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Chapter 8

Chapter 8. Precambrian Earth and Life History—The Archean Eon. Archean Rocks. The Twin Lakes in Beartooth Plateau, MT and WY is called one of the most scenic in the U.S. Most of the rocks are Archean-aged gneiss. . Precambrian. The Precambrian lasted for more than 4 billion years!

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Chapter 8

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

  2. Archean Rocks • The Twin Lakes • in Beartooth Plateau, MT and WY • is called one of the most scenic in the U.S. • Most of the rocks are Archean-aged gneiss.

  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 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

  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 of little use in stratigraphy • Subdivisions of the Precambrian • have been difficult to establish • Two eons for the Precambrian • are the Archean and Proterozoic

  7. 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 • Eoarchean refers to all time • from Earth’s origin to the Archean • 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

  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 comets and 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 comets and meteorites • with no continents, intense cosmic radiation • and widespread volcanism

  10. 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 and Greenland • some continental crust had evolved by Eoarchean time • Sedimentary rocks in Australia contain detrital zircons (ZrSiO4) dated at 4.4 billion years old • so source rocks at least that old existed • These rocks indicted that some kind • of Eoarchean crust was certainly present, • but its distribution is unknown

  11. Eoarchean Crust • Early Eoarchean crust was probably thin • and made up of ultramafic rock • igneous rock with less than 45% silica • This ultramafic crust was disrupted • by upwelling mafic magma at ridges, • and the first island arcs formed at subduction zones • 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. Dynamic Processes • During the Eoarchean, various dynamic systems • similar to ones we see today, • became operative, • but not all at the same time nor in their present forms • Once Earth differentiated • into core, mantle and crust, • million of years after it formed, • internal heat caused interactions among plates • as they diverged, converged • and slid past each other in transform motion • Continents began to grow by • accretion along convergent plate boundaries

  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 • 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 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

  18. Canadian Shield Rocks • Outcrop of Archean gneiss in the Canadian Shield in 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 • are made up of numerous units or smaller cratons • that amalgamated along deformation belts • during the Paleoproterozoic • 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, • exposed only in places by deep erosion or uplift

  21. Archean Rocks Beyond the Shield • Archean rocks found • in areas of uplift in the Teton Range, WY

  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 • Other rocks range from peridotite • to various sedimentary rocks • all of which have been metamorphosed • Greenstone belts are subordinate in quantity • but are important in unraveling Archean tectonic events

  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 • green (chlorite, actinolite, epidote)

  25. 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

  26. 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

  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 • sandstone with abundant clay and rock fragments • and argillite • 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, • metamorphosed, • and intruded by granitic magma • The Sea of Japan • is a modern example • of a back-arc basin

  35. Another Model • In another model • although not nearly as widely accepted, • greenstone belts formed • over rising mantle plumes in intracontinental rifts • The plume serves as the source • of the volcanic rocks in the lower and middle units • of the developing 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

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

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

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

  39. 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

  40. 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, • komitiites, • were more common

  41. 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

  42. The Origin of Cratons • Certainly several small cratons • existed during the Archean • and grew by periodic continental accretion • during the rest of that eon • 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

  43. 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

  44. 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 Archean event in North America • and resulted in the formation of several sizable cratons • now in the older parts of the Canadian shield

  45. 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

  46. 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

  47. 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

  48. 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

  49. 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

  50. 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

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