Chapter 20: The Earth Through Time

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					Chapter 20: The Earth Through Time
Introduction: Tracking Past Plate
Motions (1)

 The evidence in support of plate motions comes
  from measurements made with the Global
  Positioning System (GPS).
 However, evidence of past plate motions is not
  obtained through GPS.
Introduction: Tracking Past Plate
Motions (2)

   Evidence includes:
       Great arc-shaped belts of metamorphic rocks formed
        by continental collisions.
       The eroded remains of island-arc volcanic rocks.
       Traces of Earth’s past magnetic field preserved in old
        lava flows.
   Plates have been moving and changing Earth’s
    surface for at least 2 billion years.
Former Ideas About Continents (1)

 In the sixteenth century, it became apparent that
  the coasts were approximately parallel.
 During the nineteenth century, the favored idea
  was that Earth was originally a molten mass that
  is cooling and contracting, with the crust being
  gradually compressed.
Former Ideas About Continents (2)

   Scientists discovered at the beginning of the
    twentieth century that the Earth’s interior is
    kept hot by radioactive decay.
       The Earth might not be cooling but heating up (and
        therefore expanding).
       Heating would cause the Earth to expand and the
        continental crust would then crack into fragments.
The New Idea: Plate Tectonics (1)

 By the middle of the twentieth century, a totally
  new approach was developed: plate tectonics.
 When great slabs of lithosphere, called plates,
  slide sideways across the asthenosphere:
       Some parts of the slabs can be in compression.
       Others can be in tension (that is, being pulled apart).
       When a plate split in two, the broken edges of
        continental crust match perfectly.
The New Idea: Plate Tectonics (2)

 The energy needed to move plates comes from
  the Earth’s internal heat energy, which causes
  great convective flows in the mantle.
 Plate tectonics is the only theory ever developed
  that explains all of the Earth’s major features.
Pangaea (1)

 Alfred Wegener’s theory of continental drift
  originated when he attempted to explain the
  match of the shorelines on the two sides of the
  Atlantic, especially along Africa and South
  America.
 He hypothesized an ancient land mass called
  Pangaea.
 The northern half of Pangaea is called Laurasia,
  the southern half Gondwanaland.
Pangaea (2)

 Laurasia includes Northern America.
 Gondwanaland includes:
       Eurasia.
       India.
       Africa.
       Antarctica.
       Australia.
       South America.
Figure 20.1 A
Figure 20.1B
Pangaea (3)

   During the late Carboniferous Period, about 300
    million years ago, a continental ice sheet covered
    parts of South America, southern Africa, India,
    and southern Australia.
       This is explained by continental drift: 300 million
        years ago, the regions covered by ice lay in high, cold
        latitudes surrounding the south pole, while north
        America and Eurasia were close to the equator
Pangaea (4)

   Many scientists remained unconvinced because
    no one could explain how the solid rock of a
    continent could possibly overcome friction and
    slide across the oceanic crust.
Apparent Polar Wandering (1)

   From the mid-1950s to the mid-1960s,
    geophysicists discovered paleomagnetism.
       Certain igneous and sedimentary rocks can preserve a
        fossil record of the Earth’s magnetic field at the time
        and place the rocks formed.
Apparent Polar Wandering (2)

   Three essential bits of information are contained
    in that fossil magnetic record:
       The Earth’s polarity (the magnetic field was normal
        or reversed at the time of rock’s formation).
       The location of the magnetic poles at the time the rock
        formed.
       The magnetic inclination (indicating how far from the
        point of rock formation the magnetic poles lay).
Figure 20.2
Apparent Polar Wandering (3)

 The paleomagnetic inclination is a record of the
  place between the pole and the equator (that is,
  the magnetic latitude) where the rock was
  formed.
 In the 1950s, geophysicists determined that the
  strange plots of paleopole positions indicated
  apparent polar wandering.
Figure 20.3
Seafloor Spreading (1)

 In 1962, Harry Hess of Princeton University
  hypothesized that the topography of the seafloor
  could be explained if the seafloor moves
  sideways, away from the oceanic ridges.
 His hypothesis came to be called the theory of
  “seafloor spreading,” and was soon proven to be
  correct.
Seafloor Spreading (2)

