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Chapter 8 Precambrian Earth and Life History—The Hadean and Archean Archean Rocks • The Teton Range – is largely Archean – gneiss, schist, and granite – Younger rocks are also present – but not visible Grand Teton National Park, Wyoming Precambrian 4 Billion Years • The Precambrian lasted for more than 4 billion years! – Such a time span is difficult for humans to comprehend Precambrian Time Span • 88% of geologic time Precambrian • The Precambrian includes – time from Earth’s origin 4.6 billion years ago – to the beginning of the Phanerozoic Eon – 545 million years ago • No rocks are known for the first – 640 million years of geologic time Rocks Difficult to Interpret • Precambrian rocks have been • • • • • altered by metamorphism complexly deformed buried deep beneath younger rocks fossils are rare the few fossils present are of little use in stratigraphy • Eon Subdivisions Archean and Proterozoic Eons of the Precambrian • The Archean Eon – Start coincides with the age of Earth’s oldest known rocks • ~ 4 billion years old – lasted until 2.5 billion years ago – the beginning of the Proterozoic Eon • Hadean is an informal designation – for time preceding the Archean Eon What Happened During the Hadean? – Earth accreted from planetesimals – differentiated into a core and mantle • and at least some crust – – – – was bombarded by meteorites ubiquitous volcanism atmosphere formed Ocean waters accumulate Hot, Barren, Waterless Early Earth • about 4.6 billion years ago • Shortly after accretion, Earth was – – – – a rapidly rotating, hot, barren, waterless planet bombarded by comets and meteorites with no continents, intense cosmic radiation and widespread volcanism Oldest Rocks 3.96-billion-year-old Acasta Gneiss in Canada and other rocks in Montana Sedimentary rocks in Australia contain detrital zircons (ZrSiO4) dated at 4.2 billion years old – so continental source rocks at least that old existed during the Hadean Hadean Crust • Early Hadean crust was probably thin, unstable – and made up of ultramafic rock • those with comparatively little silica • This ultramafic crust was disrupted – by upwelling basaltic magma at ridges – and consumed at subduction zones • Hadean continental crust may have formed – – – – – by evolution of sialic material Sialic crust contains considerable silicon, oxygen and aluminum as in present day continental crust Only sialic-rich crust, because of its lower density, is immune to destruction by subduction Second Crustal Evolution Stage • Subduction and partial melting – of earlier-formed basaltic crust – resulted in the origin of andesitic island arcs • Partial melting of lower crustal andesites, – in turn, yielded silica-rich granitic magmas – that were emplaced in the andesitic arcs Second Crustal Evolution Stage • Several sialic continental nuclei – had formed by the beginning of Archean time – by subduction and collisions – between island arcs 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 Cratons • A shield and platform make up a craton, – a continent’s ancient nucleus and its foundations • 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 metamorphism, igneous activity and mountain building • Cratons have experienced little deformation – since the Precambrian 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 Canadian Shield • 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 Canadian Shield Rocks • Gneiss, a metamorphic rock, Georgian Bay Ontario, Canada Canadian Shield Rocks • Basalt (dark, volcanic) and granite (light, plutonic) on the Chippewa River, Ontario Amalgamated Cratons • Actually the Canadian shield and adjacent platform – is made up of numerous units or smaller cratons – that amalgamated along deformation belts – during the Early Proterozoic • Absolute ages and structural trends – help geologists differentiate – among these various smaller cratons • Drilling and geophysical evidence indicate – that Precambrian rocks underlie much – of North America, – in places exposed by deep erosion or uplift Archean Rocks Beyond the Shield Rocky Mountains, Colorado • Archean metamorphic rocks found – in areas of uplift in the Rocky Mtns Archean Rocks Beyond the Shield • Archean Brahma Schist in the deeply eroded parts of the Grand Canyon, Arizona Archean Rocks • The most common Archean Rock associations – are granite-gneiss complexes • The rocks vary from granite to peridotite – to various sedimentary rocks – all of which have been metamorphosed • Greenstone belts are subordinate in quantity – but are important in unraveling Archean tectonism Greenstone Belts • An ideal greenstone belt has 3 major rock units – volcanic rocks are most common – in the lower and middle units – the upper units are mostly sedimentary • The belts typically have synclinal structure – Most were intruded by granitic magma – and cut by thrust faults • Low-grade metamorphism – makes many of the igneous rocks – greenish (chlorite) Greenstone Belt Volcanics • Abundant pillow lavas in greenstone belts – indicate that much of the volcanism was – under water, probably