Chapter 8

1. Chapter 8 Precambrian Earth and Life History—The Archean Eon

2. Archean Rocks • The Beartooth Mountains • on the Wyoming and Montana border • consists of Archaean-age gneisses, • some of the oldest rocks in the US.

3. Precambrian • The Precambrian lasted for more than 4 billion years! • This large time span is difficult for humans to comprehend • Suppose that a 24-hour clock represented • all 4.6 billion years of geologic time • then the Precambrian would be • slightly more than 21 hours long, • constituting about 88% of all geologic time

4. Precambrian Time Span • 88% of geologic time

5. Precambrian • The term Precambrian is informal • but widely used, referring to both time and rocks • The Precambrian includes • time from Earth’s origin 4.6 billion years ago • to the beginning of the Phanerozoic Eon • 542 million years ago • It encompasses • all rocks below the Cambrian system • No rocks are known for the first • 600 million years of geologic time • The oldest known rocks on Earth • are 4.0 billion years old

6. Rocks Difficult to Interpret • The earliest record of geologic time • preserved in rocks is difficult to interpret • because many Precambrian rocks have been • altered by metamorphism • complexly deformed • buried deep beneath younger rocks • fossils are rare, and • the few fossils present are not of any use in biostratigraphy • Subdivisions of the Precambrian • have been difficult to establish • Two eons for the Precambrian • are the Archean and Proterozoic • which are based on absolute ages

7. Eons of the Precambrian • Eoarchean refers to all time • from Earth’s origin to the Paleoarchean • 3.6 billion years ago • Earth’s oldest body of rocks • the Acasta Gneiss in Canada • is about 4.0 billion years old • We have no geologic record • for much of the Archaen • Precambrian eons have no stratotypes • unlike the Cambrian Period, for example

8. What Happened During the Eoarchean? • Although no rocks of Eoarchean age are present on Earth, • except for meteorites, • we do know some events that took place then • Earth accreted from planetesimals • and differentiated into a core and mantle • and at least some crust was present • Earth was bombarded by meteorites • Volcanic activity was ubiquitous • An atmosphere formed, quite different from today’s • Oceans began to accumulate

9. Hot, Barren, Waterless Early Earth • about 4.6 billion years ago • Shortly after accretion, Earth was • a rapidly rotating, hot, barren, waterless planet • bombarded by meteorites and comets • with no continents, intense cosmic radiation • and widespread volcanism

10. Oldest Rocks • Continental crust was present by 4.0 billion years ago • Sedimentary rocks in Australia contain detrital zircons (ZrSiO4) dated at 4.4 billion years old • so source rocks at least that old existed • The Eoarchean Earth probably rotated in as little as 10 hours • and the Earth was closer to the Moon • By 4.4 billion years ago, the Earth cooled sufficiently for surface waters to accumulate

11. Eoarchean Crust • Early crust formed as upwelling mantle currents • of mafic magma, • and numerous subduction zones developed • to form the first island arcs • Eoarchean continental crust may have formed • by collisions between island arcs • as silica-rich materials were metamorphosed. • Larger groups of merged island arcs • protocontinents • grew faster by accretion along their margins

12. Origin of Continental Crust • Andesitic island arcs • form by subduction • and partial melting of oceanic crust • The island arc collides with another

13. Continental Foundations • Continents consist of rocks • with composition similar to that of granite • Continental crust is thicker • and less dense than oceanic crust • which is made up of basalt and gabbro • Precambrian shields • consist of vast areas of exposed ancient rocks • and are found on all continents • Outward from the shields are broad platforms • of buried Precambrian rocks • that underlie much of each continent

14. Cratons • A shield and its platform make up a craton, • a continent’s ancient nucleus • Along the margins of cratons, • more continental crust was added • as the continents took their present sizes and shapes • Both Archean and Proterozoic rocks • are present in cratons and show evidence of • episodes of deformation accompanied by • igneous activity, metamorphism, • and mountain building • Cratons have experienced little deformation • since the Precambrian

15. Distribution of Precambrian Rocks • Areas of exposed • Precam-brian rocks • constitute the shields • Platforms consist of • buried Pre-cambrian rocks • Shields and adjoining platforms make up cratons

16. Canadian Shield • The exposed part of the craton in North America is the Canadian shield • which occupies most of northeastern Canada • a large part of Greenland • parts of the Lake Superior region • in Minnesota, Wisconsin, and Michigan • and the Adirondack Mountains of New York • Its topography is subdued, • with numerous lakes and exposed Archean • and Proterozoic rocks thinly covered • in places by Pleistocene glacial deposits

17. Evolution of North America • North America evolved by the amalgamation of Archean cratons that served as a nucleus around which younger continental crust was added.