 Hess postulated that magma rose from Earth’s
  interior and formed new oceanic crust along the
  midocean ridges.
 Three geophysicists (Frederick Vine, Drummond
  Matthews, and Lawrence Morley) proposed that
  lava extruded at any midocean ridge becomes
  magnetized and acquires the magnetic polarity
  that exists at the time the lava cools.
Seafloor Spreading (3)

 The oceanic crust contains a continuous record
  of the Earth’s changing magnetic polarity.
 Successive strips of oceanic crust are magnetized
  with normal and reversed polarity.
 The magnetic striping allows the age of any place
  on the seafloor to be determined.
 Magnetic striping also provides a means of
  estimating the speed with which the seafloor had
  moved.
Figure 20.4
Magnetic Record And Plate Velocities
(1)

 The most recent magnetic reversal occurred
  730,000 years ago.
 The oldest reversals so far found date back to the
  middle Jurassic, about 175 million years ago.
 From the symmetrical spacing of magnetic time
  lines it appears that both plates move away from
  a spreading center at equal rates.
Figure 20.5
Magnetic Record And Plate Velocities
(2)

 All that can be deduced from magnetic time lines
  is the relative velocity of two plates.
 Absolute velocities requires information from
  GPS measurements (for present-day plate
  motions), or hot spot tracks, or past plate
  motions;
       Plates with only oceanic lithosphere tend to have high
        relative velocities (Pacific and Nazca plates).
Magnetic Record And Plate Velocities
(3)

     Plates with a great deal of thick continental
      lithosphere, such as the African, North American, and
      Eurasian plates, have low relative velocities.
     Plate velocities vary with the geometry of motion of a
      sphere.
          Plates of lithosphere are pieces of a shell on a spherical
           Earth.
Magnetic Record And Plate Velocities
(4)

     One consequence of different plates velocities is that
      the width of new oceanic crust bordering a spreading
      center increases with the distance from the rotation
      pole.
     Each transform fault lies on a line analogous to a line
      of latitude around the rotation pole.
Figure20.6
Figure 20.7
Figure 20.8
Relict Plate Boundaries In The
Geologic Record (1)

 Seafloor magnetic strips help us reconstruct
  plate motion only as far back in time as the
  Jurassic, some 175 million years ago.
 All large expanses of older oceanic crust have
  been subducted back into the mantle at
  convergent plate boundaries.
Relict Plate Boundaries In The
Geologic Record (2)

 The paleomagnetism of continental rock can be
  used to follow plate motion further back in time,
  but without the breadth and continuity of
  seafloor data.
 Today’s continents were assembled from many
  distinct plates or plate fragments.
 Small fragments of continental crust that have
  drifted as a single unit in the Earth history are
  called terranes.
Ophiolites (1)

 The igneous rock formed at a spreading center,
  known as midocean ridge basalt (MORB), has a
  distinctive chemistry;
 When MORB is found on land, it usually lies
  within a body of rock that appears to be a
  fragment of oceanic crust caught up in a
  continental collision.
Ophiolites (2)

 The minerals that characterize basalt, if buried
  deep within the collision zone, transform into a
  assemblage dominated by a distinctive green
  fibrous mineral called serpentine.
 These serpentine-dominated fragments of
  oceanic crust found on continents are called
  ophiolites, from the Greek word for serpent,
  ophis.
Ophiolites (3)

   Their structure matches well the crustal
    structure expected at a midocean ridge:
       At the top is a thin veneer of sediment that was
        deposited on the ocean floor.
       Beneath the sediment is a layer of pillowed basalt.
       Still deeper are sills of gabbro, the plutonic equivalent
        of basalt.
Ophiolites (4)

     Many ophiolites also contain the apparent source
      rocks of the basalts and gabbros.
     Beneath the gabbro sills there is often a layer of
      peridotite.
     The contact between gabbro and peridotite is
      interpreted to be the former Moho at the base of what
      was formerly oceanic crust.
Subduction Mélange And Blueschists
(1)

 Along convergent margins, a distinctive feature
  is the development of a mélange: a chaotic
  mixture of broken, jumbled, and thrust-faulted
  rock.
 A sinking plate drags the sedimentary rock
  formed from accumulated sediment downward
  beneath the overriding plate.
Subduction Mélange And Blueschists
(2)

