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170 Chapter 20 The Precambrian Record GUIDED STUDY The text chapter should be studied one section at a time. Before you read, preview each section by skimming it, noting headings and boldface items. Then read the appropriate section objectives from the following outline. Keep these objectives in mind and, as you read the chapter section, search for the information that will enable you to meet each objective. Once you have finished a section, write out answers for its objectives. Archean Geology (pp. 404-409) 4. Describe the vertical succession of rocks that characterize a greenstone belt. What evidence suggests that such sequences formed underwater? The Divisions of Precambrian Time (p. 400) 1. What characteristics of Precambrian rocks caused geologists in the eighteenth and nineteenth centuries to consider them as Earth’s oldest rocks? The Hadean Eon (pp. 401-404) 5. Describe four processes by which isolated patches of continental crust could be assembled into larger cratons. 6. Archean sedimentary rocks are mostly graywackes, cherts and volcanic sediments. Why were shallow marine and terrestrial sedimentary rocks rarely formed at this time? 2. What information about the Moon was provided by the lunar samples collected in the 1960s and 1970s? Proterozoic Geology (pp. 409-413) 7. Describe how changes in tectonic activity during the Proterozoic Eon provided new environments for deposition of shallow marine sediments. 3. What does the occurrence of the ultramafic rock komatiite of Archean age suggest about the composition of the Earth’s first crust? Copyright © Houghton Mifflin Company. All rights reserved. 171 8. 9. Describe the three-part sequence of events involved in the “modern” style of plate tectonics, termed a Wilson cycle. What evidence suggests that Laurentia experienced extensive tectonic rifting during the Proterozoic Eon? Precambrian Life (pp. 416-419) 14. What environment is now thought to be the most likely site for the origin of life? 15. What evidence suggests that eukaryotic cells originated from more primitive prokaryotic forms? CHAPTER REVIEW The Precambrian Atmosphere and Climate (pp. 413-416) 10 What process was responsible for generation of the Earth’s first permanent atmosphere? 11. What two sources provided water for the Earth’s oceans? When you have finished reading the chapter, work through the material that follows to review it. Complete the sentences. As you proceed, evaluate your performance for each section by consulting the answers on pages 177-178. Do not continue with the next section until you understand each answer. If you need to, review or reread the appropriate section in the textbook before continuing. Introduction (pp. 399-400) 1. ________________ is the scientific method used to determine the age of Earth’s oldest rocks. 2. An age of 4.6 billion years comes from measurements made of ________________. 3. 12. What two processes were responsible for the dramatic increase in oxygen levels during the Proterozoic Eon? Tectonic processes destroy the oceanic crust, so Earth’s oldest crustal rocks occur on the ________. 4. _______________ is the region that can claim the Earth’s oldest exposures of surface rocks. 5. Earth’s oldest mineral grains are _________ found in Australia that date between 4.0-4.4 billion years. 13. What is a possible mechanism to explain the late Proterozoic glaciation that apparently covered even tropical land masses? The Divisions of Precambrian Time (p. 400) 6. The rocks of presumed great age found in deep gorges and mountain belt cores were termed _____________. 7. Lack of ___________ in these metamorphosed and deformed Precambrian rocks made their correlation impossible. Copyright © Houghton Mifflin Company. All rights reserved. 172 8. Divisions used by geologists that study Precambrian 21. The rock associations that contain interlayered rocks are based upon changes in _________ volcanic and sedimentary rocks that are _______ and the ______________ of the rocks in metamorphosed and folded into broad troughs are each interval. called_______ ______. 9. The most reliable method for correlation of Precambrian rocks is ___________ ________. 22. The oldest volcanic rocks at the base of a greeenstone belt sequence are ______________. 23. Sedimentary formations interlayered with the The Hadean Eon (pp. 401-404) volcanic rocks contain dark shales and turbidites 10. The earliest events of the Hadean involved the formed in ________ environments. ____________ and_______________ of the Earth. 11. The Moon’s lower density indicates that it has a much smaller metallic________ than that of Earth. 12. The Moon is hypothesized to have been acquired by the Earth’s gravitational field according to the old idea of _______________ ____________. 13. The chemical differences between Earth and the Moon do not support the hypothesis of ______ ___________. 