at or near a spreading ridge • Pyroclastic materials probably erupted – where large volcanic centers built above sea level Pillow lavas in Ispheming greenstone at Marquette, Michigan Ultramafic Lava Flows • The most interesting rocks – in greenstone belts cooled – from ultramafic lava flows • Ultramafic magma has less than 45% silica – and requires near surface magma temperatures – of more than 1600°C—250°C hotter – than any recent flows • During Earth’s early history, – radiogenic heating was higher – and the mantle was as much as 300 °C hotter • This allowed ultramafic magma – to reach the surface Ultramafic Lava Flows • As Earth’s production – of radiogenic heat decreased, – the mantle cooled – and ultramafic flows no longer occurred • They are rare in rocks younger – than Archean and none occur now Sedimentary Rocks of Greenstone Belts • Sedimentary rocks are found – throughout the greenstone belts – although they predominate – in the upper unit • Many of these rocks are successions of – graywacke • a sandstone with abundant clay and rock fragments – and argillite • a slightly metamorphosed mudrock Sedimentary Rocks of Greenstone Belts • Small-scale cross-bedding and – graded bedding indicate an origin – as turbidity current deposits • Quartz sandstone and shale, – indicate delta, tidal-flat, – barrier-island and shallow marine deposition Relationship of Greenstone Belts to Granite-Gneiss Complexes • Two adjacent greenstone belts showing synclinal structure • They are underlain by granite-gneiss complexes • and intruded by granite Canadian Greenstone Belts • In North America, – most greenstone belts – (dark green) – occur in the Superior and Slave cratons – of the Canadian shield Evolution of Greenstone Belts • Models for the formation of greenstone belts – involve Archean plate movement • In one model, plates formed volcanic arcs – by subduction – and the greenstone belts formed – in back-arc marginal basins Evolution of Greenstone Belts • According to this model, – volcanism and sediment deposition – took place as the basins opened Evolution of Greenstone Belts • Then during closure, – the rocks were compressed, deformed, – cut by faults, – and intruded by rising magma • The Sea of Japan – is a modern example – of a back-arc basin Archean Plate Tectonics • Most geologists are convinced – that some kind of plate tectonics – took place during the Archean • BUT, Plates must have moved faster – with more residual heat from Earth’s origin – and more radiogenic heat, – Thus, magma was generated more rapidly Archean Plate Tectonics • As a result of the rapid movement of plates, – – – – continents, no doubt, 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 – were more common – due to the higher temperatures Archean World Differences • The Archean world was markedly different than later – We have little evidence of Archean rocks deposited on broad, passive continental margins – but associations of passive continental margin sediments are widespread in Proterozoic terrains – Deformation belts between colliding cratons indicate that Archean plate tectonics was active – but the ophiolites so typical of younger convergent plate boundaries are rare, – although Late Archean ophiolites are known The Origin of Cratons • Certainly several small cratons – existed by the beginning of the Archean – and grew by periodic continental accretion • 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 Southern Superior Craton Evolution Geologic map • Plate tectonic model for evolution of the southern Superior craton • North-south cross section • Greenstone belts (dark green) • Granite-gneiss complexes (light green Canadian Shield • This deformation was – the last major Late Archean event in North America – and resulted in the formation of several sizable cratons now in the older parts of the Canadian shield Present-day Atmosphere Composition • Variable gases • Nonvariable gases Water vapor H2O Nitrogen N2 78.08% Carbon dioxide CO 2 Oxygen O2 20.95 Ozone O3 Argon Ar 0.93 Other gases Neon Ne 0.002 • Particulates Others 0.001 normally trace in percentage by volume 0.1 to 4.0 0.034 0.0006 Trace 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 • Wthout a magnetic field, – the solar wind would have swept away – any atmospheric gases Outgassing • Once a core-generated magnetic field – protected the gases released during volcanism • called outgassing – they began to accumulate to form a new atmosphere • Water vapor – is the most common volcanic gas today – but volcanoes also emit – carbon dioxide, sulfur dioxide, – carbon monoxide, sulfur, hydrogen, chlorine, and nitrogen Hadean-Archean Atmosphere • Hadean 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 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 Introduction of Free Oxygen • Two processes account for – introducing free oxygen into the atmosphere, • one or both of which began during the Hadean 1. Photochemical dissociation involves 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 organism that practiced photosynthesis Photosynthesis • Photosynthesis is a metabolic process – in