Chapter 8

1. Chapter 8 Precambrian Earth and Life History—The Hadean and Archean -platforms and shield areas= together make craton -early planet: volcanic, toxic gases (CO2), -Hadean geology: rare rocks preserved!! -Archaen geology: v little evidence, greenstone belts-volcanics and sedimentary rocks. -atmosphere and ocean evolve thru time: outgassing from volcanoes and development of photosynthesis -early life forms: single celled bacteria, algae: stromatolites - Archaen mineral resources: gold, chrome, platinum, iron

2. Precambrian Time Span • 88% of geologic time

3. 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

4. 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 • 545 million years ago • It encompasses • all rocks older than Cambrian-age rocks • 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 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 • the few fossils present are of little use in stratigraphy • Subdivisions of the Precambrian • have been difficult to establish • eons for the Precambrian • are the Hadean, 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 • and lasted until 2.5 billion years ago • the beginning of the Proterozoic Eon • Hadean is an informal designation • for the time preceding the Archean Eon • Precambrian eons have no stratotypes • unlike 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 lecture following discussions of Precambrian history, • but the U.S. Geological Survey (USGS) uses different terms • Their 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. Archaen: 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

9. Oldest Rocks • Judging from the oldest known rocks on Earth, • the 3.96-billion-year-old Acasta Gneiss in Canada and other rocks in Montana • 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

10. 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 (granite!) • 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

11. 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

12. 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

13. 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

14. Canadian Shield • The craton in North America includes 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 • It’s topography is subdued, • with numerous lakes and exposed Archean • and Proterozoic rocks thinly covered • in places by Pleistocene glacial deposits

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

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

17. 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

18. 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)

19. 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

20. Ultramafic Lava Flows • The most interesting rocks • in greenstone belts cooled • from ultramafic lava flows • Ultramafic magma has less than 40% 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 • than it is now • This allowed ultramafic magma • to reach the surface

21. 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

22. 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

23. 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

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

25. 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

26. 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 • with more residual heat from Earth’s origin • and more radiogenic heat, • and magma was generated more rapidly

27. 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

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

29. 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

30. Canadian Shield • Deformation of the southern Superior craton • was part of a more extensive orogenic episode • that formed the Superior and Slave cratons • and some Archean rocks in Wyoming, Montana, • and the Mississippi River Valley • 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

31. 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 • Without a magnetic field, • the solar wind would have swept away • any atmospheric gases

32. Outgassing • Once a core generated, the 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

33. 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

34. 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

35. 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

36. 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

37. 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

38. First Organisms • Today, Earth’s biosphere consists • of millions of species of bacteria, fungi, • protistans, plants, and animals, • whereas 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

39. How Did Life First Originate? • To originate by natural processes, • life must have passed through a pre-biotic stage • in which it showed signs of living organisms • but was not truly living • In 1924, the great Russian biochemist, • A.I. Oparin, postulated that life originated • when Earth’s atmosphere had little or no free oxygen • Oxygen is damaging to Earth’s • most primitive living organisms • Some types of bacteria must live • where free oxygen is not present

40. How Did Life First Originate? • With little or no oxygen in the early atmosphere • and no ozone layer to block ultraviolet radiation, • life could have come into existence from nonliving matter • The origin of life has 2 requirements • a source of appropriate elements for organic molecules • energy sources to promote chemical reactions

41. Elements of Life • All organisms are composed mostly of • carbon (C) • hydrogen (H) • nitrogen (N) • oxygen (O) • all of which were present in Earth’s early atmosphere as • Carbon dioxide (CO2) • water vapor (H2O) • nitrogen (N2) • and possibly methane (CH4) • and ammonia (NH3)

42. Proteinoid Microspheres • Proteinoid microspheres produced in experiments • Proteinoids grow and divide much as bacteria do

43. Next Critical Step • Not much is known about the next critical step • in the origin of life • the development of a reproductive mechanism • The microspheres divide • and may represent a proto-living system • but in today’s cells nucleic acids, • either RNA or DNA • are necessary for reproduction • The problem is that nucleic acids • cannot replicate without protein enzymes, • and the appropriate enzymes • cannot be made without nucleic acids, • or so it seemed until fairly recently

44. Much Remains to Be Learned • The origin of life has not been fully solved • but considering the complexity of the problem • and the fact that scientists have been experimenting • for only about 50 years • remarkable progress has been made • Debate continues • Many researchers believe that • the earliest organic molecules • were synthesized from atmospheric gases • but some scientists suggest that life arose instead near hydrothermal vents on the seafloor

45. Azoic (“without life”) • Prior to the mid-1950s, scientists • had little knowledge of Precambrian life • They assumed that life of the Cambrian • must have had a long early history • but the fossil record offered little • to support this idea • A few enigmatic Precambrian fossils • had been reported but most were dismissed • as inorganic structures of one kind or another • The Precambrian, once called Azoic • (“without life”), seemed devoid of life

46. Stromatolites • Present-day stromatolites form and grow • as sediment grains are trapped • on sticky mats • of photosynthesizing blue-green algae (cyanobacteria) • although now they are restricted • to environments where snails cannot live • The oldest known undisputed stromatolites • are found in rocks in South Africa • that are 3.0 billion years old • but probable ones are also known • from the Warrawoona Group in Australia • which is 3.3 to 3.5 billion years old

47. Oldest Known Organisms • Charles Walcott (early 1900s) described structures • from the Early Proterozoic Gunflint Iron Formation of Ontario, Canada • that he proposed represented reefs constructed by algae • Now called stromatolites, • not until 1954 were they shown • to be products of organic activity Present-day stromatolites Shark Bay, Australia

48. Stromatolites • Different types of stromatolites include • irregular mats, columns, and columns linked by mats

49. Other Evidence of Early Life • Carbon isotopes in rocks 3.85 billion years old • in Greenland indicate life was perhaps present then • The oldest known cyanobacteria • were photosynthesizing organisms • but photosynthesis is a complex metabolic process • A simpler type of metabolism • must have preceded it • No fossils are known of these earliest organisms

50. Earliest Organisms • The earliest organisms must have resembled • tiny anaerobic bacteria • meaning they required no oxygen • They must have totally depended • on an external source of nutrients • that is, they were heterotrophic • as opposed to autotrophic organisms • that make their own nutrients, as in photosynthesis • They all had prokaryotic cells • meaning they lacked a cell nucleus • and lacked other internal cell structures typical of eukaryotic cells(to be discussed later in the term)