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Earth History GEOL 2110

Earth History GEOL 2110. Lecture 11 Origin and Early Evolution of the Earth Part 2: Differentiation of the Earth’s Spheres. Major Concepts.

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Earth History GEOL 2110

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  1. Earth History GEOL 2110 Lecture 11 Origin and Early Evolution of the Earth Part 2: Differentiation of the Earth’s Spheres

  2. Major Concepts • Early Earth became strongly heated by gravitational condensation, radioactive heating of short-lived isotopes, and impacting of asteroids; this resulted in the differentiation of the dense iron core and the light crust from the primordial mantle • Partial melting of the earth’s ultramafic mantle created the chemically distinctive crust of mafic and felsic composition • Outgassing of volatiles and photochemical dissociation during early crust formation created an early hydrosphere and atmosphere that quickly evolved to create the dominant chemistry we observe today • The evolution of the biosphere has subsequently modified the atmosphere’s composition by enriching it in oxygen • The chemical balance of the earth’s spheres have a great capacity to buffer pertubations to the system

  3. Layers of the Earth • --Mohorovicic • Discontinuity OCEANIC CONTINENTAL CRUST CRUST SiO2 47% 56% Al2O3 16% 18% FeO 13% 9% MgO 10% 3% CaO 10% 4% Na2O 2% 5.5% K2O 0.7% 2.5% TiO2 1.1% 1.3% P2O5 0.2% 0.7% MANTLE SiO2 – 45% MgO – 37% FeO – 8% Al2O3 – 4% CaO – 3% others – 3% CORE Fe – 86% S – 10% Ni – 4% Compositional Layers Structural Layers = chondritic meteorites

  4. Igneous Mineralogy of the Earth’s Crust Continental Crust Ocean Crust Mantle

  5. Oceanic CrustDistillation of the Mantle by Partial Melting Magmatism at Mid-Ocean Ridges Peridotite

  6. Oceanic CrustDistillation of the Mantle by Partial Melting Incongruent Melting – mineral phases don’t melt in equal proportions Pl Sp Gt Normal Ocean Geotherm Upwelling Geotherm Komatiite

  7. Oceanic CrustDistillation of the Mantle by Partial MeltingEvidence in Solid Solution Minerals OCEANIC CRUSTMANTLE SiO2 47% 45% Al2O3 16% 4% FeO 13% 8% MgO 10% 37% CaO 8% 3% Na2O 2.5% 0.2% K2O 0.7% 0.1% TiO2 1.1% 0.3 % P2O5 0.2% 0.05% Ni 200ppm 5000ppm Cr 100ppm 7000ppm 76% 24% 94% 6% Olivine/Pyroxene Basalt/ Gabbro Na Fe Ca Mg

  8. Continental CrustDistillation of Ocean Crust by Partial Melting Evidence in Solid Solution Minerals OCEANIC CONTINENTAL CRUST CRUST SiO2 47% 56% Al2O3 16% 18% FeO 13% 9% MgO 10% 3% CaO 10% 4% Na2O 2% 5.5% K2O 0.7% 2.5% TiO2 1.1% 1.3% P2O5 0.2% 0.7%

  9. Present-day Formation of CrustA Double Distillation Process Melting the Mantle Makes Mafic Magma – ALWAYS Melting Crust Makes Intermediate to Felsic Magma

  10. Crust Formation in Early Earth Not likely that Plate Tectonics, as we know it today, existed in the early Earth (>3.0Ga). Ocean crust too thick and too hot (plastic) to behave rigidly. Probably overthickened in compression zones causing partial melting of basal zones

  11. Evidence of Early Continental Crust 4.3-4.4 Ga zircon grains found in Sedimentary Rocks in Western Australia Zircons are rare in mafic rocks but plentiful in granitic rocks

  12. Formation of the Hydrosphere and Atmosphere

  13. Evidence of Early Formation of the Hydrosphere Acta Gneiss of the Slave Province in northwestern Canada 4.03 Ga metamorphosed sedimentary rocks

  14. Origin of Seawater and the AtmosphereVolcanic Outgassing

  15. Photochemical Evolution of the Atmosphere Prior to development of the Ozone layer, ultaviolet radiation caused dissociation of water vapor to create oxygen: 2H2O + UV rad2H2(lost) + O2 Oxygen then transformed methane and ammonia to carbon dioxide and nitrogen gas CH4 + 2O2  CO2 + 2H2O

  16. Biogentic Build-up of Oxygen in the Atmosphere Banded Iron-formation deposition Fixing biogenic oxygen in chemical sediments

  17. The Global Chemostat and ThermostatThe Earth’s Capacity to Buffer

  18. Summary or Early Earth Events

  19. Next Lecture Theory of Plate Tectonics Part 1: Toward the Development of the Grand Paradigm Quiz : Chapters 6 & 7

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