BIG BANG Big Bang 15Ga – elementary particles, light elements as H and He – stars and galaxies of the first generation – white dwarfs. Neutron stars, black holes, further light elements – supernovas – heavy elements, stars of the second generation with planets – chemical evolution – geological and biologic evolutiom EARTH DIFFERENTIATION • How did the layering occur? – Two hypotheses • Homogenous accretion (cold) – uniform density at beginning – warming melts iron and nickel etc. – warming from? • bombardment by particles • radioactive decay • compression Homogeneous Accretion Model uniform density at beginning •Material with carbonaceous chondrite composition is heated, melted, and fractionated. Chondrite - rich in the silicite minerals olivine and pyroxene. •Fe-Ni settles to core of body (gravity) •Volatiles are degassed •Si is concentrated in a "crust" Heterogeneous Accretion Model •FeNi and other refractory elements condense first. •If accretion times are rapid compared to condensation times, an FeNi core accretes first, followed by silicate mantle earthdiff3.jpg (30752 bytes) After the initial segregation into a central iron (+nickel) core and an outer silicate shell, further differentiation occurred into an inner (solid) and outer (liquid) core (a pressure effect: solid iron is more densely packed than liquid iron), the mantel (Fe+Mg silicates) and the crust (K+Na silicates). The magma ocean would have cooled to form a layer of basaltic crust (such as is present beneath the oceans today). Continental crust would have formed form later. It is probable that the Earth’s initial crust was remelted several times due to impacts with large asteroids. Impact hypothesis 20_05c Planetary capture hypothesis • 20_05a Speed and approach angle unlikely Dual accretion hypothesis 20_05b Chemical composition of the Moon suggests that it could not have co-formed with the earth. earthdiff3.jpg (30752 bytes) After the initial segregation into a central iron (+nickel) core and an outer silicate shell, further differentiation occurred into an inner (solid) and outer (liquid) core (a pressure effect: solid iron is more densely packed than liquid iron), the mantel (Fe+Mg silicates) and the crust (K+Na silicates). The magma ocean would have cooled to form a layer of basaltic crust (such as is present beneath the oceans today). Continental crust would have formed form later. It is probable that the Earth’s initial crust was remelted several times due to impacts with large asteroids. The Precambrian The Precambrian is an informal name given to the age of the first three eons of Earth history. 1.Hadean (Eon) (Early Archean) 4.6 - 3.8 bya (or 4.6 - 3.96 bya) No rock record. This is the time of the origin of the Earth. Earth was mostly hot molten rock at that time. 2.Archean Eon3.8-2.5 bya 3.Proterozoic Eon 2.5-0.544 bya International Stratigraphical Chart Geologic time scale 19_02 This block compressed 88% of Earth History Key Events of Precambrian time 20_03 Acasta Gneiss is dated at 3.96 bya. It is near Yellowknife Lake , NWT Canada Zircons possibly a bit older in Australia Hadean earthstep1 earthstep1 •Shortly after accretion, Earth was –a rapidly rotating, hot, barren, waterless planet –bombarded by comets and meteorites –with no continents, intense cosmic radiation –widespread volcanism Hot, Barren, Waterless Early Earth WICHG30801 http://www.utdallas.edu/~janokb/Files/GEOS_1304/Web_pages/K_Precambrian_files/slide0014_image023.gi f http://www.utdallas.edu/~janokb/Files/GEOS_1304/Web_pages/K_Precambrian_files/slide0014_image024.gi f http://www.utdallas.edu/~janokb/Files/GEOS_1304/Web_pages/K_Precambrian_files/slide0014_image025.gi f Precambrian Early Atmosphere •Early permanent earth atmosphere mostly Nitrogen (inert) and CO2 Post-differentiation start of liquid core dynamo •Liquid water is required to remove CO2 from atmosphere. –Mars is too cold to have liquid water. –Venus is too hot. –Both have CO2 atmospheres. •On Earth, most of the world’s CO2 is locked up in limestones, dolomites, and life! Mars Earth Venus •Early Hadean crust was probably thin, unstable and made up of ultramafic rock •rock with comparatively little silica Hadean Crust •This ultramafic crust was disrupted •by upwelling basaltic magma at ridges •and consumed at subduction zones •Later Hadean continental crust may have formed by evolution of felsic material •only felsic crust, because of its lower density, is immune to destruction by subduction igclsht2 Oldest rock – Hudson Bay, Nuvvuagittuq greenstone belt, amphibolites Nuvvuagittuq greenstone belt The Nuvvuagittuq greenstone belt, originally named the Porpoise Cove greenstone belt, is a greenstone belt on the eastern shore of Hudson Bay in northern