THE PRECAMBRIAN

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THE PRECAMBRIAN. HADEAN ARCHEAN PROTEROZOIC. The Big Bang. Event that occurred approximately 13.7 BILLION years ago All the mass and energy concentrated at a point The universe began expanding and continues to expand - PowerPoint PPT Presentation

Text of THE PRECAMBRIAN

  • THE PRECAMBRIANHADEAN ARCHEAN PROTEROZOIC

  • The Big BangEvent that occurred approximately 13.7 BILLION years agoAll the mass and energy concentrated at a pointThe universe began expanding and continues to expandAfter 1 million years matter began to cool enough to form atoms- Hydrogen- the building block of stars

  • Galaxies and StarsGalaxy- huge rotating aggregation of stars, dust, gas held together by gravityEarth, the sun and our solar system is part of the Milky WayStars are massive spheres of incandescent gases (hydrogen and helium)

  • The Solar SystemOur solar system is located away from the galaxys centerOur sun and the planets originated from a solar nebula that had been enriched with heavy elements from nearby super novae (Stellar Synthesis)Solar system is approximately 5 Billion years oldComposition is 75% hydrogen, 23% helium and 2% other materials

  • Formation of aProtostar

  • Center contracts

    Center continues to heat up

    Protostar radiates more heat

  • Fusion begins in the stars core

  • Shockwaves radiate outward releasing material

    Material coalesces into planets, moons or comets

    Other material is ejected to the periphery

  • Our Solar System

    4 inner planets (terrestrial)

    4 outer planets (gaseous)

  • Solar nebula photographed by Hubble

  • Structure of the EarthSolid Inner CoreOuter Liquid CoreLower MantleUpper MantleAsthenosphereBrittle MantleLithosphereCrust

  • Structure of the EarthRefraction of Seismic WavesChanges in Velocity due to density

  • A Magma OceanLunar evidenceTextures, Uniform Composition, AgeCrystallization of well-mixed magma ocean produces uniform layered crustTerrestrial Magma OceanExistence of large amount initial heatOuter part of Earth melt during accretionDepth estimates 100 to >1000 KmUltramafic (high Fe & Mg)Crystallization complete in 100 my

  • Composition of the Early CrustUltramaficHigh Fe & MgKomatiites: volcanic, extrusive rocksRapid break-up and recycling of crustDue to vigorous convectionImpactsExistence of Plate Tectonics

  • Solidifying Basalt- Hawaii

  • The EarthCore is composed of mixtures or alloys of iron (pressure is more than a million times that at the surface and temperature is estimated to be at 4000C); has a solid inner core and a liquid outer core (earth's magnetic field may be produced by the motion of the liquid material in the iron-rich outer core) Layer outside the earth's core is the mantle; it is solid but very hot, near the melting point of rocks, so it flows almost like a liquid, though much slower; it is 70% of the earth's volume

  • The EarthOutermost layer is the crust; it is extremely thin (is thinner under the oceans than under the continents) Oceanic crust is made of basalt (low in silica and high in iron and magnesium) and has a higher density compared to continental crust, which is made of granite (high content of aluminum and magnesium silicate with quartz and feldspar) and has a lower density Thus, continents lie above sea level and oceanic crust lies below sea level because of density differences

  • Hotspots and Flood Basalts

  • The Lithosphere and Mantle ReservoirsArcheanhotter Earth >> thinner lithospheresteeper geothermal gradient >> Komatiites (require higher temperatures)lithospheric plates smaller and less stableMantle ReservoirsBetween 4 by and 2 by mantle separated into reservoirs which have remained homogeneous since formation

  • The Origin of the CrustAge based on lunar rocks and meteorites 4.4 to 4.5 byArchean rocks of Canadas Slave Province 3962 +/- 3 my based on zircon mineral crystalsHadean rocks of Australias Pilbara region 4400 my based on zirconRecycled due to rapid convection

  • Composition of the Early CrustComposition largely SpeculativeOldest lunar crustal rocks- representation of early earthGranitic?Too buoyant, resists subduction, no evidenceLunar Highlands (4.4 bybp)Fractional crystallization of basaltic magmaGabbros and anorthosites, rich in mafics mineralsKomatiites and Basalts

  • Anorthosite vs KomatiiteAnorthositedry magma forms crust (moon)wet magma plagioclase sinks and does not form crust (earth)Komatiite or Basaltabundant in Archean terraneshigh density and convective drag forces makes for easy recyclingformed as localized islands

  • Lower CrustMetamorphismlow grade for at shallow depths (
  • The First ContinentsContinental crust resists recycling due to buoyancyProduced by partial melting of oceanic crust in subduction zonesTonalites- abundant plagioclase, quartz, high in Ca,Na, AlAccretion of small islands into bigger continentsOldest remnants 3.8 to 4.0 bydetrital zircons 4.2-4.4 by< 500km diameterTonalites and granodiorites

