GOL 106 LAB 7 PALEOGEOGRAPHY Group ________ Member Names (1)_________________________________________________ (2)_________________________________________________ (3)_________________________________________________ Page Question(s) 89 1 and 2 90 3, 4, 5, and 6 91 7 92 All 93 4 and 5 Precambrian Earth and Life History Part I (The Archean Eon) 1 Archean Rocks • The Beartooth Mountains on the Wyoming and Montana border consists of Archaean-age gneisses – some of the oldest rocks in the US. 2 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 3 Precambrian 4 Precambrian • The term Precambrian is informal but widely used when 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 below the Cambrian system • No rocks are known for the first 600 million years of geologic time – The oldest known rocks on Earth are 4.0 billion years old 5 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 not useful in biostratigraphy • Subdivisions of the Precambrian have been difficult to establish • Two eons for the Precambrian are the Archean and Proterozoic which are based on absolute ages 6 Eons of the Precambrian • Eoarchean refers to all time from Earth’s origin to the Paleoarchean, 3.6 billion years ago • Earth’s oldest body of rocks, the Acasta Gneiss in Canada is about 4.0 billion years old • We have no geologic record for much of the Archaen • Precambrian eons have no stratotypes – unlike the Cambrian Period, for example 7 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 core and mantle • and at least some crust was present – – – – Earth was bombarded by meteorites Volcanic activity was widespread An atmosphere formed, quite different from today’s Oceans began to accumulate 8 Hot, Barren, Waterless Early Earth • about 4.6 billion years ago • Shortly after accretion, Earth was – – – – http://www.youtube.com/watch?v=QDqskltCixA a rapidly rotating, hot, barren, waterless planet bombarded by meteorites and comets with no continents, intense cosmic radiation and widespread volcanism 9 Oldest Rocks • Continental crust was present by 4.0 billion years ago – Sedimentary rocks in Australia contain detrital zircons (ZrSiO4) dated at 4.4 billion years old – so source rocks at least that old existed • The Eoarchean Earth probably rotated in as little as 10 hours – and the Earth was closer to the Moon • By 4.4 billion years ago, the Earth cooled sufficiently for surface waters to accumulate 10 Eoarchean Crust • Early crust formed as upwelling mantle currents of mafic magma, and numerous subduction zones developed to form the first island arcs • 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 11 Origin of Continental Crust • Andesitic island arcs – form by subduction – and partial melting of oceanic crust • The island arc collides with another 12 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 13 Cratons • A shield and its 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 igneous activity, metamorphism, and mountain building • Cratons have experienced little deformation since the Precambrian 14 Distribution of Precambrian Rocks • Areas of exposed – Precambrian rocks – constitute the shields • Platforms consist of – buried Precambrian rocks – Shields and adjoining platforms make up cratons 15 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 16 Evolution of North America • North America evolved by the amalgamation of Archean cratons that served as a nucleus around which younger continental crust was added. 17 Archean Rocks • Only 22% of Earth’s exposed Precambrian crust is Archean • The most common Archean rock associations are granitegneiss complexes • Other rocks range from peridotite to various sedimentary rocks – all of which have been metamorphosed • Greenstone belts are subordinate in quantity, – account for only 10% of Archean rocks – but are important in unraveling Archean tectonic events • Outcrop of Archean gneiss cut by a granite dike from a granitegneiss complex in Ontario, Canada 18 Archean Rocks • Shell Creek in the Bighorn Mountains of Wyoming has cut a gorge into this 2.9 billion year old granite 19 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 20 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, – komatites, were more common 21 The Origin of Cratons • Certainly several small cratons existed during the Archean and grew by accretion along their margins • 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 22 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 23 Present-day Atmosphere Composition • Nonvariable gases Nitrogen N2 78.08% Oxygen O2 20.95 Argon Ar 0.93 Neon Ne 0.002 Others 0.001 • Variable gases Water vapor H2O Carbon dioxide CO2 Ozone O3 Other gases 0.1 to 4.0 0.038 0.000006 Trace • Particulates normally trace in percentage by volume 24 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 25 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 26 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 27 Archean 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 28 Introduction of Free Oxygen • Two processes account for introducing free oxygen into the atmosphere, • one or both of which began during the Eoarchean. 