Chapter 8 Precambrian Earth and Life History—The Hadean and Archean.
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Transcript of Chapter 8 Precambrian Earth and Life History—The Hadean and Archean.
Chapter 8
Precambrian Earth and Life History—The Hadean and
Archean
• The Teton Range – is largely
Archean
– gneiss, schist, and granite
– Younger rocks are also present
– but not visible
Archean Rocks
Grand Teton National Park, Wyoming
• The Precambrian lasted for more than 4 billion years!– Such a 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
Precambrian 4 Billion Years
• 88% of geologic time
Precambrian Time Span
• The term Precambrian is informal – but widely used, 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 – 545 million years ago
• It encompasses – all rocks older than Cambrian-age rocks
• No rocks are known for the first – 640 million years of geologic time– The oldest known rocks on Earth – are 3.96 billion years old
Precambrian
• 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• the few fossils present are of little use in stratigraphy
• Subdivisions of the Precambrian – have been difficult to establish
• Two eons for the Precambrian – are the Archean and Proterozoic
Rocks Difficult to Interpret
• The onset of the Archean Eon coincides – with the age of Earth’s oldest known rocks
• approximately 4 billion years old
– and lasted until 2.5 billion years ago– the beginning of the Proterozoic Eon
• Hadean is an informal designation – for the time preceding the Archean Eon
• Precambrian eons have no stratotypes– unlike the Cambrian Period, for example,
• which is based on the Cambrian System, • a time-stratigraphic unit with a stratotype in Wales
– Precambrian eons are strictly terms denoting time
Eons of the Precambrian
• Archean and Proterozoic are used in our – following discussions of Precambrian history,– but the U.S. Geological Survey (USGS) uses
different terms• Their Precambrian W begins within the Early Archean
– and ends at the end of the Archean
• Precambrian X corresponds to the Early Proterozoic, – 2500 to 1600 million years ago
• Precambrian Y, – from 1600 to 800 million years ago, – overlaps with the Middle and part of the Late Proterozoic
• Precambrian Z is – from 800 million years to the end of the Precambrian, – within the Late Proterozoic
US Geologic Survey Terms
• Although no rocks of Hadean age are present on Earth, – except for meteorite,
• we do know some events that took place then– Earth accreted from planetesimals – and differentiated into a core and mantle
• and at least some crust– Earth was bombarded by meteorites– Volcanic activity was ubiquitous– Atmosphere formed, quite different from today’s– Oceans began to accumulate
What Happened During the Hadean?
• 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
Hot, Barren, Waterless Early Earth
• about 4.6 billion years ago
• Judging from the oldest known rocks on Earth, – the 3.96-billion-year-old Acasta Gneiss in Canada
and other rocks in Montana– some continental crust had evolved by 4 billion
years ago– Sedimentary rocks in Australia contain detrital
zircons (ZrSiO4) dated at 4.2 billion years old– so source rocks at least that old existed
• These rocks indicted that some kind – of Hadean crust was certainly present, – but its distribution is unknown
Oldest Rocks
• Early Hadean crust was probably thin, unstable – and made up of ultramafic rock
• rock 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
Hadean Crust
• This second stage in crustal evolution – began as Earth’s production
– of radiogenic heat decreased
• 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
Second Crustal Evolution Stage
• During the Hadean, various dynamic systems • similar to ones we see today,
– became operative, – but not all at the same time nor in their present forms
• Once Earth differentiated – into core, mantle and crust, – million of years after it formed,– internal heat caused interactions among plates – as they diverged, converged – and slid past each other in transform motion
• Continents began to grow by – accretion along convergent plate boundaries
Dynamic Processes
• 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
Continental Foundations
• 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
Cratons
Distribution of Precambrian Rocks
• Areas of exposed – Precam-
brian rocks – constitute
the shields• Platforms
consist of – buried Pre-
cambrian rocks
– Shields and adjoining platforms make up cratons
• 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
• It’s topography is subdued, – with numerous lakes and exposed Archean – and Proterozoic rocks thinly covered – in places by Pleistocene glacial deposits
Canadian Shield
• Gneiss, a metamorphic rock, Georgian Bay Ontario, Canada
Canadian Shield Rocks
• Basalt (dark, volcanic) and granite (light, plutonic) on the Chippewa River, Ontario
Canadian Shield Rocks
• 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
Amalgamated Cratons
• Archean metamorphic rocks found – in areas of uplift in the Rocky Mtns
Archean Rocks Beyond the Shield
Rocky Mountains, Colorado
• Archean Brahma Schist in the deeply eroded parts of the Grand Canyon, Arizona
Archean Rocks Beyond the Shield
• 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
Archean Rocks
– 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 Belts• An ideal greenstone belt has 3
major rock units
• Abundant pillow lavas in greenstone belts – indicate that much of the volcanism was – under water, probably at or near a spreading ridge
Greenstone Belt Volcanics
• Pyroclastic materials probably erupted – where large
volcanic centers built above sea level
Pillow lavas in Ispheming greenstone at Marquette, Michigan
• The most interesting rocks – in greenstone belts cooled – from ultramafic lava flows
• Ultramafic magma has less than 40% 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 – than it is now
• 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
Ultramafic Lava Flows
• 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
Sedimentary Rocks of Greenstone Belts
• Quartz sandstone and shale, – indicate delta, tidal-flat, – barrier-island and shallow marine
deposition
• Two adjacent greenstone belts showing synclinal structure
Relationship of Greenstone Belts to Granite-Gneiss Complexes
• They are underlain by granite-gneiss complexes
• and intruded by granite
• In North America,– most
greenstone belts
– (dark green) – occur in the
Superior and Slave cratons
– of the Canadian shield
Canadian Greenstone Belts
• Models for the formation of greenstone belts – involve Archean plate movement
• In one model, plates formed volcanic arcs – by subduction