14. Because it explains the composition and orbit of the Moon, the _______________hypothesis is 24. The tectonic location of greenstone belts is probably in forearc or backarc basins near _________ _____. 25. Small patches of continental crust were joined together to form larger _______ by a variety of tectonic processes. 26. Larger continental blocks were unable to form because the elevated temperature of the crust produced widespread _______________. 27. Archean sedimentary rocks do not include sandstone or limestone because the continental blocks lacked broad, shallow_____________. currently considered the most likely explanation for the origin of the moon. 15. The entire ________ of the Earth may have melted, due to heat from accretion, impacts and radioactive decay from the interior. 16. The composition of the Earth’s first crust was __________ , as indicated by the presence of very old Archean lavas called komatiites. 17. Intermediate to felsic magmas that formed the Earth’s earliest continental crust may have formed by the process of ________ _____________. 18. Zircons 4 –4.4 billion years old from Australia indicate the oldest possible ___________ _______. Proterozoic Geology (pp. 409-413) 28. Proterozoic rocks are more commonly exposed and less ___________ than Archean rocks. 29. Slower Proterozoic mantle convection produced larger continents with sediment deposition on broad shelves at the _____________ _______. 30. Proterozoic marine sediments were also deposited in shallow seaways called ________ _______, which covered large parts of the cratons. 31. The accumulation of coarse detrital sediments and volcanic rocks in fault-bounded basins indicates the process of _______________ ____________, the initial stage of a Wilson cycle. Archean Geology (pp. 404-409) 19. Archean rocks are exposed in the ________areas of the continents. 32. The accumulation of marine sediments along passive margins indicates _____________ of rifted continental fragments. 20. The rock association that represents the original Archean continental crust is called_______ ______. Copyright © Houghton Mifflin Company. All rights reserved. 173 33. Deformation of passive margins rocks by subduction and continental collision represents __________ of ocean basins. 34. The elongated belt of deformed rocks that forms at the plate edge from the tectonic processes of a Wilson cycle is called a(n) __________. 35. The oldest known Wilson cycle is preserved in a belt of folded, metamorphosed, and intruded rocks in Canada called the __________ __________. 36. The assembly of all major shield provinces of _____________. 45. Although cooling of the Earth’s crust is probably the source of most water found in the oceans, impact of _________ may have provided additional volumes of water. 46. Splitting of water molecules by ultraviolet radiation in the atmosphere releases free oxygen by the process of ____________ ________________. 47. Oxygen produced by the process of _________________ dissolved in the ocean waters Proterozoic age in North America formed the and combined with other elements to form oxide continent of _______________. minerals. 37. The southwestern portion of North America is 48. Significant levels of dissolved oxygen in the early called the _______________ province. It represents Proterozoic seas are indicated by the abundance of a series of volcanic island arcs that were accreted to _______ ______ ____________, which form when the southern margin of Laurentia. dissolved iron combines with oxygen on the ancient 38. The youngest addition to the North American craton during the Proterozoic occurred during the sea floor. 49. Precambrian ice ages seem to have resulted from ___________ ________, where North America plunging levels of atmospheric carbon dioxide, apparently collided with South America. which may have been caused by excessive 39. Crustal stretching during the middle Proterozoic formation of ______________ rocks. formed the ___________ _______, exposed at the surface only in the Great Lakes region. 40. The late Proterozoic supercontinent of __________ was composed of Africa, South America, Europe, North America, Australia, Antarctica, and India. 41. Basaltic lavas extruded in the Great Lakes region Precambrian Life (pp. 416-419) 50. Living things are composed of organic molecules with the ability to metabolize and ___________. 51. Because of igneous activity and the constant bombardment from space, the environment most during the middle Proterozoic were rich in the likely to have fostered life was probably the deep metal __________. ocean near _______________ _______. The Precambrian Atmosphere and Climate 52. The earliest fossils from 3.5 billion year-old cherts (pp. 413-416) of Australia resemble modern prokaryotes called 42. The Earth’s first atmosphere of nebular gases was __________________. probably swept away by the blast of particles generated from the sun known as the _____ _____. 43. The release of large volumes of gases and water vapor to the surface from volcanic activity is called _________ ____________. 