Quebec, Canada. They measured tiny variations in the isotopes (or species of an element that have different numbers of neutrons) of the rare earth elements neodymium and samarium in the rocks and determined that the samples were from 3.8 to 4.28 billion years old. The Nuvvuagittuq greenstone belt is mainly composed of cummingtonite-plagioclase-biotite-garnet mafic amphibolites called the Ujaraaluk unit (formerly called the "Faux-amphibolite" due to its unusual color). Rocks from the Ujaraaluk unit have a neodymium-142 isotopic signature that can only be acquired in the Hadean, prior to 4 billion years ago. This isotopic tool has been used to date these rocks at up to 4.3 billion years old. The mafic rocks from the Nuvvuagittuq belt are interpreted to be volcanic rocks that were hydrothermally altered and includes a banded iron formation between a lower basalt and an upper unit which includes basalt and andesite. In 2014, a detailed geochemical analysis, revealing layered gradients of ytterbium and niobium, suggested that this formation consists of pillow lavas from a tectonic subduction zone, similar to the modern Mariana trench. img019 Oldest continental crust Originally only oceanic crust - subducted The Earth’s Oldest Crustal Rocks acasta The Acasta gneiss in Canada’s NWT was formed 4.0 Byr ago. Along with similar metamorphic rocks in southern Greenland, these are the most ancient pieces of crust remaining on Earth. –3.96 Ga +/- 3 Ma Oldest Rock on Earth acasta •Acasta Gneiss Origin of Continental Crust •3.9 to 4.2 Bya •Acasta Gneiss –3.96 Ga +/- 3 Ma • Oldest Rock on Earth •Zircon from an Australian sedimentary rock indicates an age of 4.4 Gyr years old. zircon_grain_crystal00sm In addition, the oxygen isotopic compositions of some of these zircons have been interpreted to indicate that more than 4.4 billion years ago there was already water on the surface of the Earth •Judging from the oldest known rocks on Earth •the 4.03-billion-year-old Acasta Gneiss in Canada, some continental crust had evolved prior to 4 billion years ago •Sedimentary rocks in Australia contain detrital zircons dated at 4.4 billion years old •These rocks indicted that some kind of Hadean crust was certainly present –distribution is unknown Oldest ContinentalRocks First continental crust switch these Density differences allow subduction and allow further felsic crust Water out First Then: http://www.utdallas.edu/~janokb/Files/GEOS_1304/Web_pages/K_Precambrian_files/slide0015_image027.gi f Precambrian Early Oceans from 4 bya •Much water vapor from volcanic degassing. •Salt in oceans is derived from weathering and carried to the oceans by rivers. •Part of the earth’s water probably came from comets. –Comets are literally large dirty snowballs. –Provide fresh water. Hadean •A time of major changes and Earth formation. No rock record. •Differentiation of the Earth to form crust, mantle and core •Origin of the atmosphere Volcanic outgassing (or degassing) H2O, H2, HCl, CO, CO2, N2, Sulfur gases Little or no free oxygen (O2); would lead to rapid oxidation of iron minerals •Condensation of water vapor formed the hydrosphere ((3,9, ?4,4 Ga) rain; runoff leads to lakes, rivers, oceans originally freshwater (rain); may have been acidic from sulfurous gases slow accumulation of salts due to weathering •Beginnning of formation of oceanic and continental crust of Earth. Archean Oldest oceanic crust Ophiolite fragment embedded in the Isua supracrustal belt in south-west Greenland. An ophiolite is a section of the Earth's oceanic crust and the underlying upper mantle that has been uplifted and exposed above sea level and often emplaced onto continental crustal rocks. Life appeared on Earth during the Archean (3.5 - 3.8 bya). Geochemical evidence of photosynthesis in rocks 3.8 billion years old on Greenland. Anomalously high C12/C13 ratio, consistent with photosynthesis Earliest cells were prokaryotic (did not have a nucleus or organelles) like bacteria The earliest cells had to form and exist in anoxic conditions. Probably chemosynthetic, producing H2S or CO2 Some of the early organisms became photosynthetic possibly due to a shortage of raw materials for energy. •Fossils and organo-sedimentary structures remaining from this early life include: •Algal filament fossils 3.5 b.y. at North Pole, western Australia •Spheroidal bacterial structures (Kingdom Monera) Fig Tree Group, South Africa 3.0 - 3.1 by prokaryotic cells; appear to show various stages of cell division •Stromatolites (cyanobacteria or blue-green algae) in carbonate sediment oldest are 3.4 - 3.5 by old also in rocks 2.8 - 3 by old more abundant in Proterozoic rocks 7. Oxygen began to build up in the atmosphere as a waste product of photosynthesis Chemosynthesis is the process by which certain microbes create energy by mediating chemical reactions. microfos 3.5 bya – first evidence of life on earth! Microfossils from Western Australia Blue-green algae Blue-green algae Photosynthesis Produces Oxygen! 