18. North American Craton • Drilling and geophysical evidence indicate • that Precambrian rocks underlie much • of North America, • exposed only in places by deep erosion or uplift

19. Archean Rocks • Only 22% of Earth’s exposed Precambrian crust is Archean • The most common Archean rock associations • are granite-gneiss complexes • Other rocks range from peridotite • to various sedimentary rocks • all of which have been metamorphosed • Greenstone belts are subordinate in quantity, • account for only 10% of Archean rocks • but are important in unraveling Archean tectonic events

20. Archean Rocks • Outcrop of Archean gneiss cut by a granite dike from a granite-gneiss complex in Ontario, Canada

21. Archean Rocks • Shell Creek in the Bighorn Mountains of Wyoming has cut a gorge into this 2.9 billion year old granite

22. Greenstone Belts • A greenstone belt has 3 major rock units • volcanic rocks are most common • in the lower and middle units • the upper units are mostly sedimentary • The belts typically have synclinal structure • Most were intruded by granitic magma • and cut by thrust faults • Low-grade metamorphism • makes many of the igneous rocks green • Because they contain chlorite, actinolite, and epidote

23. Greenstone Belts and Granite-Gneiss Complexes • Two adjacent greenstone belts showing synclinal structure • They are underlain by granite-gneiss complexes • and intruded by granite

24. Greenstone Belt Volcanics • Pillow lavas in greenstone belts • indicate that much of the volcanism was • subaqueous • Pyroclastic materials probably erupted • where large volcanic centers built above sea level Pillow lavas in Ispheming greenstone belt at Marquette, Michigan

25. Ultramafic Lava Flows • The most interesting rocks • in greenstone belts are komatiites, • cooled from ultramafic lava flows • Ultramafic magma (< 45% silica) • requires near surface magma temperatures • of more than 1600°C • 250°C hotter than any recent flows • During Earth’s early history, • radiogenic heating was greater • and the mantle was as much as 300 °C hotter • than it is now • This allowed ultramafic magma • to reach the surface

26. Ultramafic Lava Flows • As Earth’s production • of radiogenic heat decreased, • the mantle cooled • and ultramafic flows no longer occurred • They are rare in rocks younger • than Archean and none occur now

27. Sedimentary Rocks of Greenstone Belts • Sedimentary rocks are found • throughout the greenstone belts • although they predominate • in the upper unit • Many of these rocks are successions of • graywacke • sandstone with abundant clay and rock fragments • and argillite • slightly metamorphosed mudrock

28. Sedimentary Rocks of Greenstone Belts • Small-scale cross-bedding and graded bedding • indicate an origin as turbidity current deposits • Other sedimentary rocks are present, but not abundant • sandstone, conglomerate, chert, carbonates • Iron-rich rocks, banded iron formations, are more typical of Proterozoic deposits

29. Canadian Greenstone Belts • In North America, • most greenstone belts • (dark green) • occur in the Superior and Slave cratons • of the Canadian shield

30. Evolution of Greenstone Belts • Greenstone belts formed in several tectonic settings • Models for the formation of greenstone belts • involve Archean plate movement • In one model, greenstone belts formed • in back-arc marginal basins

31. Evolution of Greenstone Belts • According to this model, • There was an early stage of extension as the back-arc marginal basin formed • volcanism and sediment deposition followed

32. Evolution of Greenstone Belts • Then during closure, • the rocks were compressed, • metamorphosed, • and intruded by granitic magma • The Sea of Japan • is a modern example • of a back-arc basin

33. Another Model • In another model accepted by some geologists, • greenstone belts formed • over rising mantle plumes in intracontinental rifts • As the plume rises beneath sialic crust • it spreads and generates tensional forces • The mantle plume is the source • of the volcanic rocks in the lower and middle units • of the greenstone belt • and erosion of volcanic rocks and flanks for the rift • supply the sediment to the upper unit • An episode of subsidence, deformation, • metamorphism and plutonism followed

34. Greenstone Belts—Intracontinental Rift Model • Ascending mantle plume • causes rifting • and volcanism

35. Greenstone Belts—Intracontinental Rift Model • Erosion of the rift flanks • accounts for sediments

36. Greenstone Belts—Intracontinental Rift Model • Closure of rift • causes compression • and deformation

37. Archean Plate Tectonics • Plate tectonic activity has operated • since the Paleoproterozoic or earlier • Most geologists are convinced • that some kind of plate tectonic activity • took place during the Archean as well • but it differed in detail from today • Plates must have moved faster • with more residual heat from Earth’s origin • and more radiogenic heat, • and magma was generated more rapidly

