Chapter 8

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

2. Archean Rocks • The Teton Range • is largely Archean • gneiss, schist, and granite • Younger rocks are also present • but not visible Grand Teton National Park, Wyoming

3. Precambrian 4 Billion Years • The Precambrian lasted for more than 4 billion years! • Such a time span is difficult for humans to comprehend

4. Precambrian Time Span • 88% of geologic time

5. Precambrian • The Precambrian includes • time from Earth’s origin 4.6 billion years ago • to the beginning of the Phanerozoic Eon • 545 million years ago • No rocks are known for the first • 640 million years of geologic time

6. Rocks Difficult to Interpret • Precambrian rocks have been • altered by metamorphism • complexly deformed • buried deep beneath younger rocks • fossils are rare • the few fossils present are of little use in stratigraphy • Eon Subdivisions Archean and Proterozoic

7. Eons of the Precambrian • The Archean Eon • Start coincides with the age of Earth’s oldest known rocks • ~ 4 billion years old • lasted until 2.5 billion years ago • the beginning of the Proterozoic Eon • Hadean is an informal designation • for time preceding the Archean Eon

8. What Happened During the Hadean? • Earth accreted from planetesimals • differentiated into a core and mantle • and at least some crust • was bombarded by meteorites • ubiquitous volcanism • atmosphere formed • Ocean waters 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 comets and meteorites • with no continents, intense cosmic radiation • and widespread volcanism

10. Oldest Rocks 3.96-billion-year-old Acasta Gneiss in Canada and other rocks in Montana Sedimentary rocks in Australia contain detrital zircons (ZrSiO4) dated at 4.2 billion years old • so continental source rocks at least that old existed during the Hadean

11. Hadean Crust • Early Hadean crust was probably thin, unstable • and made up of ultramafic rock • those with comparatively little silica • This ultramafic crust was disrupted • by upwelling basaltic magma at ridges • and consumed at subduction zones • Hadean continental crust may have formed • by evolution of sialic material • Sialic crust contains considerable silicon, oxygen • and aluminum as in present day continental crust • Only sialic-rich crust, because of its lower density, • is immune to destruction by subduction

12. Second Crustal Evolution Stage • Subduction and partial melting • of earlier-formed basaltic crust • resulted in the origin of andesitic island arcs • Partial melting of lower crustal andesites, • in turn, yielded silica-rich granitic magmas • that were emplaced in the andesitic arcs

13. Second Crustal Evolution Stage • Several sialic continental nuclei • had formed by the beginning of Archean time • by subduction and collisions • between island arcs

14. 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

15. Cratons • A shield and platform make up a craton, • a continent’s ancient nucleus and its foundations • 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 • metamorphism, igneous activity • and mountain building • Cratons have experienced little deformation • since the Precambrian

16. 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

17. Canadian Shield • 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

18. Canadian Shield Rocks • Gneiss, a metamorphic rock, Georgian Bay Ontario, Canada

19. Canadian Shield Rocks • Basalt (dark, volcanic) and granite (light, plutonic) on the Chippewa River, Ontario

20. Amalgamated Cratons • Actually the Canadian shield and adjacent platform • is made up of numerous units or smaller cratons • that amalgamated along deformation belts • during the Early Proterozoic • Absolute ages and structural trends • help geologists differentiate • among these various smaller cratons • Drilling and geophysical evidence indicate • that Precambrian rocks underlie much • of North America, • in places exposed by deep erosion or uplift

21. Archean Rocks Beyond the Shield • Archean metamorphic rocks found • in areas of uplift in the Rocky Mtns Rocky Mountains, Colorado

22. Archean Rocks Beyond the Shield • Archean Brahma Schist in the deeply eroded parts of the Grand Canyon, Arizona

23. Archean Rocks • The most common Archean Rock associations • are granite-gneiss complexes • The rocks vary from granite to peridotite • to various sedimentary rocks • all of which have been metamorphosed • Greenstone belts are subordinate in quantity • but are important in unraveling Archean tectonism