  • Early Continental CrustAmitsoq GneissIsua Greenland4.0-3.8 by

  • Growth of Crust- MechanismVertical growth- thickeningunderplating (intrusion of magma to lower crust)Lateral growthriftingHigh grade granulites (representing 35-40 km) on the surface underlain by normal 40km thick crust impliesunderplating kept pace with uplift and erosion

  • Mechanisms Continental Growth Magma addition in arcsTerrane accretionContinental collisionWelding of marginal sediments

  • Mechanism for Continental Growth(a) Magma addition in arcs(b) Seaward migration of ocean plate(c) Terrane accretion through suturing(d) Continental collision(e) Welding of marginal sediments

  • Continental Growth RatesRapid early growthrecycling not feasibleLinear growthEpisodic growth2.7by, 2.0 by, 1.0 by correspond to major orogenic episodes in North Americatimevol

  • Crustal ProvincesLarge segments >107 km2Provinces are identified by geologic history and isotopic datesmost gneisses and granitesRecognized several large Precambrian provinces in North America >2.5 byNain; Rae; Slave; Hearne; Wyoming: Superior5 provinces 0.9 byWopmay; Yavapai-Mazatzal; Trans-Hudson; Mid-Continent; Grenville

  • Precambrian Provinces of N. Am.

  • The Assembling of North AmericaCollision and suturing of provinces to make a continentAssembly of Archean plates took only 10 my50% Late Archean (2.5-3.0 by)30% Early Proterozoic (1.6-2.0 by)
  • CratonsStable part of continentOldest part of continentsComposed of Shield and PlatformAll continents contain at least one cratonic massSmall
  • North American Craton- shield, and platform

  • The Hadean EonNo direct record of the first 800 myFormation of core 4.4 to 4.5 byCreation of Magma Ocean and cooling of a komatiite crustMosaic of small rapidly moving platesRecycling of crust at subduction zonesPartial melting of crust gives rise to tonalite magmas

  • Hadean Crust(a) 4.6 to 4.3 by; rapid recycling of an unstable crust(b) 4.3 to 3.8 by; the formation of continental islands

  • The Archean&The Proterozoic

  • Subdivisions of the PrecambrianMajor EventsOrigin of the EarthMajor outgassing development of internal structureOrigin of LifeBIFsKenoran OrogenyRed BedsGlaciationsGrenville Orogeny

  • Precambrian BasementIgneous & Metamorphic RocksAssociation of rocks based on Superposition and Cross-cutting relationshipsDivided into ARCHEAN and PROTEROZOICArchean 3.96 b.y. to 2.5 b.yProterozoic 2.5 b.y to 0.544 b.y.Hadean No record >3.96 b.y.Differentiation of EarthNo free OxygenRich in CO2 & H2O vaporMeteoric impacts for 100 m.y.No evidence on Earth, But evidence on Moon & Mars

  • ShieldsGeologic Stable Regions- Every continent has 1 or moreCanadian Shield, center of North America around Hudson BayExposed by Pleistocene GlaciationSurrounded by Platforms Thin Blankets of Sedimentary RocksShield + Platform = CRATON

  • Canadian Shield 11 ProvincesSuperior, Wyoming, Slave, Nam, Hearne, Rae & Grenville, Wopmay,Based on Faults and FoldsBased on Age of RocksBoundaries marked by Truncations in Structural LineationsBands of severely deformed rocksSuture zones consolidated by 1.9 b.y

  • Precambrian Provinces of N. Am.

  • Archean RocksThe Granitoid- Greenstone AssociationBroad basins, subsiding, subaqueous volcanicsShallow water deposition > stromatolitesGreenstone Beltsfolded and metamorphosedlinear to irregular-shaped successionschlorite, amphiboles, pillow basaltssome chert, BIFs, komatiites, felsic and intermediate volcanics, greywackesGranitoid Gneissintrusive granitic rocks > metamorphosed to gneiss

  • GREENSTONE BELTS

  • Greenstone Showing Well Developed Pillow Structures

  • Greenstone Belts of the Superior Province

  • Greenstone Belt: Barberton Mountain Land, South AfricaRiches of Greenstone BeltsCopper, Zinc, Silver and GoldWitswatersrand: Placer Gold Deposit

  • Plate Tectonic Model for the Development of Greenstone Belts and Growth of Continental Crust

  • Origin of AtmosphereAtmosphere evolved in 4 steps: primordial gases, later lost from sun's radiation exhalations from the molten surface (volcanic venting); bombardment from icy comets steady additions of carbon dioxide, water vapor, carbon monoxide, nitrogen, hydrogen, hydrogen chloride, ammonia, and methane from volcanic activity addition of oxygen by plant/bacterial life

  • ATMOSPHEREPresent Composition78% Nitrogen; 21% Oxygen; trace amounts of CO2, Argon, ect.Atmosphere Unique Among Other PlanetsVenus & Mars CO2 Gaseous planets H, He, CH4 Pressure in Venus 100x Earth on Mars 1/100Surface Temperature 450-500oC Venus; -130-25oC MarsAtmospheric Gases Controlled by volcanoes and interactions between gases and the solid Earth & Oceans as well as biotic componentOzone (O3): produced by