 Caught between the overriding plate and the
  sinking plate, the sediment becomes shattered,
  crushed, sheared, and thrust-faulted, forming a
  mélange.
 As the mélange thickens, it undergoes
  metamorphism, common in many mélange
  zones, to form low-temperature metamorphism
  blueschists and eclogites.
Figure 20.10
Back-arc Basins And Plate Extension
(1)

 When the sinking of a subducting plate is faster
  than the forward motion of the overriding plate,
  the margin of the overriding plate can be
  subjected to tensional (pulling) stress.
 If the overriding plate is oceanic or if the
  extension of a continental margin has progressed
  to an extreme state, an arc-shaped basin forms
  behind and parallel to the magmatic arc of the
  subduction zone.
Figure 20.11
Figure 20.12
Back-arc Basins And Plate Extension
(2)

   Basaltic magma may rise into such a back-arc
    basin at a newly formed spreading center, and
    new oceanic crust may form.
Supercontinents And Vanished Oceans
(1)

 The average composition of continental
  lithosphere is quartz-rich, compared to olivine
  and pyroxene-rich oceanic lithosphere.
 Continental lithosphere is always less dense than
  oceanic lithosphere and is not subducted.
 Seismic evidence suggests that a root of mantle
  rock is attached to the base of old, cool
  continental crust.
Supercontinents And Vanished Oceans
(2)

 Continental buoyancy can be enhanced by the
  detachment and loss of this dense mantle root
  during the final stages of a continental collision.
 When plate movement bunches continental
  fragments together, the heat of the mantle
  beneath still must escape Earth’s interior.
       A supercontinent impedes heat flow from the deep
        mantle to the surface, effectively forming a layer of
        insulation.
Supercontinents And Vanished Oceans
(3)

   Convective motion within Earth’s mantle:
       Will accumulate heat and cause thermal buoyancy at
        the base of the continental lithosphere.
       Warms and softens the lithosphere, which begins to
        rift.
          The opening of the Atlantic ocean was heralded by the
           eruption of large basalt flows;
          In some cases, these eruptions persisted and became hot spots
           (example: the Parana flood basalt in Brazil).
Supercontinents And Vanished Oceans
(4)

   Two supercontinents existed in the past:
       Pangaea, formed roughly 350 million years ago,
       Rodinia, an earlier Proterozoic continent formed
        roughly 1100 million years ago and split apart
        roughly 750 million years ago.
   The sutures between continental fragments can
    be found in the many mountain ranges and belts
    of ophiolites that formed in the late Paleozoic
    period, roughly between 450 and 350 million
    years ago.
Supercontinents And Vanished Oceans
(5)

   Examples include:
       The Appalachian mountains in eastern North
        America.
       The Atlas mountains in Morocco.
       The Ural mountains in Russia.
       The Hercynian mountains in Europe.
   Each of these mountain ranges is heavily eroded
    now.
Supercontinents And Vanished Oceans
(6)

   As Pangaea formed, sediment sequences on
    continental shelves around the world record
    evidence that global sea level fell, draining
    shallow continental seas and exposing large areas
    of continental shelves.
Supercontinents And Vanished Oceans
(7)

   Geologists interpret the sea level drop as
    evidence for:
       A general slowdown in plate movement.
       A slowdown in the rate of formation of young,
        buoyant oceanic lithosphere at midocean ridges.
   The oceanic lithosphere at the time of Pangaea
    was, on average, older, colder, and denser,
    leading to a deeper world ocean.
Supercontinents And Vanished Oceans
(8)

 Pangaea lasted as a supercontinent for at least
  150 million years.
 New oceans formed when Pangaea separated:
       The Atlantic.
       Southern Oceans.
Figure 20.13
Ice Ages in Earth History (1)
   Large continental ice sheets have existed several
    times during Earth’s history:
       In the Carboniferous Period, 286 to 360 million years
        ago.
       In the Ordovician and Silurian periods, roughly 450
        to 425 million years ago.
   Both of these glacial periods occurred at times
    when large continental landmasses lays over
    Earth’s south pole, providing a platform for the
    accumulation of vast quantities of snow and ice
    at high latitudes.
Figure 20.14 A
Figure 20.14 B
Figure 20.14 C
Ice Ages in Earth History (2)