which carbon dioxide and water – combine into organic molecules – and oxygen is released as a waste product CO2 + H2O ==> organic compounds + O2 • 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 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 Earth’s Surface Waters • Outgassing was responsible – for the early atmosphere – and also for Earth’s surface water • the hydrosphere – most of which is in the oceans • more than 97% • However, some but probably not much – of our surface water was derived from icy comets • Probably at some time during the Hadean, – the Earth had cooled sufficiently – so that the abundant volcanic water vapor – condensed and began to accumulate in oceans • Oceans were present by Early Archean times Ocean water • The volume and geographic extent – of the Early Archean oceans cannot be determined • Nevertheless, we can envision an early Earth – with considerable volcanism – and a rapid accumulation of surface waters • 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 • Much of volcanic water vapor today – is recycled surface water 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 – With less heat outgassing decreased • Heat production 4 billion years ago was 4 to 6 times as great as it is now First Organisms • Today, Earth’s biosphere consists – of millions of species of bacteria, fungi, – protistans, plants, and animals, – whereas only bacteria are found in Archean rocks • We have fossils from Archean rocks – 3.3 to 3.5 billion years old • Carbon isotope ratios in rocks in Greenland – that are 3.85 billion years old – convince some investigators that life was present then What Is Life? • Minimally, a living organism must reproduce – and practice some kind of metabolism • Reproduction insures – the long-term survival of a group of organisms • whereas metabolism – such as photosynthesis, for instance – insures the short-term survival of an individual • 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 What Is Life? • Comparatively simple organic (carbon based) molecules known as microspheres – form spontaneously – show greater organizational complexity – than inorganic objects such as rocks – 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 How Did Life First Originate? • To originate by natural processes, – life must have passed through a prebiotic stage • in which it showed signs of living organisms • but was not truly living • In 1924, the great Russian biochemist, – A.I. Oparin, postulated that life originated – when Earth’s atmosphere had little or no free oxygen • Oxygen is damaging to Earth’s – most primitive living organisms • Some types of bacteria must live – where free oxygen is not present How Did Life First Originate? • With little or no oxygen in the early atmosphere – and no ozone layer to block ultraviolet radiation, – life could have come into existence from nonliving matter • The origin of life has 2 requirements – a source of appropriate elements for organic molecules – energy sources to promote chemical reactions 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) Basic Building Blocks of Life • Energy from • lightning • and ultraviolet radiation – probably promoted chemical reactions – during which C, H, N and O combined – to form monomers • comparatively simple organic molecules • such as amino acids • Monomers are the basic building blocks – of more complex organic molecules 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 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 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 some of which consist of more than 200 linked amino acids when heating dehydrated concentrated amino acids Proteinoids • The heated dehydrated 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 Proteinoid Microspheres • Proteinoid microspheres produced in experiments • Proteinoids grow and divide much as bacteria do Protobionts • Protobionts are intermediate between – inorganic chemical compounds – and living organisms • Because of their life-like properties – the proteinoid molecules can be referred to – as protobionts Monomer and Proteinoid Soup • The origin-of-life experiments are interesting, – but what is their relationship to early Earth? • Monomers likely formed continuously and by the billions – and accumulated in the early oceans into a “hot, dilute soup” (J.B.S. Haldane, British biochemist) • 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 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, 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 and unsolved problem Much Remains to Be Learned • The origin of life has not been fully solved – – – – but considering the complexity of the problem and the fact that scientists have been experimenting for only about 50 years remarkable progress has been made • Debate continues • 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 Azoic (“without life”) • Prior to the mid-1950s, scientists – had little knowledge of Precambrian life • They assumed that life of the Cambrian – must have had a long early history – but the fossil record offered little – to support this idea • A