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Precambrian Earth and Life History


Chapter 8 Precambrian Earth and Life History The Hadean and Archean – PowerPoint PPT presentation

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Title: Precambrian Earth and Life History

Chapter 8
Precambrian Earth and Life HistoryThe Hadean and
Time check
  • The Precambrian lasted for more than 4 billion
  • Such a time span is almost impossible for us
  • If a 24-hour clock represented all 4.6 billion
    years of geologic time
  • the Precambrian would be slightly more than 21
    hours long,
  • It constitutes about 88 of all geologic time

Precambrian Time Span
  • The term Precambrian is informal term referring
    to both time and rocks
  • It includes time from Earths 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
  • The oldest known rocks on Earth are 3.96 billion
    years old

Rocks of the 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
  • Because of this subdivisions of the Precambrian
    have been difficult to establish
  • Two eons for the Precambrian
  • the Archean and Proterozoic

Eons of the Precambrian
  • The onset of the Archean Eon coincides with the
    age of Earths oldest known rocks
  • approximately 4 billion years old
  • lasted until 2.5 billion years ago (the beginning
    of the Proterozoic Eon)
  • The Hadean is an informal designation for the
    time preceding the Archean Eon
  • Precambrian eons have no stratotypes
  • 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

US Geologic Survey Terms
  • Archean and Proterozoic are used in our
    discussions of Precambrian history, but the U.S.
    Geological Survey (USGS) uses different terms
  • 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

The Hadean?
  • Except for meteorites no rocks of Hadean age are
    present on Earth, however we do know some events
    that took place during this period
  • Earth was accreted
  • Differentiation occurred, creating a core and
    mantle and at least some crust

Earth beautiful Earth.
about 4.6 billion years ago
  • Shortly after accretion, Earth was a rapidly
    rotating, hot, barren, waterless planet
  • bombarded by comets and meteorites
  • There were no continents,
  • intense cosmic radiation
  • widespread volcanism

Oldest Rocks
  • Judging from the oldest known rocks on Earth, the
    3.96-billion-year-old Acasta Gneiss in Canada
    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

Hadean Crust
  • 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
  • Only sialic-rich crust, because of its lower
    density, is immune to destruction by subduction

Crustal Evolution
  • A second stage in crustal evolution began as
    Earths 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

Crustal Evolution
  • Several sialic continental nuclei had formed by
    the beginning of Archean time by subduction and
    collisions between island arcs

Dynamic Processes
  • During the Hadean, various dynamic systems
    similar to ones we see today, became operative,
  • not all at the same time nor in their present
  • Once Earth differentiated into core, mantle and
  • internal heat caused interactions among plates
  • they diverged, converged and slid past each other
  • Continents began to grow by accretion along
    convergent plate boundaries

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
  • 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

  • A shield and platform make up a craton
  • a continents 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
  • Platforms consist of buried Precambrian rocks

Shields and adjoining platforms make up cratons
Canadian Shield
  • The craton in North America is the Canadian
  • Occupies most of northeastern Canada, a large
    part of Greenland, parts of the Lake Superior
    region in Minnesota, Wisconsin, Michigan, and the
    Adirondack Mountains of New York
  • Its topography is subdued, with numerous lakes
    and exposed Archean and Proterozoic rocks thinly
    covered in places by Pleistocene glacial deposits

Canadian Shield Rocks
  • Gneiss, a metamorphic rock, Georgian Bay Ontario,

Canadian Shield Rocks
  • Basalt (dark, volcanic) and granite (light,
    plutonic) on the Chippewa River, Ontario

Amalgamated Cratons
  • The Canadian shield and adjacent platform
    consists of numerous units or smaller cratons
    that were welded together along deformation belts
    during the Early Proterozoic
  • Absolute ages and structural trends help
    geologists differentiate among these various
    smaller cratons

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
  • 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

Greenstone Belt Volcanics
  • Abundant pillow lavas in greenstone belts
    indicate that much of the volcanism was under
  • 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 40 silica
  • requires near surface magma temperatures of more
    than 1600C250C
  • hotter than any recent flows
  • During Earths 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

Sedimentary Rocks of Greenstone Belts
  • Sedimentary rocks are found throughout the
    greenstone belts
  • Mostly found 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