Evolution of Greenstone Belts
– and the greenstone belts formed
– in back-arc marginal basins
• According to this model, – volcanism and sediment deposition – took place as
the basins opened
Evolution of Greenstone Belts
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
• In another model– although not nearly as widely accepted, – greenstone belts formed – over rising mantle plumes in intracontinental rifts
• The plume serves as the source – of the volcanic rocks in the lower and middle units – of the developing greenstone belt– and erosion of the rift margins supplied – the sediment to the upper unit
• An episode of subsidence, deformation, – metamorphism and plutonism followed
Another Model
• Plate tectonic activity has operated – since the Early Proterozoic or earlier
• Most geologists are convinced – that some kind of plate tectonics – 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
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 Plate Tectonics
• The Archean world was markedly different than later
Archean World Differences
– 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
• Certainly several small cratons – existed by the beginning of the Archean – and grew by periodic continental accretion– during the rest of that eon
• They amalgamated into a larger unit – during the Early 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
The Origin of Cratons
Southern Superior Craton Evolution
Geologic map • Greenstone belts (dark green)
• Granite-gneiss complexes (light green
• Plate tectonic model for evolution of the southern Superior craton
• North-south cross section
• Deformation of the southern Superior craton– was part of a more extensive orogenic episode– that formed the Superior and Slave cratons – and some Archean rocks in Wyoming, Montana, – and the Mississippi River Valley
• 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
Canadian Shield
• 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)
• oxygen not combined with other elements • such as in carbon dioxide (CO2)
– water vapor (H2O)– ozone (O3)
• which is common enough in the upper atmosphere • to block most of the Sun’s ultraviolet radiation
Atmosphere and Hydrosphere
• Nonvariable gases
Nitrogen N2 78.08%
Oxygen O2 20.95
Argon Ar 0.93
Neon Ne 0.002
Others 0.001
in percentage by volume
Present-day Atmosphere Composition
• Variable gases
Water vapor H2O 0.1 to 4.0
Carbon dioxide CO2 0.034
Ozone O3 0.0006
Other gases Trace
• Particulates normally trace
• 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
Earth’s Very Early Atmosphere
• 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,
Outgassing
– carbon monoxide, sulfur, hydrogen, chlorine, and nitrogen
• 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
Hadean-Archean 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
Evidence for an Oxygen-Free Atmosphere
• 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
Introduction of Free Oxygen
• 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
Photosynthesis
• Photochemical dissociation and photosynthesis – added free oxygen to the atmosphere
Oxygen Forming Processes
– Once free oxygen was present
– an ozone layer formed
– and blocked incoming ultraviolet radiation
• 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
Earth’s Surface Waters
• 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
Ocean water
• Ratio of radiogenic heat production in the past to the present
Decreasing Heat
– The width of the colored band
– indicates variations in ratios
– from different models
• Heat production 4 billion years ago was 4 to 6 times as great as it is now
– With less heat outgassing decreased
• 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
First Organisms
• 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
What Is Life?
– 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
• 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
How Did Life First Originate?
• 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)
Elements 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
Basic Building Blocks of Life
• Is it plausible that monomers – originated in the manner postulated?– Experimental evidence indicates that it is
Experiment on the Origin of Life
• During the late 1950s– Stanley Miller – synthesized several
amino acids – by circulating gases
approximating – the early atmosphere – in a closed glass
vessel
• This mixture was subjected to an electric spark– to simulate lightning
Experiment on the Origin of Life
• 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
• 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
Polymerization
• 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
Proteinoids
• Proteinoid microspheres produced in experiments
Proteinoid Microspheres
• Proteinoids grow and divide much as bacteria do
• 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
Protobionts
• 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
Monomer and Proteinoid Soup
• 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
Next Critical Step
• 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
RNA World?
• 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
Much Remains to Be Learned
• 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
Azoic (“without life”)
• Charles Walcott (early 1900s) described structures – from the Early Proterozoic Gunflint Iron Formation of
Ontario, Canada – that he proposed represented reefs constructed by
algae
Oldest Know Organisms
• Now called stromatolites, – not until 1954
were they shown
– to be products of organic activity
Present-day stromatolites Shark Bay, Australia
• 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
Stromatolites
• 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
Other Evidence of Early Life
• 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
Earliest Organisms
• 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
Fermentation
• 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
Photosynthesis
• Photomicrographs from western Australia’s– 3.3- to 3.5-billion-year-old Warrawoona Group, – with schematic restoration shown at the right of each
Fossil Prokaryotes
• 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 Mineral Resources
• 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
Archean Sulfide Deposits
• 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
• 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
Chrome and Platinum
• 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
Iron
• 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
Pegmatites
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