3.5-2.0 bya only prokaryotes lived chapter26 stromatolites_hamelin_pool1 a-c-stromato1 stromats At right is a layered stromatolite, produced by the activity of ancient cyanobacteria. The layers were produced as calcium carbonate precipitated over the growing mat of bacterial filaments; photosynthesis in the bacteria depleted carbon dioxide in the surrounding water, initiating the precipitation. The minerals, along with grains of sediment precipitating from the water, were then trapped within the sticky layer of mucilage that surrounds the bacterial colonies, which then continued to grow upwards through the sediment to form a new layer. As this process occured over and over again, the layers of sediment were created. This process still occurs today; Shark Bay in western Australia is well known for the stromatolite "turfs" rising along its beaches. Photosynthesis Produces Oxygen! Photosynthesis Produces Oxygen! Photosynthesis Produces Oxygen! Photosynthesis Produces Oxygen! The earth began in a state similar to the moon with a crustal (lithospheric) surface composed dominantly of mafic/ultramafic igneous rocks and anorthosites. With the formation of the oceans the early earth would have been a relatively simple world compared with today - oceans from pole to pole, with occasional scattered hot spot volcanos. Quickly, however, convection cells established divergent and convergent plate boundaries which began the fractionation processes that would build the continents. The sequence of cross sections below illustrate the kinds of processes by which initial volcanic arcs could increase in size to form protocontinents, then through cordilleran orogenies and collisions form protocontinets, which would grow to form microcontinents, which would eventually grow to form supercontinents. All of these processes constitute variations on the Wilson cycle. The combinations and permutations of relationships is virtually endless. Anything that could reasonably happen, probably happened. Archaean Crustal Evolution and the Formation of Continents igclsht2 The sequence of cross sections below illustrate the kinds of processes by which initial volcanic arcs could increase in size to form protocontinents, then through cordilleran orogenies and collisions form protocontinets, which would grow to form microcontinents, which would eventually grow to form supercontinents. All of these processes constitute variations on the Wilson cycle. Growth of the early continents 20_12 (a) Magmatism from Subduction Zones causes thickening Growth of the early continents 20_12 (b) Island Arcs and other terranes accrete as intervening ocean crust is subducted Little Archean ocean crust survives, nearly all subducted Growth of the early continents 20_12 (c) Sediments extend continental materials seaward Growth of the early continents 20_12 (d) •Continent-Content collisions result in larger continents •Again, not very big in Archean, Plate Tectonics too fast The period, from about 3.0 to 2.5 billion years ago, was the period of maximum continent formation. 70% of continental landmasses date from this period (Thus, most of the continents are extremely ancient). Modern Earth sciences recognize that the present continents are built around cores of extremely ancient rock, called "shields". A large part of Australia is a "shield", as is much of Canada, India, Siberia, and Scandinavia. Archean Rocks 1.Granulites, gneisses high grade metamorphic rocks – continental crust, originally granodiorites, tonalites and anorthosites. Anorthosite is a intrusive igneous rock characterized by a predominance of plagioclase feldspar (90–100%), and a minimal mafic component (0–10% 2. Greenstone belts - volcanic and sedimentary rocks commonly metamorphosed chlorite produces green color Areas of granitic rock (now gneisses) separated by greenstone belts: bands of sequences of weakly metamorphosed komatiites -> basalts -> felsic volcanics -> marine sediments (turbidites, cherts, banded iron formations, etc.). 