   Factors leading to an ice sheet:
       The amount of carbon dioxide and other greenhouse
        gases in the atmosphere.
       The tilt of the Earth’s axis.
       Changes in the Earth’s orbit around the sun.
       The height of continents and their positions in relation
        to the north and south poles.
Phanerozoic Glaciations (1)

   Three major periods of high-latitude glaciation
    have been recognized in the Phanerozoic:
       Pleistocene (0 to 5 million years ago).
       Carboniferous (roughly 300 million years ago).
       Ordovician (roughly 455 million years ago).
   Rapid carbon burial and reduced volcanic
    activity lower concentrations of CO2 in the
    atmosphere, reducing the greenhouse effect and
    encouraging glaciation.
Phanerozoic Glaciations (2)

 Carboniferous coal deposits occur within
  repetitive layered sedimentary sequences of
  marine and nonmarine sediments called
  cyclothems.
 Cyclothems indicate repeated transgressions by
  the sea.
       Timing connected to the longer Milankovitch orbital
        cycles of 100,000 years.
       glacial and interglacial climate may have alternated in
        a regular pattern.
Phanerozoic Glaciations (3)

   New continental glaciers:
       Would have stolen water from the oceans, causing sea
        level to drop.
       Drained Coastal coal swamps as the sea level fell.
       Decreased carbon consumption.
   Atmospheric CO2 levels could return to normal
    levels.
       An increase in greenhouse warming would melt the
        glaciers and raise the sea level, rejuvenating the coal
        swamps and repeating the glacial cycle.
Phanerozoic Glaciations (4)

   There is evidence for continental glaciation near
    the end of the Proterozoic Eon, between 600 and
    800 million years ago.
       The extensive low-latitude glaciation occurred during
        a climatic extreme in which the whole Earth must
        have experienced glaciation.
   Extensive low-latitude glaciation did not occur at
    any time during the Phanerozoic.
Regional Structures Of Continents (1)

   Because the most ancient oceanic crust known to
    exist in the ocean dates only from the mid-
    Jurassic Period, the only direct evidence
    concerning geologic events more ancient than the
    mid-Jurassic comes from the continental crust.
Regional Structures Of Continents (2)

   Two kinds of structural units can be
    distinguished within the continental crust:
       The craton is a core of very ancient rock that has
        attained tectonic and isostatic stability.
       Orogens are elongate regions of crust that have been
        intensely folded and faulted during continental
        collision.
          Only the youngest orogens are mountainous today.
          Ancient orogens reveal their history through the kinds of
           rock they contain and the kind of deformation present.
Figure 20.15
Regional Structures Of Continents (3)

 An assemblage of cratons and ancient orogens
  that has reached isostatic equilibrium is called a
  continental shield.
 Because it is a stable platform, a continental
  shield is covered by a thin layer of little-
  deformed sediments.
Regional Structures Of Continents (4)

   North America has a huge continental shield at
    its core, and around the shield are four younger
    orogens:
       The Caledonide.
       Appalachian.
       Cordilleran.
       Innuitian.
       Each is younger than 600 million years.
Regional Structures Of Continents (5)

   Because the North American shield crops out in
    Canada, and is mostly covered by flat-lying
    sedimentary rocks in the United States,
    geologists often refer to it as the Canadian
    Shield.
Regional Structures Of Continents (6)

   Geologists have identified several ancient cratons
    and orogens in the Canadian Shield;
       Within the cratons, all rocks are older than 2.0 billion
        years.
       The small cratons within the Canadian Shield were
        probably minicontinents during the Archean Eon and
        early part of the Proterozoic Eon.
       By about 1.6 billion years ago, these minicontinents
        had become welded together.
Regional Structures Of Continents (7)

     Each time two cratonic fragments collided, an orogen
      was formed between them.
     The existence of ancient collision belts is the best
      evidence that plate tectonics operated at least as far
      back as 2 billion years ago.
Regional Structures Of Continents (8)

   The fragmentation, drift, and welding together
    of pieces of continental are responsible for the
    five types of continental margins we know of
    today:
       Passive.
       Convergent.
       Collision.
       Transform fault.
       Accreted terrane.
Passive Continental Margins (1)