44. The composition of Earth’s early atmosphere can be estimated from the gases exhaled from modern Copyright © Houghton Mifflin Company. All rights reserved. 53. _______________ are the most important fossil type of the Archean and Proterozoic eons. 54. Coiled ribbons of multicellular _______ represent the oldest known eukaryote fossils. 55. Eukaryotes may have formed by the union of ingested prokaryotic components in a process called _________________. 174 56. Multicellular organisms in which groups of cells perform specialized tasks are called __________. 57. Disk or frond-shaped fossil impressions of late Proterozoic age found represent the __________ ________. 58. Late Proterozoic locomotion is indicated by ______ __________ of crawling animals. 59. The first appearance of hard skeletal fossils is represented by small _________ _______. 60. Diversity of metazoans in the late Proterozoic was c. the dual formation hypothesis d. the planetary capture idea 6. The first crust of the Earth was destroyed by: a. plate tectonics. b. meteorite impacts. c. melting of the Earth’s mantle. d. All of the above. 7. The hottest lavas that ever erupted on the Earth’s surface formed ultramafic volcanic rocks called: a. basalts. b. komatiites. c. andesites. d. rhyolites. due to a warming of the oceans and an increase in the levels of ___________. PRACTICE TESTS After you thoroughly understand the correct answers of the Chapter Review, answer the following questions and check them with the answers on pages 178-179. If your answer is incorrect, consult the appropriate pages of the text. Multiple Choice Questions Circle your answers to the following questions. 1. The oldest rocks on Earth would be: a. the rocks of the ocean basins. b. the rocks of the folded mountain belts. c. the rocks of the shield areas. d. fragments of meteorites that landed on Earth. 2. Earth’s oldest known crustal rocks are found in: a. Canada. c. Antarctica. b. South Africa. d. China. 3. Primary rock formations were considered to be: a. uncorrelatable. b. devoid of fossils c. deformed and metamorphosed. d. All of the above. 4. The eons of the Precambrian combined account for: a. 27% of all geologic time. b. 42% of all geologic time. c. 87% of all geologic time. d. 97% of all geologic time. 5. Which hypothesis concerning the origin of the Moon best explains its differences in composition from that of Earth? a. the nebular hypothesis b. the impactor hypothesis 8. The oldest rock formation that represents continental crust consists of a rock called: a. sandstone. b. komatiite. c. tonalite. d. greenstone. 9. Zircon mineral grains from Australia that are dated at 4.0-4.4 billion years old indicate a felsic continental crust that is: a. over 4 billion years old. b. about 3.5 billion years old. c. about 3 billion years old. d. about 2.5 billion years old. 10. The most common Archean continental crust rocks are found: a. in greenstone belts. b. in granulite gniess associations. c. volcanic island arcs. d. continental shelves. 11. Rocks of the greenstone belt associations indicate: a. deposition on land. b. deposition along continental margins. c. deposition in deep water of backarc basins. d. deposition in oceanic trenches 12. Small patches of continental crust were enlarged by: a. intrusions of magma along plate edges. b. accretion of volcanic arcs at subduction zones. c. welding of sediment wedges at plate margins. d. all of the above. 13. Plate tectonics in the Archean Eon was characterized by: a. rapid plate movement and frequent collisions. b. rifting of large continents. c. drifting of large continents. d. collision of large continents. Copyright © Houghton Mifflin Company. All rights reserved. 175 14. The small, narrow continental blocks of the Archean were ideal depositional sites for: a. sandstones. c. graywackes. b. limestones. d. conglomerates. 15. Formation of broad shallow shelves and epeiric seas was afforded by: a. an increase in mantle plume activity. b. a decrease in the rate of mantle convection. c. an increase in rifting of continental blocks. d. an increase in collision of continental blocks. 16. The Wilson cycle illustrates: a. the “old” Archean tectonic style of plate motion. b. the “old” Archean style of mantle convection. c. the “modern” style of plate motion. d. the “modern” style of mantle convection. 17. The first stage of a Wilson cycle involves: a. rifting of continents to form ocean basins. b. drifting of rifted continental fragments. c. closure of ocean basins by subduction and continental collision. d. accretion of volcanic arcs to plate margins. 18. The deposition of quartz-rich sandstones and carbonates of the Odjick formation in the Wopmay orogen indicates which stage of a Wilson cycle? a. rifting of continents to form ocean basins. b. drifting of rifted continental fragments. c. closure of ocean basins by subduction and continental collision. d. accretion of volcanic arcs to plate margins. 