1. Photochemical dissociation involving ultraviolet radiation in the upper atmosphere • The radiation disrupts water molecules and releases their oxygen and hydrogen • This could account for 2% of present-day oxygen • but with 2% oxygen, ozone forms, creating a barrier against ultraviolet radiation 2. More important were the activities of organisms that practiced photosynthesis 29 Photosynthesis • Photosynthesis is a metabolic process – in which carbon dioxide and water are used in making organic molecules – and oxygen is released as a waste product 6CO2 + 6H2O ==> C6H12O6 + 6O2 • Even with photochemical dissociation and photosynthesis, – probably no more than 1% of the free oxygen level of today was present by the end of the Archean 30 Oxygen Forming Processes • Photochemical dissociation and photosynthesis added free oxygen to the atmosphere – Once free oxygen was present, an ozone layer formed – and blocked incoming ultraviolet radiation 31 Earth’s Surface Waters • Outgassing was responsible for the early atmosphere and also for some of Earth’s surface water • the hydrosphere, most of which is in the oceans > 97% • Another source of our surface water was meteorites and icy comets • Numerous erupting volcanoes, and an early episode of intense meteorite and comet bombardment accounted for rapid rate of surface water accumulation 32 Ocean Water • Volcanoes still erupt and release water vapor – Is the volume of ocean water still increasing? – Perhaps it is, but if so, the rate has decreased considerably because the amount of heat needed to generate magma has diminished 33 Decreasing Heat • Ratio of radiogenic heat production in the past to the present – The width of the colored band indicates variations in ratios from different models • Heat production 4 billion years ago was 3 to 6 times as great as it is now • With less heat outgassing decreased 34 First Organisms • Today, Earth’s biosphere consists of millions of species of Archea, Bacteria, Fungi, Protists, Plants, and Animals, – whereas only bacteria and archea are found in Archean rocks • We have fossils from Archean rocks – 3.5 billion years old • Chemical evidence in rocks in Greenland that are 3.8 billion years old convince some investigators that organisms were present then 35 What Is Life? • Minimally, a living organism must reproduce – and practice some kind of metabolism • Reproduction ensures the long-term survival of a group of organisms • whereas metabolism maintains the organism • The distinction between living and nonliving things is not always easy • Are viruses living? – When in a host cell they behave like living organisms – but outside, they neither reproduce nor metabolize 36 What Is Life? • Comparatively simple organic (carbon based) molecules known as microspheres – form spontaneously – can even grow and divide in a somewhat organism-like fashion – but their processes are more like random chemical reactions, so they are not living 37 How Did Life First Originate? • To originate by natural processes, from non-living matter (abiogenesis), life must have passed through a prebiotic stages – in which it showed signs of living – but was not truly living • The origin of life has 2 requirements – a source of appropriate elements for organic molecules – energy sources to promote chemical reactions 38 Elements of Life • All organisms are composed mostly of – – – – carbon (C) hydrogen (H) nitrogen (N) oxygen (O) • all of which were present in Earth’s early atmosphere as – – – – – carbon dioxide (CO2) water vapor (H2O) nitrogen (N2) and possibly methane (CH4) and ammonia (NH3) 39 Basic Building Blocks of Life • Energy from • Lightning, volcanism, • and ultraviolet radiation – probably promoted chemical reactions during which C, H, N, and O combined – to form monomers • such as amino acids • Monomers are the basic building blocks of more complex organic molecules 40 Experiment on the Origin of Life • Is it plausible that monomers originated in the manner postulated? – Experimental evidence indicates that it is • During the late 1950s – Stanley Miller synthesized several amino acids – by circulating gases approximating the early atmosphere – in a closed glass vessel 41 Experiment on the Origin of Life • This mixture was subjected to an electric spark – to simulate lightning • In a few days – it became