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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