3. Sedimentary rocks clastic, altered to metasedimentary rocks metagraywackes, slates, schists, metaconglomerates, diamictites some relict sedimentary structure Banded Iron Formations red chert (jasper) and unoxidized iron-rich sedimentary rocks. First appear 3.8 Ga; much more common in Proterozoic. Rare after 1.9 Ga, last appear c. 720 Ma. Major iron ore. Shield Australia 2 Precambrian Early Continents (Cratons) •Archean cratons consist of regions of light-colored felsic rock (gneisses) • surrounded by pods of dark-colored greenstone (chlorite rich metamorphic rocks). –Pilbara Shield, Australia –Canadian Shield –South African Shield. Mafic Greenstone Belts Felsic Islands 40km Komatiite? •Ultramafic lava flow •Rich in high Mg olivine •Olivine alters to serpentine •Only occurs in Archaean terrains, rare in Proterozoic •High temperatures - associated with mantle plumes •Archaean greenstone belts – occurs as altered komatiite flows •Oldest 4.03 Ga, disappear around 2.5 Ga. • wall Greenstone belts consist primarily of volcanic rocks, typically basalts that have been altered by low grade metamorphism which produces chlorite - a greenish mineral. Some belts include ultramafic lava flows which require near surface temperatures of 1600°C (komatiites). This means that the early mantle was nearly 300°C hotter than today. This suggests that the earth has cooled, probably as a result of a loss of radiogenic heat. Two models have been proposed for the evolution of greenstone belts. 1) Back-arc spreading in marginal basins and 2) Plate tectonic model 2) mantle plumes Back-arc model Plate tectonic model Biot218PhotoJ Mantle plume model plume Formation of greenstone belts 20_11 •Early continents formed by collision of felsic proto-continents. •Greenstone belts represent volcanic rocks and sediments that accumulated along subduction zones and then were sutured to the protocontinents during collisions. •Protocontinents small, rapid convection breaks them up Stratigraphic Sequence of a Greenstone belt 20_09 Younger lavas richer in silica Komatiites form at very high temps Increasingly Silica-rich extrusives, some rhyolites with granites below them. •Komatiite: Ultramafic volcanics, very common in Archean, very rare afterwards. Require temperatures of greater than 1600 şC (modern lavas max out at 1350 şC). Hint at the extreme activity of Archean mantle. • Clinopyroxen crystals and spinifex texture in komatiite komatiite1 komatiite1a Komatiites are ultramafic volcanic rocks, having very low silica contents (~40-45%) and very high MgO contents (~18%. These ancient lava flows erupted at a time when the Earth's internal heat was much greater than today, thus generating exceptionally hot, fluid lavas with calculated eruption temperatures in excess of 1,600 degrees C (2,900 degrees F). In comparison, typical basaltic lavas erupting today have eruption temperatures of about 1,100 degrees C. gneiss_8x6 Archean To Proterozoic Sedimentary Rocks •Archean Mostly deep water clastic deposits such as mudstones and muddy sandstones. –high concentration of eroded volcanic minerals. •Absence of shallow water shelf carbonates. –Mostly chert. •low oxygen levels, free iron was much more common in the Archean. –Free iron formed “chemical sinks” that consumed much of the early planetary oxygen. –Formed banded ironstones, commonly with interbedded chert. – Proterozoic – Carbonates become important File written by Adobe Photoshop® 5.0 compevol bif Banded Iron Formation, Alternating bands of red jasper and black hematite, about 2250 million years old (2.55 billion years old) Jasper Knob, Ishpeming, Michigan bif Banding of BIF: record of episodic growth of microbes - precipitation of Fe oxides followed by depletion of O2 - cycle repeats many times continental_shields_8x6 Archean-Age Surface Rocks 20_07 • • Origin of Life Origin of Archaebacteria 3.5 bya •Archaebacteria are the most primitive fossil life forms –Likely ancestors of all life. •Primitive Archaebacteria are hyperthermophiles that thrive in boiling point of water. –Modern Archaebacteria live in deep-sea volcanic vents. •Some Archaebacteria feed directly on sulfur (chemoautotrophs). –Archean life probably arose in deep oceans hydrothermal, volcanic vents that would have dotted the ocean floor near rifting zones. –Vents provide: •chemical and heat energy, •abundant chemical and mineral compounds, including sulfur •protection from oxygen and ultraviolet radiation. Some of the early organisms became photosynthetic possibly due to a shortage of raw materials for energy. Photosynthesis was an adaptive advantage. Produced their own raw materials. Autotrophs. Examples = cyanobacteria (stromatolites) Oxygen was a WASTE PRODUCT. • •Autotrophs use an abiotic source of energy to convert inorganic material into organic compounds for growth and reproduction. •Autotrophs produce food, and are known as “primary producers”. •Inorganic vs. organic material. –Inorganic = CO2, NH3, NO32-, PO43-, etc –Organic = living, or derived from living tissue (proteins, lipids, carbohydrates, nucleic acids, or containing C-C bonds (petroleum products). – •Plants are autotrophs and the primary producers in most