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