19. Thrusting of the Coronation Supergroup formations eastward over the Slave craton illustrates which stage of a Wilson cycle? a. rifting of continents to form ocean basins. b. drifting of rifted continental fragments. c. closure of ocean basins by subduction and continental collision. d. accretion of volcanic arcs to plate margins. 20. The continent of Laurentia was assembled during the early Proterozoic by the process of: a. rifting. b. drifting. c. subduction and continental collision. d. accretion of volcanic arcs to plate margins. 21. The total, subsurface extent of the Midcontinent rift can be determined by which rock type? a. basaltic lavas. b. conglomerates. c. sandstones. d. graywackes. Copyright © Houghton Mifflin Company. All rights reserved. 22. The assembly of Rodinia was accomplished in part by the: a. Midcontinent rift. b. Trans-Hudson orogen. c. Mazatal orogen. d. Grenville orogeny. 23. To survive, the earliest atmosphere would have required: a. intense surface igneous activity. b. formation of the Earth’s magnetic field. c. bombardment by comets. d. the presence of plants performing photosynthesis. 24. Salinity in the oceans developed from: a. magmas with a high salt content. b. comets with a high salt water content c. rainwater with a high salt content. d. chemical weathering of the continents by highly acidic surface waters. 25. Elevated oxygen levels on land in the later Proterozoic are best indicated by the formation of: a. banded iron formations. b. glacial deposits. c. redbeds. d. salt deposits. 26. Solar energy during the Precambrian may have been only 70% of today’s levels. How did the Earth stay warm? a. limestone production warmed the Earth b. elevated carbon dioxide levels warmed the Earth c. the Earth was closer to the Sun d. the continents were located on the equator 27. Proteins, the building blocks of cells, are composed of: a. the nucleic acid DNA b. the nucleic acid RNA. c. protenoids. d. amino acids. 28. Stromatolites are sedimentary structures formed by: a. sticky cyanobacteria mats that bind sediments. b. shallow waves c. swiftly-moving streams. d. impressions of the Ediacara metazoans. 29. The best evidence for endosymbiosis lies with the: a. organelles that contain their own proteins. b. organelles that contain their own amino acids. c. organelles that contain their own DNA. d. organelles that contain their own single cell wall. 176 30. The Ediacaran fauna represents: a. unicellular eukaryotes. b. multicellular eukaryotes c. unicellular prokaryotes. d. multicellular prokaryotes. True or False Items CHALLENGE TEST Answer these questions the day before an exam as a final check on your understanding of the chapter’s terms and concepts. Check your responses with the answers on pages 179-180. If your answer is incorrect, consult the appropriate pages of the text (in parentheses following the correct answer). Write true or false on the line in front of each statement. Completion _____ 1. Fossils provide us with accurate determinations of the age of the Earth. _____ 2. The oldest Earth materials are 4.0-4.4 billion-year old zircon grains from Australia. _____ 3. Precambrian rocks encompass 87% of all geologic time. _____ 4. The dual formation hypothesis best explains the origin of the Moon. _____ 5. Formation of Earth’s crust followed the formation of the Moon. _____ 6. The first continents were composed of ultramafic and mafic rocks. _____ 7. Archean continental crust is represented by rocks contained in greenstone belts. _____ 8. Archean continents remained small because of frequent rifting. _____ 9. Large Proterozoic continents developed because of slower rates of mantle convection. _____ 10. A Wilson cycle describes the tectonic processes responsible for growth of small Archean continents. _____ 11. Laurentia was formed by a series of continental rifting events. _____ 12. Earth’s present atmosphere is characterized by the presence of the ozone layer. _____ 13. Higher levels of carbon dioxide in the Precambrian atmosphere kept temperatures warm because of a greenhouse effect. _____ 14. Prokaryotes are primitive, nonnucleated cells. _____ 15. Ediacaran metazoans were capable of making hard skeletons. Essay Questions Write a brief essay on a separate sheet of paper answering each of the following questions. 1. List and briefly discuss several hypotheses that explain the Moon’s formation. 2. What evidence is there to suggest that the Earth’s early atmosphere was compositionally very different from that of today? What changes took place to cause the modern atmosphere to form? 3. What could have caused the extreme glacial ages of the late Proterozoic ? Fill in the correct answers. 1. The three subdivisions of the Precambrian are the: ___________________, ___________________, and ____________________ eons. 2. The three ideas used to explain the origin of the Moon are the _____________ ____________, ________ ______________, and __________ hypotheses. 3. The composition of Earth’s first crust is believed to be ____________. 