Chapter 8

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

2. Time check • The Precambrian lasted for more than 4 billion years! • Such a time span is almost impossible for us comprehend • If a 24-hour clock represented all 4.6 billion years of geologic time • the Precambrian would be slightly more than 21 hours long, • It constitutes about 88% of all geologic time

3. Precambrian Time Span

4. Precambrian • The term Precambrian is informal term referring to both time and rocks • It 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 • The oldest known rocks on Earth are 3.96 billion years old

5. Rocks of the Precambrian • 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 • the few fossils present are of little use in stratigraphy • Because of this subdivisions of the Precambrian have been difficult to establish • Two eons for the Precambrian • the Archean and Proterozoic

6. Eons of the Precambrian • The onset of the Archean Eon coincides with the age of Earth’s oldest known rocks • approximately 4 billion years old • lasted until 2.5 billion years ago (the beginning of the Proterozoic Eon) • The Hadean is an informal designation for the time preceding the Archean Eon • Precambrian eons have no stratotypes • the Cambrian Period, for example, which is based on the Cambrian System, a time-stratigraphic unit with a stratotype in Wales • Precambrian eons are strictly terms denoting time

7. US Geologic Survey Terms • Archean and Proterozoic are used in our discussions of Precambrian history, but the U.S. Geological Survey (USGS) uses different terms • Precambrian W begins within the Early Archean and ends at the end of the Archean • Precambrian X corresponds to the Early Proterozoic, 2500 to 1600 million years ago • Precambrian Y, from 1600 to 800 million years ago, overlaps with the Middle and part of the Late Proterozoic • Precambrian Z is from 800 million years to the end of the Precambrian, within the Late Proterozoic

8. The Hadean? • Except for meteorites no rocks of Hadean age are present on Earth, however we do know some events that took place during this period • Earth was accreted • Differentiation occurred, creating a core and mantle and at least some crust

9. Earth beautiful Earth…. about 4.6 billion years ago • Shortly after accretion, Earth was a rapidly rotating, hot, barren, waterless planet • bombarded by comets and meteorites • There were no continents, • intense cosmic radiation • widespread volcanism

10. Oldest Rocks • Judging from the oldest known rocks on Earth, the 3.96-billion-year-old Acasta Gneiss in Canada some continental crust had evolved by 4 billion years ago • Sedimentary rocks in Australia contain detrital zircons (ZrSiO4) dated at 4.2 billion years old • so source rocks at least that old existed • These rocks indicted that some kind of Hadean crust was certainly present, but its distribution is unknown

11. Hadean Crust • Early Hadean crust was probably thin, unstable and made up of ultramafic rock • rock 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. Crustal Evolution • A second stage in crustal evolution began as Earth’s production of radiogenic heat decreased • 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. Crustal Evolution • Several sialic continental nuclei had formed by the beginning of Archean time by subduction and collisions between island arcs

14. Dynamic Processes • During the Hadean, various dynamic systems similar to ones we see today, became operative, • not all at the same time nor in their present forms • Once Earth differentiated into core, mantle and crust, • internal heat caused interactions among plates • they diverged, converged and slid past each other • Continents began to grow by accretion along convergent plate boundaries

15. 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

16. 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

17. Distribution of Precambrian Rocks • Areas of exposed Precambrian rocks constitute the shields • Platforms consist of buried Precambrian rocks Shields and adjoining platforms make up cratons

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

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

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

21. Amalgamated Cratons • The Canadian shield and adjacent platform consists of numerous units or smaller cratons that were weldedtogether along deformation belts during the Early Proterozoic • Absolute ages and structural trends help geologists differentiate among these various smaller cratons

22. 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

23. Greenstone Belts • 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) • An ideal greenstone belt has 3 major rock units

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

25. Ultramafic Lava Flows • The most interesting rocks in greenstone belts cooled from ultramafic lava flows • Ultramafic magma has less than 40% 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 higher and the mantle was as much as 300 °C hotter than it is now • This allowed ultramafic magma to reach the surface

26. Sedimentary Rocks of Greenstone Belts • Sedimentary rocks are found throughout the greenstone belts • Mostly found 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

27. 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

28. 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

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

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

32. 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

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

34. 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 accretionas plates collided with island arcs and other plates • Also, ultramafic extrusive igneous rocks were more common due to the higher temperatures

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

36. The Origin of Cratons • Certainly several small cratons existed by the beginning of the Archean • During the rest of that eon they amalgamated into a larger unit • during the Early 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

37. 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

38. 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) • oxygen not combined with other elements • such as in carbon dioxide (CO2) • water vapor (H2O) • ozone (O3) • which is common enough in the upper atmosphere to block most of the Sun’s ultraviolet radiation

39. Present-day Atmosphere 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 • 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

40. 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. • Also because Earth had no magnetic field until its core formed the solar wind would have swept away any atmospheric gases

41. Outgassing • Once a core-generated magnetic field protected Earth, gases released during volcanism began to accumulate • Calledoutgassing • Water vapor is the most common volcanic gas today • also emitted • carbon dioxide • sulfur dioxide • Hydrogen Sulfide • carbon monoxide • Hydrogen • Chlorine • nitrogen

42. 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

43. 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) • Oxidized iron becomes increasingly common in Proterozoic rocks

44. Introduction of Free Oxygen • Two processes account for introducing free oxygen into the atmosphere, 1.Photochemical dissociation involves ultraviolet radiation in the upper atmosphere • The radiation breaks up water molecules and releases 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

45. Photosynthesis • Photosynthesis is a metabolic process in which carbon dioxide and water combine into organic molecules and oxygen is released as a wasteproduct 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

46. Earth’s Surface Waters • Outgassing was responsible for the early atmosphere and also for Earth’s surface water • the hydrosphere • Some but probably not much of our surface water was derived from icy comets • At some point 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

47. 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? • Much of volcanic water vapor today is recycled surface water

48. First Organisms • Today, Earth’s biosphere consists of millions of species of bacteria, fungi, protistans, plants, and animals, • only bacteria are found in Archean rocks • We have fossils from Archean rocks • 3.3 to 3.5 billion years old • Carbon isotope ratios in rocks in Greenland that are 3.85 billion years old convince some investigators that life was present then

49. What Is Life? • Minimally, a living organism must reproduce and practice some kind of metabolism • Reproduction insures the long-term survival of a group of organisms • whereas metabolism such as photosynthesis, for instance insures the short-term survival of an individual • The distinction between living and nonliving things is not always easy • Are viruses living? • When in a host cell they behave like living organisms • but outside they neither reproduce nor metabolize

50. What Is Life? • Comparatively simple organic (carbon based) molecules known as microspheres • form spontaneously • show greater organizational complexity than inorganic objects such as rocks • can even grow and divide in a somewhat organism-like fashion • but their processes are more like random chemical reactions, so they are not living