Relationship of Greenstone Belts to
Granite-Gneiss Complexes
  • Two adjacent greenstone belts showing synclinal
  • 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
  • the greenstone belts formed in back-arc marginal

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
  • The Sea of Japan is a modern example of a
    back-arc basin

Archean Plate Tectonics
  • 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
  • residual heat from Earths origin
  • more radiogenic heat
  • 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
  • but associations of passive continental margin
  • are widespread in Proterozoic terrains
  • We have little evidence of Archean rocks
  • deposited on broad, passive continental margins
  • 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
  • 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

Southern Superior Craton Evolution
  • Greenstone belts (dark green)
  • Granite-gneiss complexes (light green
  • Geologic map
  • Plate tectonic model for evolution of the
    southern Superior craton
  • North-south cross section

Atmosphere and Hydrosphere
  • Earths early atmosphere and hydrosphere were
    quite different than they are now
  • They also played an important role in the
    development of the biosphere
  • Todays 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 Suns ultraviolet radiation

Present-day Atmosphere
  • 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
  • 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

Earths Very Early Atmosphere
  • Earths 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 Earths gravity is insufficient to retain
  • Also because Earth had no magnetic field until
    its core formed the solar wind would have swept
    away any atmospheric gases

  • Once a core-generated magnetic field protected
    Earth, gases released during volcanism began to
  • Called outgassing
  • Water vapor is the most common volcanic gas today
  • also emitted
  • carbon dioxide
  • sulfur dioxide
  • Hydrogen Sulfide
  • carbon monoxide
  • Hydrogen
  • Chlorine
  • 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

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)
  • Oxidized iron becomes increasingly common in
    Proterozoic rocks

Introduction of Free Oxygen
  • Two processes account for introducing free oxygen
    into the atmosphere,
  • 1. Photochemical dissociation involves
    ultraviolet radiation in the upper atmosphere
  • The radiation breaks up water molecules and
    releases 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 is a metabolic process in which
    carbon dioxide and water combine into organic
    molecules and oxygen is released as a waste
  • CO2 H2O gt 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

Earths Surface Waters
  • Outgassing was responsible for the early
    atmosphere and also for Earths surface water
  • the hydrosphere
  • Some but probably not much of our surface water
    was derived from icy comets
  • At some point during the Hadean, the Earth had
    cooled sufficiently so that the abundant volcanic
    water vapor condensed and began to accumulate in
  • 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?
  • Much of volcanic water vapor today is recycled
    surface water

First Organisms
  • Today, Earths biosphere consists of millions of
    species of bacteria, fungi, protistans, plants,
    and animals,
  • 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
  • The distinction between living and nonliving
    things is not always easy
  • Are viruses living?
  • When in a host cell they behave like living
  • 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
  • it showed signs of living organisms but was not
    truly living
  • In 1924 A.I. Oparin postulated that life
    originated when Earths atmosphere had little or
    no free oxygen
  • Oxygen is damaging to Earths 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
  • 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 Earths 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
  • 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
  • During the late 1950s
  • Stanley Miller synthesized several amino acids by
    circulating gases approximating the early
    atmosphere in a closed glass vessel

  • The molecules of organisms are polymers
  • proteins
  • nucleic acids
  • RNA-ribonucleic acid and DNA-deoxyribonucleic
  • consist of monomers linked together in a specific
  • 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

  • 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
  • are bounded by cell-like membranes
  • grow and divide much as bacteria do

Proteinoid Microspheres
  • Proteinoid microspheres produced in experiments
  • 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

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
  • The amino acids in the soup might have washed
    up onto a beach or perhaps cinder cones where
    they were concentrated by evaporationand
    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 todays cells nucleic
    acids, either RNA or DNA, are necessary for
  • 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

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
  • from the Early Proterozoic Gunflint Iron
    Formation of Ontario, Canada
  • that he proposed represented reefs constructed by
  • 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

  • Present-day stromatolites form and grow as
    sediment grains are trapped on sticky mats of
    photosynthesizing blue-green algae
  • 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
  • The oldest known cyanobacteria were
    photosynthesizing organisms but photosynthesis is
    a complex metabolic process
  • A simpler type of metabolism must have preceded
  • 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
  • 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

Fossil Prokaryotes
  • Photomicrographs from western Australias 3.3- to
    3.5-billion-year-old Warrawoona Group
  • with schematic restoration shown at the right of