4. The felsic continental crust was probably formed by the process of________ ___________. 5. The greenstone belts probably formed under water because they contain _______ __________. 6. The Archean continents remain small because of continual _______________. 7. Archean sedimentary rocks were deposited maily in _______________ environments, because the continents were small, narrow, and steep-sided. 8. Proterozoic tectonics facilitated the growth of larger continents, and the appearance of broad, shallow shelves that favored deposition of _______________ (rock type). 9. The last phase of a Wilson cycle produces folding and metamorphism, which are produced by subduction and _____________ ____________. 10. An________ is an elongated belt of deformed rocks that forms at the edge of a plate through tectonic processes. 11. In North America, the assembly of Rodinia involved the __________ __________, in which Copyright © Houghton Mifflin Company. All rights reserved. 177 western South America and eastern North America collided. 12. Proterozoic glacial episodes are indicated by the presence of varved sediments and _________. 13. The most popular theory for the origin of eukaryotes is called _____________________. Multiple-Choice Questions Circle the correct answer. _____ 2. The age of Precambrian rocks can be determined from the severity of their deformation. _____ 3. The overall density of the Moon is much less than that of Earth. _____ 4. Komatiite lavas indicate a much hotter Archean Earth. _____ 5. The “modern” style of plate tectonics began in the Proterozoic with a slower rate of mantle convection. _____ 6. Stromatolites were the most advanced fossil forms throughout the entire Precambrian interval. 1. Earth’s oldest rocks are technically: a. meteorites. c. tonalites. b. granulites. d. sandstones. ANSWERS 2. The oldest interval of the Earth is the: a. Phanerozoic Eon. c. Hadean Eon. b. Proterozoic Eon. d. Archean Eon. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 3. The Moon was probably derived by impact from the Earth’s: a. core c. mantle. b. crust. d. asthenosphere. 4. The Archean rocks of the Canadian Shield have been assigned to distinct crustal provinces based on: a. different folding types. c. different ages. b. different compositions. d. all of the above. 5. Archean ultramafic lavas that indicate a hotter Earth surface are called: a. tonalite. c. quratzite. b. komatiite. d. granulite. 6. Sedimentary rocks characteristic of Archean deposition are: a. sandstones. c. graywackes. b. conglomerates. d. limestones. 7. The second phase of a Wilson cycle would be: a. rifting continents. c. colliding continents. b. drifting continents. d. shifting continents. 8. The most advance Proterozoic organisms were: a. unicellular prokaryotes. b. multicellular prokaryotes. c. unicellular eukaryotes. d. multicelular eukaryotes. True or False Items Write true or false on the line in front of each statement. _____ 1. Earth’s oldest crust is exposed in rocks of Greenland. Copyright © Houghton Mifflin Company. All rights reserved. CHAPTER REVIEW radioactivity meteorites continents Canada zircons primary fossils tectonic activity; composition radiometric dating accretion; differentiation core planetary capture dual formation impactor mantle ultramafic partial melting continental crust shield granulite gniess greenstone belts komatiites deep-water subduction zones cratons rifting shelves deformed continental margins epeiric seas continental rifting drifting closure orogen Wopmay orogen Laurentia Mazatal Grenville orogeny Midcontinent rift Rodinia copper 178 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52. 53. 54. 55. 56. 57. 58. 59. 60. solar wind volcanic outgassing volcanoes comets photochemical dissociation photosynthesis banded iron formations limestone reproduce hydrothermal vents cyanobacteria stromatolites algae endosymbiosis metazoans Ediacaran fauna trace fossils calcite tubes oxygen PRACTICE TESTS Multiple-Choice Questions 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. d a d c b d b c a b c d a c b c a b c c a d b d c b d a c b True or False Items 1. False. Radiometric dating of meteorites provides accurate determinations of the age of the Earth. 2. True. 3. True. 4. False. The impactor hypothesis best explains the origin of the Moon. 5. True. 6. False. The first oceanic crust was composed of ultramafic rocks. 7. False. Archean continental crust is represented by rocks of the granulite gniess association. 8. True. 9. True. 10. False. A Wilson cycle describes the modern sequence of plate tectonic processes. 11. False. Laurentia was built by a series of continental collisions. 