cloudy • Analysis showed that – several amino acids typical of organisms had formed • Since then, – scientists have synthesized all 20 amino acids found in organisms 42 Polymerization • The molecules of organisms are polymers – such as proteins – and nucleic acids • RNA (ribonucleic acid) and DNA (deoxyribonucleic acid) consisting of monomers linked together in a specific sequence • How did polymerization take place? • Water usually causes depolymerization, – however, researchers synthesized molecules known as proteinoids or thermal proteins – some of which consist of more than 200 linked amino acids – when heating dehydrated concentrated amino acids 43 Proteinoids • These concentrated amino acids – spontaneously polymerized to form proteinoids • Perhaps similar conditions for polymerization existed on early Earth, – but the proteinoids needed to be protected by an outer membrane or they would break down • Experiments show that proteinoids spontaneously aggregate into microspheres – which are bounded by cell-like membranes and grow and divide much as bacteria do 44 Proteinoid Microspheres • Proteinoid microspheres produced in experiments • Proteinoids grow and divide much as 45 bacteria do Protobionts • These proteinoid molecules can be referred to as protobionts – These are intermediates between inorganic chemical compounds and living organisms 46 Monomer and Proteinoid Soup • The origin-of-life experiments are interesting, – but what is their relationship to early Earth? • Monomers likely formed continuously and in billions – They accumulated in the early oceans into a “hot, dilute soup” – The amino acids in the “soup” might have washed up onto a beach or perhaps cinder cones – where they were concentrated by evaporation and polymerized by heat • The polymers then washed back into the ocean – where they reacted further 47 QUESTIONS? 48 Precambrian Earth and Life History Part II (The Archean Eon) 49 Next Critical Step • Not much is known about the next critical step in the origin of life • the development of a reproductive mechanism • The microspheres divide and may represent a protoliving system – but in today’s cells, nucleic acids, • either RNA or DNA – are necessary for reproduction • The problem is that nucleic acids cannot replicate without protein enzymes, – and the appropriate enzymes cannot be made without nucleic acids, 50 – or so it seemed until fairly recently RNA World? • Now we know that small RNA molecules – can replicate without the aid of protein enzymes • Thus, the first replicating systems – may have been RNA molecules • Some researchers propose – an early “RNA world” in which these molecules were intermediate between • inorganic chemical compounds • and the DNA-based molecules of organisms • How RNA was naturally synthesized – remains an unsolved problem 51 Much Remains to Be Learned • Scientists agree on some basic requirements for the origin of life, – but the exact steps involved and significance of results are debated • Many researchers believe that – the earliest organic molecules were synthesized from atmospheric gases – but some scientist suggest that life arose instead near hydrothermal vents on the seafloor 52 Submarine Hydrothermal Vents • Seawater seeps into the crust near spreading ridges, becomes heated, rises and discharges • Black smokers – Discharge water saturated with dissolved minerals – Life may have formed near these in the past 53 Submarine Hydrothermal Vents • Several minerals containing zinc, copper, and iron precipitate around them • Communities of organisms – previously unknown to science, are supported here. – Necessary elements, sulfur, and phosphorus are present in seawater – Polymerization can take place on surface of clay minerals – Protocells were deposited on the ocean floor 54 Oldest Known Organisms • The first organisms were archaea and bacteria – both of which consist of prokaryotic cells, – cells that lack an internal, membrane-bounded nucleus and other structures • Prior to the 1950s, scientists assumed that life – must have had a long early history – but the fossil record offered little to support this idea • The Precambrian, once called Azoic – (“without life”), seemed devoid of life 55 Oldest Know Organisms • Charles Walcott (early 1900s) described structures from the Paleoproterozoic Gunflint Iron Formation of Ontario, Canada – that he proposed represented reefs constructed by algae • Now called stromatolites, – not until 1954 were they shown to be products of organic activity 56 Present-day stromatolites (Shark Bay, Australia) Stromatolites • Different types of stromatolites include – irregular mats, columns, and columns linked by mats 57 Stromatolites • Present-day stromatolites form and grow as