Evolution of the earth

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Copyright ©The McGraw-Hill Companies, Inc Permission required for reproduction or display Evolution of the Earth Seventh Edition Prothero • Dott Chapter Cryptozoic History: Introduction to the Origin of Continental Crust Figure 4.7 Figure 4.8 Complete Geologic Time Scale Hadean to Recent Phanerozoic – “visible life” Geologic Time Scale for 1st 3.8 Billion Years of Earth Existence Proterozoic - “hidden life” Archean – life first appears (?) and remains viable Hadean – meteorite bombardment, life started and restarted? Chap - Origin of Continental Crust • Main Topics – Earth cooled sufficiently to permit formation of early continental (granitic) material – Isotopic age dates within continents “cluster” suggesting several periods of “orogeny” – Early continents seem to represent “partial melts” of andesitic volcanics or early sediments – Most of the present-day volume of continental material had formed by ~2.5 billion yrs ago Chap - Origin of Continental Crust • Main Topics (cont.) – Archean (3800 – 2500 Bya) rocks characterized by “greenstone” belts and texturally immature sediments (graywackes), largely form oceanic arcs Suggesting plate tectonics may have started? – Proterozoic (2500 – 540 Bya) rocks are texturally and compositionally mature, include chemical sediments (carbonates and evaporites) Stromatolites are present showing life had evolved while evaporites suggest that sea water had also evolved to its present composition Fig 8.1 Atrists conception of what surface of earth looked like during its first 500 million years Surface was largely molten, with a few of the original microcontinents beginning to form Intense meteorite bombardment heated surface to melting Moon was twice as close, exerting a very strong gravitational pull Early atmosphere had no O2, but probably consisted of N2, CH4, NH3, CO2 and H2O Note no oceans Evidence of Crustal Development from Igneous and Metamorphic Rocks • Importance of Granite • Rock-types surviving from early Cryptozic are mainly granitic in composition and they are arrangemed in highly deformed orogenic belts • This has led to hypothesis of continential accretion of early granitic masses into protocontinents and then continents Evidence of Crustal Development from Igneous and Metamorphic Rocks • However field evidence suggests that granitic continental crust was not original and must have increased in volume through time • Original crust was thin and mainly basalt Weathering, erosion and igneous activity converted some of the original crust to granite to form embryonic continents • Embryonic continents persisted on surface of earth and accreted slowly to form larger continents Modern habitat of ooids • Jolter’s Cay in Bahamas (Island in center of picture) Modern ooids form in the warm, shallow waters in the lee of the island Fig 8.29 SEM photographic of chert showing the sponge spicules that make up the bulk of the rock Magnification 160x Fig 8.23 Fig 8.24 Continental growth by accretion of small plates (“strange terrains”) Note the “suture” zone between the two colliding granitic masses The following slides of E Africa show a modern “aulacogen” in the process of developing Fig 8.26 Another product of a failed rift, the midcontinent gravity high thought to be a result of a failed arm back in the Keweenawan (1Bya) The floor of the high is largely dense basalts that poured out of the upper mantle before the arm failed, again similar to what is happening in E Africa today Fig 8.33 Global distribution of late Proterozoic (Varangian) glacial deposits (triangles) showing their occurrence in equatorial regions The glacial deposits are interbedded with limestones which further suggest a low latitude origin The Earth may have narrowly escaped freezing over completely in the Varangian Fig 8.31 Mud cracks in red shales in the Chuar Group of the Grand Canyon 1.8 Bya Rocks like these indicate hot, dry conditions (mudcracks) while the red color indicates that there was not enough oxygen in the atmosphere to turn the rocks rusty red Fig 8.32 Laminated mudstone with scattered pebbles and sand grains dropped from above Gowganda Formation, Blind River Ontario This textures suggests the stones dropped from a drifting iceberg Fig 8.34a Fig 8.34b Fig 8.5 Pillow basalts in Archean “greenstones” 15 km west of Marquette, MI “Protusions” on lower side of several of the pillows indicate (point to) bottom Fig 8.21 Fig 8.25 Fig 8.27 Fig 8.4 Early field geologists working on Lake Mistassini, Quebec, 1885 [...]... onto a core of older, mostly granitic, rock • Thus the modern continents have a history of growth by addition of smaller granitic masses, which persisted through time because of their greater buoyancy Fig 8.3 Map showing locations of all Cryptozoic and early Paleozoic rocks in the world Numbers refer to age in By Fig 8.11 These geologic provinces form the core of the North American craton The older... visible evidence of life, as early as 3.465 billion years ago • These rocks are actually comples colonies of different types of bacteria, each type occuping a special niche in the colony The most important are the photosynthetic cyanobacteria (formerly blue green algae) common pond scum • These amazing life forms are highly adaptable and form the base of the first food chain Oh yes, they also are responsible... responsible for all the oxygen in the air O2 is a waste product of their photosynthesis • Plants later likely simply incorporated a version of cyanobaterial to carry out their photosynthesis Nature rarely reinvents a wheel Fig 8.22 Outcrop of a stromatolite “reef” from 1.6-billion year old Proterozoic carbonate in the Wopmay orogen These reefs were formed by colonies of photosynthetic “blue-green”... accreted, the ocean floors filled with these basalts and graywackes collapsed, forming greenstone belts that also accreted to the growing protocontinent • Thus some of the early seafloor survived destruction (by subduction) and became part of the stable craton Fig 8.12 Evolution of greenstone belts A Basins between protocontinents fill with basalts, B when protocontinents collide, they “collapse” the oceans... by the hypothetical hydrated aluminum and iron oxides as the end member), and silica (silicon dioxide) Sediments and sedimentary rocks have the same ranges of composition Iron-rich laterites and aluminum-rich bauxites are the products of intense weathering Sandstones are primarily composed of indurated sandy sediments, in many cases dominantly quartz Argillaceous rocks are formed by lithification of. .. origin of cross-stratification by migration of ripples Cross-bedding reveals top and bottom as well as current direction Fig 8.19 Comparison of relative sorting of sand grain sizes by different sedimentary processes Sorting can help determine the origin of a sandstone Origin of Life - Stromatolites • A special type of rock exists throughout the geologic record, called stromatolites, which record the. .. a stable craton where it could spend a lot of time (millions (?) of years ) washing around as loose grains on a beach This rock could be 2nd or 3rd cycle from pre-existing sediments as they were buried, consolidated and then uplifted and eroded • • Sediment composition triangle The diagram shows the range of sedimentary rock types represented as mixtures of three components: calcium (plus magnesium)... Greenland Age of rocks in this picture are ~3.8 By High-grade metamorphic rock (gneiss) typical of ancient “shield” regions Fig 8.6 Cross-section from N Shore of L Superior to northern Michigan Numbers refer to relative age (1 = oldest) Development of a Cryptozoic Chronology • Age dating of ancient rocks showed patterns of old rocks bounded by younger rocks in patterns that suggested accretion of younger... clay-rich muds Sediments or sedimentary rocks rarely, if ever, have compositions represented by the white area of the triangle One example of a classification chart for sedimentary rocks • Cherts are the sedimentary rock equivalent of biologically deposited siliceous deposits During the transformation into rock, the amorphous silica, originally deposited by diatoms and radiolarians, is transformed into... Steps in the evolution of a mature sand from initial weathering of a granite Texturally mature sand is mono-minerallic (quartz), well-rounded and of a uniform grain size This indicates a long time spent in transport or washing around on a beach It may also be 2nd or even 3rd cycle Graywacke suggests rapid transport and burial (why?) while arkosic sands suggest longer transport or more intense weathering
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