12. True. 13. True. 14. True. 15. False. Organisms that lived at the same time as the Ediacaran metazoans made hard, calcite skeletons. Essay Questions 1. Several hypotheses have been proposed to explain the origin of the Moon. a. The oldest idea is that of planetary capture, which suggests that the Moon was a planetary body that formed elsewhere and was later captured by the Earth’s gravitational field. The likelihood of such an event is very small, considering the small size of the Moon and the approach necessary to permit capture without causing a catastrophic collision. b. The dual accretion hypothesis proposes simultaneous formation of the Earth and the Moon with the orbital dynamics such that the smaller Moon orbits the more massive Earth. The compositional differences between these two bodies does not support such an origin, as the Earth is so much more dense. The inclination of the Moon’s orbit is also significantly different from what would be expected had the two bodies formed together. c. The impactor hypothesis imagines a collision between the very young Earth and a Mars-sized body, which derived enough material from Earth’s mantle to form a smaller body composed of less dense, mantle composition. The core of the impactor may have sunk into the Earth’s core to contribute to its greater overall density. This last hypothesis most successfully explains compositional differences between the Earth and the Moon, and currently is most widely accepted. 2. Volcanic activity even today releases large volumes of gases and water vapor, and the composition of these gases provides clues to the composition of Copyright © Houghton Mifflin Company. All rights reserved. 179 Earth’s first permanent atmosphere. These gases were mainly water vapor, carbon dioxide, and nitrogen, with lesser amounts of carbon monoxide, hydrochloric acid, and sulfur dioxide. Methane and ammonia would have formed by chemical reactions in the atmosphere. Oxygen was completely absent in this early atmosphere, as evidenced by the existence of sedimentary pyrite in Archean rocks. Two processes brought about significant oxygen production in the Archean Eon. In the atmosphere, water molecules are split by ultraviolet radiation into free hydrogen that is lost to space, and free oxygen, which cannot escape the Earth’s gravity and remains. Free oxygen was produced in much greater amounts by the process of photosynthesis, in which plants use water and carbon dioxide to make a simple sugar compound and free oxygen. The oxygen produced was dissolved in the oceans and rapidly bound to metals to precipitate as insoluble iron oxides, forming the widespread banded iron formations of the late Archean and early Proterozoic. By late Proterozoic times, oxygen levels due to rampant photosynthesis had bound most of the dissolved iron in the oceans, and iron oxides began to appear on land in sediments known as redbeds. Elevated oxygen levels also formed the ozone layer in the upper parts of the atmosphere, which thereafter protected the Earth’s surface from ultraviolet radiation. 3. Glacial episodes of the late Proterozoic appear to have been the most extreme Earth has ever faced, with tropical glaciation and possibly frozen oceans. This “snowball” Earth may have developed because of the reduced energy output of the early Sun, and the cyclic imbalance between chemical weathering that utilizes carbon dioxide, and limestone formation that locks up carbon dioxide. The Proterozoic atmosphere was much richer in carbon dioxide than it is today, and this probably caused a significant greenhouse effect on Earth, which kept it warm despite the decreased solar energy output. As continents rifted apart, plate margins favorable for limestone production appeared, and levels of carbon dioxide fell as limestone formation soared. With dropping temperatures, the oceans froze and limestone production ceased as glaciation became widespread. Volcanic activity continued with continental rifting, raising levels of carbon dioxide. The Earth warmed and the oceans melted, causing limestone production to begin again. In tectonic situations of continental collisions such as the formation of Rodinia, environments favorable for limestone formation would be reduced and the greenhouse effect would continue unchecked. The fact that glacial deposits are interbedded with shallow marine carbonates provides evidence for this late Proterozoic climatic rollercoaster. Copyright © Houghton Mifflin Company. All rights reserved. CHALLENGE TEST Completion 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. Hadean; Archean; Proterozoic planetary capture; dual accretion; impactor ultramafic partial melting pillow lavas rifting deep-water sandstone continental collision orogen Grenville Orogeny tillites endosymbiosis Multiple-Choice Questions 1. 2. 3. 4. 5. 6. 7. 8. a c c d b c b d True or False Items 1. False Earth’s oldest crust is exposed in rocks of the Northwest Territories in Canada. 2. False The age of Precambrian rocks is determined by radiometric dating. 3. True 4. True 5. True 6. False Although stromatolites are the most common Archean and early Proterozoic fossils, the Ediacaran fauna of late Proterozoic age represents advanced multicellular eukaryotes called metazoans. In these organisms, groups of cells perform specialized tasks.