sediment grains are trapped on sticky mats of photosynthesizing cyanobacteria – although now they are restricted to environments where snails cannot live • The oldest known undisputed stromatolites are found in rocks in South Africa – that are 3.0 billion years old • But probable ones are also known from the Warrawoona Group in Australia – which is 3.3 to 3.5 billion years old 58 Other Evidence of Early Life • Chemical evidence in rocks 3.85 billion years old in Greenland indicate life was perhaps present then • The oldest known cyanobacteria were photosynthesizing organisms – but photosynthesis is a complex metabolic process • A simpler type of metabolism must have preceded it • No fossils are known of these earliest organisms 59 Earliest Organisms • The earliest organisms must have resembled – tiny anaerobic bacteria – meaning they required no oxygen • They must have totally depended on an external source of nutrients – that is, they were heterotrophic, as opposed to autotrophic organisms • that make their own nutrients, as in photosynthesis • They all had prokaryotic cells 60 Earliest Organisms • The earliest organisms, then, were anaerobic, heterotrophic prokaryotes • Their nutrient source was most likely – adenosine triphosphate (ATP) from their environment which was used to drive the energyrequiring reactions in cells • ATP can easily be synthesized from simple gases and phosphate – so it was available in the early Earth environment 61 Fermentation • Obtaining ATP from the surroundings could not have persisted for long – because more and more cells competed for the same resources • The first organisms to develop a more sophisticated metabolism – probably used fermentation to meet their energy needs • Fermentation is an anaerobic process in which molecules such as sugars are split, releasing carbon dioxide, alcohol, and energy 62 Photosynthesis • A very important biological event occurring in the Archean was the development of the autotrophic process of photosynthesis • This may have happened as much as 3.5 billion years ago • These prokaryotic cells were still anaerobic, – but as autotrophs they were no longer dependent on preformed organic molecules as a source of nutrients 63 Fossil Prokaryotes • Photomicrographs from western Australia’s – 3.3- to 3.5-billion-year-old Warrawoona Group, – with schematic restoration shown at the right of each 64 Archean Mineral Resources • A variety of mineral deposits are of Archean-age – but gold is the most commonly associated, although it is also found in Proterozoic and Phanerozoic rocks • This soft yellow metal is prized for jewelry, – but it is or has been used as a monetary standard, in glass making, electric circuitry, and chemical industry • About half the world’s gold since 1886 has come from Archean and Proterozoic rocks in South Africa • Gold mines also exist in Archean rocks of the Superior craton in Canada 65 Archean Sulfide Deposits • Archean sulfide deposits of • zinc, • copper • and nickel – occur in Australia, Zimbabwe, and in the Abitibi greenstone belt in Ontario, Canada • Some, at least, formed as mineral deposits – next to hydrothermal vents on the seafloor, much as they do now around black smokers 66 Chrome • About 1/4 of Earth’s chrome reserves are in Archean rocks, especially in Zimbabwe • These ore deposits are found in – the volcanic units of greenstone belts – where they appear to have formed when crystals settled and became concentrated in the lower parts of plutons – such as mafic and ultramafic sills • Chrome is needed in the steel industry • The United States has very few chrome deposits – so must import most of what it uses 67 Chrome and Platinum • One chrome deposit in the United States is in the Stillwater Complex in Montana • Low-grade ores were mined there during war times, – but they were simply stockpiled and never refined for chrome • These rocks also contain platinum, – a precious metal, that is used • in the automotive industry in catalytic converters • in the chemical industry • for cancer chemotherapy 68 Iron • Banded Iron formations are sedimentary rocks – consisting of alternating layers of silica (chert) and iron minerals • About 6% of the world’s banded iron formations were deposited during the Archean Eon • Although Archean iron ores are mined in some areas – they are neither as thick nor as extensive as those of the Proterozoic Eon, which constitute the world’s major source of iron 69 Pegmatites • Pegmatites are very coarsely crystalline igneous rocks, commonly associated with granite plutons • Some Archean pegmatites, – such in the Herb Lake district in Manitoba, Canada, – and Rhodesian Province in Africa, – contain valuable minerals • In addition to minerals of gem quality, – Archean pegmatites contain minerals mined for lithium, beryllium, rubidium, and cesium 70 QUESTIONS? 