   A passive continental margin occurs in the stable
    interior of a plate.
       The eastern coast of North America, for example, is in
        the stable interior of the North American Plate.
    Passive continental margins develop when a new
    ocean basin forms by the rifting of continental
    crust (for example, the Red Sea).
Figure 20.16
Figure 20.17
Passive Continental Margins (2)

 Passive continental margins are places where a
  great thickness of sediment accumulates.
 This accumulation apparently occurs in the
  following manner:
       Basaltic magma, associated with formation of the new
        spreading edge, splits the continent.
       A plateau forms as the lithosphere expands, with an
        elevation of as much as 2.5 km above sea level.
Passive Continental Margins (3)

     Tensional forces cause normal faults and form a rift
      (pronounced topographic relief between the plateau
      and the floor of the rift).
     Before the rift floor sinks low enough for sea water to
      enter, clastic nonmarine sediments shed from the
      steep valley walls accumulate in the rift.
     Basaltic lavas, dikes, and sills form by magma rising
      up the normal faults.
Passive Continental Margins (4)
       As the rift widens, a point is reached where seawater
        enters.
       The high rate of evaporation results in the deposition
        of a strata of evaporate salts laid down on top of the
        clastic nonmarine sediments
   The formation of three-armed rifts with one of
    the arms not developing into an ocean is a
    characteristic feature of passive continental
    margins.
Figure 20.18
Passive Continental Margins (5)

   The Gulf of Aden, the Red Sea, and the northern
    end of the African Rift Valley meet at angles of
    1200.
       Such a meeting point formed by three spreading edges
        is called a plate triple junction.
Continental Convergent Margins (1)

 At a continental convergent margin, the edge of
  a continent coincides with a convergent plate
  margin in which oceanic lithosphere is being
  subducted beneath continental lithosphere.
 Subduction produces intense deformation of a
  continental margin.
Continental Convergent Margins (2)

   The Andean coast of South America is an
    example.
       The Nazca Plate (capped by oceanic crust) is being
        subducted beneath the South American Plate.
       Wet partial melting in the mantle activated by water
        released by the subducted Nazca Plate produced the
        andesitic magma that formed the Andes ( a
        continental volcanic arc).
Continental Convergent Margins (3)

   Sediment subjected to deformation in such a
    setting forms a mélange.
       Sediments subjected to high-pressure and low-
        temperature metamorphism.
       Adjacent and parallel to the belt of mélange, the crust
        beneath the continental volcanic arc is
        metamorphosed, but here the metamorphism is
        regional. One distinctive feature of a continental
        convergent margin, therefore, is a pair of parallel
        metamorphic belts.
Figure 20.19
Continental Collision Margins (1)

   The edges of two continents, each on a different
    plate, come into collision at a continental
    collision margin.
       This results in intense folding and thrust faulting.
            The Himalayan Mountain chain.
   Within a fold-and-thrust mountain system,
    strata are compressed, faulted, folded, and
    crumpled, commonly in an exceedingly complex
    manner.
Figure 20.20
Continental Collision Margins (2)

   Metamorphism and igneous activity are always
    present.
       The Alps, the Himalayas, and the Carpathians are all
        young fold-and thrust mountain systems formed
        during the Mesozoic and Cenozoic eras.
   The Appalachians and the Urals are older,
    Paleozoic-aged, examples.
Fold-And Thrust Mountain System (1)

   These systems develop from piles of sedimentary
    strata commonly 15,000 m or more in thickness.
       Sediments are predominantly marine, accumulated
        along passive continental margins.
   The American geologist J.D. Dana coined the
    term geosyncline to describe a great trough that
    received thick deposits of sediment during slow
    subsidence through long geologic periods.
Fold-And Thrust Mountain System (2)

   One distinctive feature of mountain systems
    formed by collision is that an ocean disappears
    and a new mountain system lies in the interior of
    a major landmass.
Transform Fault Margins

   A transform fault continental margin occurs
    when the margin of a continent coincides with a
    transform fault boundary of a plate.
       The San Andreas Fault.
   The San Andreas Fault apparently arose when
    the westward-moving North American continent
    overrode part of the East Pacific Rise.
       The San Andreas is the transform fault that connects
        the two remaining segments of the old spreading
        center.
Figure 20.21
Accreted Terrane Margins (1)

