegases, and thus an atmosphere developed, but one lackingfree oxygen and an ozone layer. It was, however, rich in car-bon dioxide, and gases reacting in this early atmosphereprobably formed ammonia (NH3) and methane (CH4).

This early oxygen-deficient but carbon dioxide-richatmosphere persisted throughout the Archean. Someof the evidence for this conclusion comes from detritaldeposits containing minerals such as pyrite (FeS2)and ura-ninite (U02), both of which oxidize rapidly in the presenceof free oxygen. So, the atmosphere was a chemically reduc-ing one rather than an oxidizing one. However, oxidizediron becomes increasingly common in Proterozoic rocks,indicating that at least some free oxygen was present then(see Chapter 9).

The Atmosphere and Hydrosphere 159

Escapes

HydrogenH2

To atmosphereNitrogen

N2

~ Figure 8.13 Outgassing and Earth's Early Atmo-sphere Erupting volcanoes emit mostly water vapor, carbon dioxide,and several other gases but no free oxygen-that is, oxygen notcombined with other elements. In addition to the gases shown here,chemical reactions in the early atmosphere probably yielded methane(CH4) and ammonia (NH3)'

Two processes account for introducing free oxygeninto the atmosphere, one or both of which began dur-ing the Hadean. The first, photochemical dissociation,involves ultraviolet radiation from the Sun disrupting

c'E

!-------------===~~ê'.s

M

~©

Oxygen#~ O,

To atmosphere

Carbon dioxideCO,

WaterH,O

water molecules in the upper atmosphere, thus releasingtheir oxygen and hydrogen (j-Eigure 8.14). This processmay eventually have supplied 2 percent of the present -dayoxygen level, but with this amount of free oxygen in theatmosphere, ozone forms, creating a barrier against ultra-violet radiation and the formation of more ozone. Evenmore important was photosynthesis, a metab.olic processin which organisms use carbon dioxide and water to makeorganic molecules and then release oxygen as a waste

ê' product (Figure 8.14). Even so, probably no more than.~ 1 percent of the free oxygen levei of today was present by.5g. the end of the Archean, 2.5 billion years ago.!M

'"Nrg The Hydrosphere-Earth's Surface

WatersAll water on Earth is part of the hydrosphere, but most ofit-more than 97 percent-is in the oceans. Where did itcome from, and how has it changed? Certainly, outgassingreleased water vapor from Earth's interior, and once theplanet cooled sufficiently, water vapor condensed and sur-face waters began to accumulate. Another source of watervapor, and eventually liquid water, was meteorites andespecially icy comets (Figure 8.3). It is not known whetheroutgassing or meteorites and comets was most important,but we do know that oceans were present by Archean time,although their volumes and geographic extent cannot bedetermined. Nevertheless, we can envision an ArcheanEarth with numerous erupting volcanoes and an earlyepisode of intense meteorite and comet bombardmentaccounting for a rapid rate of surface water accumulation.

Ultraviolet radiation

~ Figure 8.14 Evolution of theAtmosphere Photochemical dissociationoccurs when ultraviolet radiation disruptswater molecules that release hydrogen(H2) and free oxygen (02)' some of which isconverted to ozone (03) that blocks most ofthe ultraviolet radiation. In the presence ofsunlight, photosynthesizing organisrns usecarbon dioxide (C02) and water (H20) tomake organíc molecules and in the processrelease free oxygen as a waste product.

Escape tospace

OxygenO,

~~~~~~;;;~=:;Organic compounds

160 CHAPTER 8 Precambrian Earth and Life History

FoUowing Earths ear1yepisode of meteorite and cometbombardment, which ended about 3.8 billion years ago,these extraterrestrial bodies have added little to the accu-mulating surface waters. However, volcanoes continue toerupt and expel water vapor (much of it recycled surfacewater), so is the amount ofwater in the oceans increasing?Probably it is, but at a considerably reduced rate, becausemuch of Earths residual heat from its origin has dissipatedand the amount of radioactive decay to generate internalheat has diminished, so volcanism is not near1y as com-monplace. Accordingly, the amount of water added to theoceans now is trivial compared to their volumes.

RecaUfrom Chapter 4 that one ear1yattempt to deter-mine Earth's age was to calculate how long it took for theoceans to reach their current salinity level-assuming, ofcourse, that the oceans formed soon after Earth did, thatthey were freshwater to begin with, and that their salinityincreased at a uniform rate. None of these assumptions iscorrect, so the ages determined were vastly different. Wenow know that the very ear1yoceans were salty, probablyabout as salty as they are now. That is, very ear1y in theirhistory, the oceans reached chemical equilibrium and haverpm~lnprl lf"'1 np-:)1'"_~rn,;l;h~,; •.•-- ---- j!L.~

kind of chemical activity (metabolism) to maintain itself,and it must be capable of reproduction to ensure thelong-term survival of the group to which it belongs. Thismetabolism-reproduction criterion might seem sufficientto decide if something is living or not, and yet the distinc-tion is not always easy to make.

Bacterial are living, but under some circumstances,they can go for long periods during which they show nosigns ofliving and then go on living again. Are viruses liv-ing? They behave like living organisms in the appropriatehost cell, but when outside a host ceU,they neither metab-olize nor reproduce. Some biologists think that virusesrepresent another way of living, but others disagree. Com-paratively simple organic molecules called microspheresform spontaneously and grow and divide in a somewhatorganism-like rnanner, but these processes are more likerandom chemieal reactions, so they are not living.

So what do viruses and microspheres have to do with theorigin oflife? First, they show that the livingversus nonlivingdistinction is not always clear.And second, if life originatedby natural processes, it must have passed through prebi-otie stages-that is, stages in which the entities woulrl h::lvP

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