38. Archean Plate Tectonics • As a result of the rapid movement of plates, • continents grew more rapidly along their margins • a process called continental accretion • as plates collided with island arcs and other plates • Also, ultramafic extrusive igneous rocks, • komatiites, • were more common

39. Archean World Differences • but associations of passive continental margin sediments • are widespread in Proterozoic terrains • The Archean world was markedly different than later • We have little evidence of Archean rocks • deposited on broad, passive continental margins • but the ophiolites so typical of younger convergent plate boundaries are rare, • although Neoarchean ophiolites are known • Deformation belts between colliding cratons • indicate that Archean plate tectonics was active

40. The Origin of Cratons • Certainly several small cratons • existed during the Archean • and grew by accretion along their margins • They amalgamated into a larger unit • during the Proterozoic • By the end of the Archean, • 30-40% of the present volume • of continental crust existed • Archean crust probably evolved similarly • to the evolution of the southern Superior craton of Canada

41. Southern Superior Craton Evolution • Greenstone belts (dark green) • Granite-gneiss complexes (light green Geologic map • Plate tectonic model for evolution of the southern Superior craton • North-south cross section

42. Canadian Shield • Deformation of the southern Superior craton • was part of a more extensive orogenic episode • during the Mesoarchean and Neoarchean • that formed the Superior and Slave cratons • and some Archean rocks in Wyoming, Montana, • and the Mississippi River Valley • By the time this Archean event ended • several cratons had formed that are found • in the older parts of the Canadian shield

43. Atmosphere and Hydrosphere • Earth’s early atmosphere and hydrosphere • were quite different than they are now • They also played an important role • in the development of the biosphere • Today’s atmosphere is mostly • nitrogen (N2) • abundant free oxygen (O2), • or oxygen not combined with other elements • such as in carbon dioxide (CO2) • water vapor (H2O) • small amounts of other gases, like ozone (O3) • which is common enough in the upper atmosphere • to block most of the Sun’s ultraviolet radiation

44. Present-day Atmosphere Composition • Variable gases Water vapor H2O 0.1 to 4.0 Carbon dioxide CO2 0.038 Ozone O3 0.000006 Other gases Trace • Particulates normally trace • Nonvariable gases Nitrogen N2 78.08% Oxygen O2 20.95 Argon Ar 0.93 Neon Ne 0.002 Others 0.001 in percentage by volume

45. Earth’s Very Early Atmosphere • Earth’s very early atmosphere was probably composed of • hydrogen and helium, • the most abundant gases in the universe • If so, it would have quickly been lost into space • because Earth’s gravity is insufficient to retain them • because Earth had no magnetic field until its core formed (magnetosphere) • Without a magnetic field, • the solar wind would have swept away • any atmospheric gases

46. Outgassing • Once a magnetosphere was present • Atmosphere began accumulating as a result of outgassing • released during volcanism • Water vapor • is the most common volcanic gas today • but volcanoes also emit • carbon dioxide, sulfur dioxide, • carbon monoxide, sulfur, • hydrogen, chlorine, and nitrogen

47. Archean Atmosphere • Archean volcanoes probably • emitted the same gases, • and thus an atmosphere developed • but one lacking free oxygen and an ozone layer • It was rich in carbon dioxide, • and gases reacting in this early atmosphere • probably formed • ammonia (NH3) • methane (CH4) • This early atmosphere persisted • throughout the Archean

48. Evidence for an Oxygen-Free Atmosphere • The atmosphere was chemically reducing • rather than an oxidizing one • Some of the evidence for this conclusion • comes from detrital deposits • containing minerals that oxidize rapidly • in the presence of oxygen • pyrite (FeS2) • uraninite (UO2) • But oxidized iron becomes • increasingly common in Proterozoic rocks • indicating that at least some free oxygen • was present then

49. Introduction of Free Oxygen • Two processes account for • introducing free oxygen into the atmosphere, • one or both of which began during the Eoarchean. 1. Photochemical dissociation involves ultraviolet radiation in the upper atmosphere • The radiation disrupts water molecules and releases their oxygen and hydrogen • This could account for 2% of present-day oxygen • but with 2% oxygen, ozone forms, creating a barrier against ultraviolet radiation 2. More important were the activities of organisms that practiced photosynthesis

50. Photosynthesis • Photosynthesis is a metabolic process • in which carbon dioxide and water • to make organic molecules • and oxygen is released as a waste product CO2 + H2O ==> organic compounds + O2 • Even with photochemical dissociation • and photosynthesis, • probably no more than 1% of the free oxygen level • of today was present by the end of the Archean