few enigmatic Precambrian fossils – had been reported but most were dismissed – as inorganic structures of one kind or another • The Precambrian, once called Azoic – (“without life”), seemed devoid of life Oldest Know Organisms • Charles Walcott (early 1900s) described structures – from the Early Proterozoic 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 Present-day stromatolites Shark Bay, Australia Stromatolites • Different types of stromatolites include – irregular mats, columns, and columns linked by mats Stromatolites • Present-day stromatolites form and grow – – – – – as sediment grains are trapped on sticky mats of photosynthesizing blue-green algae (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 Other Evidence of Early Life • Carbon isotopes 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 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 – meaning they lacked a cell nucleus – and lacked other internal cell structures typical of eukaryotic cells (to be discussed later in the term) 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 energy-requiring reactions in cells • ATP can easily be synthesized – from simple gases and phosphate – so it was doubtless available – in the early Earth environment 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 which is used by most living prokaryotic cells 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 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 • These anaerobic, autotrophic prokaryotes – belong to the Kingdom Monera, – represented today by bacteria and cyanobacteria 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 Archean Mineral Resources • A variety of mineral deposits are Archean – but gold is the most notably Archean, – 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 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 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 various 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 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 time, – 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 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 Pegmatites • Pegmatites are very coarsely crystalline igneous rocks, – commonly associated with granite plutons, – composed of quartz and feldspars • Some Archean pegmatites, – such in the Herb Lake district in Manitoba, Canada, – and Zambia in Africa, contain valuable minerals • In addition to minerals of gem quality, – Archean pegmatites contain minerals mined – for lithium, beryllium, rubidium, and cesium Summary • Precambrian encompasses all geologic time – – – – from Earth’s origin to the beginning of the Phanerozoic Eon The term also refers to all rocks that lie stratigraphically below Cambrian rocks • Terms for Precambrian time include – an informal one, the Hadean, – followed by two eons, the Archean and Proterozoic • Some Hadean crust must have existed, – but none of it has been preserved • By the beginning of the Archean Eon, – several small continental nuclei were present Summary • All continents have an ancient stable nucleus – or craton made up of • an exposed shield • and a buried platform • The exposed part of the North American craton – is the Canadian shield, – and is make up of smaller units – delineated by their ages and structural trends • Archean greenstone belts are linear, – syncline-like bodies found within – much more extensive granite-gneiss complexes Summary • Greenstone belts typically consist of – two lower units dominated by igneous rocks – and an upper unit of mostly sedimentary rocks • They probably formed by plate movements – responsible for opening – and then closing back-arc marginal basins • Widespread deformation took place – during the Late Archean – as parts of the Canadian shield evolved Summary • Many geologists are convinced – some type of Archean plate tectonics occurred, – but it probably differed – from the tectonic style of the present • For one thing, Earth had more heat – and for another, plates probably moved faster • The early atmosphere and hydrosphere – formed as a result of outgassing, – but this atmosphere lacked free oxygen and – contained abundant water vapor and carbon dioxide Summary • Models for the origin of life – – – – – by natural processes require an oxygen deficient atmosphere, the appropriate elements for organic molecules, and energy to promote the synthesis of organic molecules • The first naturally formed organic molecules – – – – were probably monomers, such as amino acids, that linked together to form more complex polymers such as proteins Summary • RNA molecules may have been – the first molecules capable of self-replication – However, how a reproductive mechanism evolved is not known • The only known Archean fossils – are of single-celled, prokaryotic bacteria – or blue-green algae (cyanobacteria) • Stromatolites formed by photosynthesizing bacteria – are found in rocks as much as 3.5 billion years old • Carbon isotopes indicate – life was present even earlier Summary • The most important – Archean mineral resources are – gold, chrome, zinc, copper, and nickel