71 Precambrian Earth and Life History (The Proterozoic Eon – Part I) 1 The Length of the Proterozoic • The Proterozoic Eon alone, – at 1.958 billion years long, – accounts for 42.5% of all geologic time – yet we review this long episode of Earth and life history in a single chapter 2 The Phanerozoic • The Phanerozoic, – consisting of • Paleozoic, • Mesozoic, • Cenozoic eras, – lasted a comparatively brief 542 million years 3 Proterozoic Rocks • The Vishnu schist in the Grand Canyon was originally lava flows and sedimentary rocks, but was intruded by the Zoraster Granite 1.7 billion years ago 4 Proterozoic Rocks • The outcrop of sandstone and mudstone 1.0 billion years old has only been slightly altered by metamorphism 5 Archean-Proterozoic Boundary • Geologists have rather arbitrarily placed the ArcheanProterozoic boundary at 2.5 billion years ago because it marks the approximate time of changes in the style of crustal evolution • However, we must emphasize “approximate”, because Archean-type crustal evolution was not completed at the same time in all areas 6 Style of Crustal Evolution • Archean crust-forming processes generated – granite-gneiss complexes and greenstone belts that were shaped into cratons • Although these same rock associations continued to form during the Proterozoic, they did so at a considerably reduced rate 7 Archean vs. Proterozoic • Many Archean rocks have been metamorphosed, • However, vast exposures of Proterozoic rocks are unaltered or nearly so • In many areas, Archean rocks are separated from Proterozoic rocks by an unconformity • Widespread associations of sedimentary rocks of passive continental margins were deposited during the Proterozoic by a plate tectonic style essentially the same as it is now 8 Other Differences • The Proterozoic was also a time in evolution of the atmosphere and biosphere as well as the origin of some important natural resources • Oxygen-dependent organisms evolved during this time • The first multicelled organisms and animals made their appearance. • The fossil record is still poor compared to the 9 Phanerozoic Evolution of Proterozoic Continents • Archean cratons assembled during collisions of island arcs and mini-continents, providing the nuclei around which Proterozoic crust accreted, thereby forming much larger landmasses • Proterozoic accretion – probably took place more rapidly than today because Earth possessed more radiogenic heat, – and the plates moved faster 10 Focus on Laurentia • Our focus here is on the geologic evolution of Laurentia, a large landmass that consisted of what is now • North America, • Greenland, • parts of northwestern Scotland, • and perhaps some of the Baltic shield of Scandinavia 11 Early Proterozoic History of Laurentia • Laurentia underwent important changes between 2.0 and 1.8 billion years ago • During this time, collisions among various plates formed several orogens, which are linear or arcuate deformation belts in which many of the rocks have been • metamorphosed and intruded by magma, thus forming plutons, especially batholiths 12 Proterozoic Evolution of Laurentia • Archean cratons were sutured along these orogens, thereby forming a larger landmass which makes up much of Greenland, central Canada, and the north-central United States 13 Wilson Cycle • Rocks of the Wopmay orogen in northwestern Canada are important because they record the opening and closing of an ocean basin or what is called a Wilson cycle • A complete Wilson cycle, named after the Canadian geologist J. Tuzo Wilson, involves • rifting of a continent, • opening and closing of an ocean basin, • and finally reassembly of the continent 14 Wilson Cycle • Some geologists think that the Wopmay orogen – represents a complete Wilson cycle 15 Accretion along Laurentia’s Southern Margin • Following the initial episode of amalgamation of Archean cratons, accretion took place along Laurentia’s southern margin as it collided with volcanic island arcs and oceanic terranes • From 1.65 to 1.76 billion years ago, the Yavapai and Mazatzal orogens were added to the evolving continent • The rocks have been deformed, altered by metamorphism, intruded by granitic batholiths, and incorporated into Laurentia. 16 Southern Margin Accretion • Laurentia grew along its southern margin – by accretion of the Central Plains, Yavapai, and Mazatzal orogens 17 BIF, Red Beds, Glacier

Do you have a similar assignment and would want someone to complete it for you? Click on the ORDER NOW option to get instant services at essayloop.com. We assure you of a well written and plagiarism free papers delivered within your specified deadline.