24. Greenstone Belts • An ideal 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 • greenish (chlorite)

25. Greenstone Belt Volcanics • Abundant pillow lavas in greenstone belts • indicate that much of the volcanism was • under water, probably at or near a spreading ridge • Pyroclastic materials probably erupted • where large volcanic centers built above sea level Pillow lavas in Ispheming greenstone at Marquette, Michigan

26. Ultramafic Lava Flows • The most interesting rocks • in greenstone belts cooled • from ultramafic lava flows • Ultramafic magma has less than 45% silica • and 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 higher • and the mantle was as much as 300 °C hotter • This allowed ultramafic magma • to reach the surface

27. 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

28. 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 • a sandstone with abundant clay and rock fragments • and argillite • a slightly metamorphosed mudrock

29. Sedimentary Rocks of Greenstone Belts • Small-scale cross-bedding and • graded bedding indicate an origin • as turbidity current deposits • Quartz sandstone and shale, • indicate delta, tidal-flat, • barrier-island and shallow marine deposition

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

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

32. Evolution of Greenstone Belts • Models for the formation of greenstone belts • involve Archean plate movement • In one model, plates formed volcanic arcs • by subduction • and the greenstone belts formed • in back-arc marginal basins

33. Evolution of Greenstone Belts • According to this model, • volcanism and sediment deposition • took place as the basins opened

34. Evolution of Greenstone Belts • Then during closure, • the rocks were compressed, deformed, • cut by faults, • and intruded by rising magma • The Sea of Japan • is a modern example • of a back-arc basin

35. Archean Plate Tectonics • Most geologists are convinced • that some kind of plate tectonics • took place during the Archean • BUT, Plates must have moved faster • with more residual heat from Earth’s origin • and more radiogenic heat, • Thus, magma was generated more rapidly

36. Archean Plate Tectonics • As a result of the rapid movement of plates, • continents, no doubt, • 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 • were more common • due to the higher temperatures

37. 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 Late Archean ophiolites are known • Deformation belts between colliding cratons indicate that Archean plate tectonics was active

38. The Origin of Cratons • Certainly several small cratons • existed by the beginning of the Archean • and grew by periodic continental accretion • 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

39. 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

40. Canadian Shield • This deformation was • the last major Late Archean event in North America • and resulted in the formation of several sizable cratons now in the older parts of the Canadian shield

41. Present-day Atmosphere Composition • Variable gases Water vapor H2O 0.1 to 4.0 Carbon dioxide CO2 0.034 Ozone O3 0.0006 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

42. 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 • Wthout a magnetic field, • the solar wind would have swept away • any atmospheric gases

43. Outgassing • Once a core-generated magnetic field • protected the gases released during volcanism • called outgassing • they began to accumulate to form a new atmosphere • Water vapor • is the most common volcanic gas today • but volcanoes also emit • carbon dioxide, sulfur dioxide, • carbon monoxide, sulfur, hydrogen, chlorine, and nitrogen

44. Hadean-Archean Atmosphere • Hadean 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

45. 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

46. Introduction of Free Oxygen • Two processes account for • introducing free oxygen into the atmosphere, • one or both of which began during the Hadean 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 organism that practiced photosynthesis

47. Photosynthesis • Photosynthesis is a metabolic process • in which carbon dioxide and water • combine into 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

48. Oxygen Forming Processes • Photochemical dissociation and photosynthesis • added free oxygen to the atmosphere • Once free oxygen was present • an ozone layer formed • and blocked incoming ultraviolet radiation

49. Earth’s Surface Waters • Outgassing was responsible • for the early atmosphere • and also for Earth’s surface water • the hydrosphere • most of which is in the oceans • more than 97% • However, some but probably not much • of our surface water was derived from icy comets • Probably at some time during the Hadean, • the Earth had cooled sufficiently • so that the abundant volcanic water vapor • condensed and began to accumulate in oceans • Oceans were present by Early Archean times

50. Ocean water • The volume and geographic extent • of the Early Archean oceans cannot be determined • Nevertheless, we can envision an early Earth • with considerable volcanism • and a rapid accumulation of surface waters • Volcanoes still erupt and release water vapor • Is the volume of ocean water still increasing? • Perhaps it is, but if so, the rate • has decreased considerably • because the amount of heat needed • to generate magma has diminished • Much of volcanic water vapor today • is recycled surface water