ecosystems. –Energy source is the Sun. – •Chemosynthetic bacteria are autotrophs and primary producers in deep vent communities –Energy source is inorganic sulfur molecules, NOT SUN! – – 10-1 Autotrophs Chemosynthetic Organisms Ø Use sulfur or sulfides Ø Use methane Ø Bacteria and cyanobacteria Life appeared on Earth during the Archean (3.5 - 3.8 bya). Geochemical evidence of photosynthesis in rocks 3.8 billion years old on Greenland. Anomalously high C12/C13 ratio, consistent with photosynthesis Earliest cells were prokaryotic (did not have a nucleus or organelles) like bacteria The earliest cells had to form and exist in anoxic conditions. Probably chemosynthetic, producing H2S or CO2 Some of the early organisms became photosynthetic possibly due to a shortage of raw materials for energy. •Fossils and organo-sedimentary structures remaining from this early life include: •Stromatolites (cyanobacteria or blue-green algae) in carbonate sediment oldest are 3.4 - 3.5 by old also in rocks 2.8 - 3 by old more abundant in Proterozoic rocks •Algal filament fossils 3.5 b.y. at North Pole, western Australia •Spheroidal bacterial structures (Kingdom Monera) Fig Tree Group, South Africa 3.0 - 3.1 by prokaryotic cells; appear to show various stages of cell division 7. Oxygen began to build up in the atmosphere as a waste product of photosynthesis When did life arise on Earth? •Probably before 3.85 billion years ago. •Shortly after end of heavy bombardment, 4.2-3.9 billion years ago. •Evidence from fossils, carbon isotopes. 18-02 Grand Canyon layers record 2 billion years of Earth’s history… http://www.utdallas.edu/~janokb/Files/GEOS_1304/Web_pages/K_Precambrian_files/slide0023_image038.gi f slide0023_image037 Fossil Bacteria •Prokaryotic archaebacteria and eubacteria are dominant. 2 bya –Eubacteria form stromatolites (photosynthetic). –More common in upper Archean as shallow water shelves began to form along margins of early continents. –Archean is the age of pond-scum. •Molds of individual bacterial cells found in Precambrian cherts. 850 million years old Chroococcalean 1.8 Palaeolyngbya 1.8 Photosynthesis Produces Oxygen! Fossil evidence for microbes 3.5 billion years ago • Already fairly complex life (photosynthesis), suggesting much earlier origin. 18-04A stromats At right is a layered stromatolite, produced by the activity of ancient cyanobacteria. The layers were produced as calcium carbonate precipitated over the growing mat of bacterial filaments; photosynthesis in the bacteria depleted carbon dioxide in the surrounding water, initiating the precipitation. The minerals, along with grains of sediment precipitating from the water, were then trapped within the sticky layer of mucilage that surrounds the bacterial colonies, which then continued to grow upwards through the sediment to form a new layer. As this process occured over and over again, the layers of sediment were created. This process still occurs today; Shark Bay in western Australia is well known for the stromatolite "turfs" rising along its beaches. img004 stromats At right is a layered stromatolite, produced by the activity of ancient cyanobacteria. The layers were produced as calcium carbonate precipitated over the growing mat of bacterial filaments; photosynthesis in the bacteria depleted carbon dioxide in the surrounding water, initiating the precipitation. The minerals, along with grains of sediment precipitating from the water, were then trapped within the sticky layer of mucilage that surrounds the bacterial colonies, which then continued to grow upwards through the sediment to form a new layer. As this process occured over and over again, the layers of sediment were created. This process still occurs today; Shark Bay in western Australia is well known for the stromatolite "turfs" rising along its beaches. Crustal provinces: Proterozoic Tectonics 20_13 Intensely folded rocks where cratons were sutured together in Early Proterozoic Slave Craton Rift and Drift Followed by Wopmay Orogen Proterozoic Tectonics: The Wilson Cycle •Proterozoic – Convection Slows •Rift Phase –Coarse border, valley and lava rocks in normal faulted basins • •Drift Phase –Passive margin sediments • •Collision Phase –Subduction of ocean floor, collision with Island Arcs Wilson Cycle 1&2 Rift & Drift Coronation Supergroup 20_14 Proterozoic 2 bya as Slave craton pulled apart 2. Passive Margin sediments 1. Rift Valley Much later stuff Near-collision phase of the Wilson Cycle in the Wopmay Orogen 20_15 (a) 3. End of Wilson cycle in the Wopmay orogen 20_15 (b) Coronation Supergroup thrust eastward over Slave Craton Note the vertical exaggeration protocon Komatiite: melts at 1600 degrees C! Upper mantle hotter during Cryptozoic?