 An accreted terrane continental margin is a
  former convergent or transform fault margin
  that has been further modified by the addition of
  rafted-in, exotic fragments of crust such as island
  arcs.
 The northwestern margin of North America,
  from central California to Alaska, is an example
  of an accreted terrane margin.
Accreted Terrane Margins (2)

 Eventually, any fragment not consumed by
  subduction is added (accreted) to a larger
  continental mass.
 In the western Pacific Ocean, there are several
  examples:
       Taiwan.
       The Philippine Islands.
       The many islands of Indonesia.
Accreted Terrane Margins (3)

 Each fragment, called a terrane, is a geologic
  entity characterized by a distinctive stratigraphic
  sequence and structural history.
 An accreted terrane is always fault-bounded and
  differs in its geologic features from adjacent
  terranes (also called suspect terranes).
 Some terranes have moved 5000 km or more.
Figure 20.22
Mountain Building

 Today’s fold-and-thrust ranges are the orogens
  that formed during the last few hundred million
  years.
 Examples include:
       The Appalachians.
       The Alps.
       The Canadian Rockies.
The Appalachians (1)

 The Appalachians are a Paleozoic fold-and-
  thrust mountain system 2500 km long, that
  borders the eastern and southeastern coasts of
  North America.
 They contain mud cracks, ripple marks, fossils of
  shallow-water organisms, and, in places, fresh
  water materials such as coal.
 The sedimentary strata thicken from west to
  east.
Figure 20.23
Figure 20.24
The Appalachians (2)

 Many of Pennsylvania’s oil pools were found in
  these gently folded strata.
 In the region known as the Valley Ridge
  Province, the strata have been bent into broad
  anticlines and synclines.
 In the west, the strata are nearly flat lying, but as
  one moves further east the strata dip more
  steeply to the east.
Figure 20.25
The Appalachians (3)

   The surface along which movement occurred is
    known as a detachment surface, and the slice that
    moved is commonly referred to by its French
    name, decollement.
Figure 20.26
The Alps (1)

 The Alps and associated mountain ranges in
  southern Europe were formed during the
  Mesozoic and Cenozoic eras, as a consequence of
  a collision between the European and African
  plates.
 The Jura Mountains, which mark the
  northwestern edge of the Alps, have the same
  folded form and origin as the Valley and Ridge
  Province in North America.
Figure 20.27
The Alps (2)

 The Jura Mountains were formed from shallow-
  water sediments deposited on an ancient
  continental shelf.
 In the high Alps, thrusting appears to have
  developed on a much grander scale than in the
  Appalachians.
 The high Alps are composed of deeper-water
  marine strata.
Figure 20.28
The Canadian Rockies

 In the Canadian Rockies, a central zone has been
  intensely metamorphosed.
 The thrust sheets in the Canadian Rockies
  moved eastward away from the core zone.
 Each sedimentary unit becomes thinner from
  west to east.
Figure 20.29
Revisiting Plate Tectonics And The
Earth System (1)

 For more than 2000 years, Arab traders sailed to
  India during the hot summer months, because
  summer winds blow from the west. They sailed
  back while winter winds blow from the east.
 The Arabic name for this seasonal reversal of
  wind and weather is mausim, from which we
  derive our word monsoon.
Revisiting Plate Tectonics And The
Earth System (2)

 The monsoon winds of India and Southeast Asia
  are a consequence of the collision between the
  Eurasian Plate and the Australian-Indian Plate.
 The high mountains and plateau divert the
  normal flow of westerly winds.
Figure 20.31
Revisiting Plate Tectonics And The
Earth System (3)

   Chinese geologists have discovered that these
    high lands are rather recent topographic
    features.
       They base their conclusion in part on evidence of
        plant fossils of the Pliocene Epoch (5.3 to 1.6 million
        years ago), collected at altitudes of 4000 to 6000 m,
        that include many subtropical forms that today exist
        only at altitudes below 2000 m;
Revisiting Plate Tectonics And The
Earth System (4)

   Sediment cores from the northern Indian Ocean
    show rates of sedimentation until about 10
    million years ago.
       Two peaks in sediment supply (9 to 6 and 4 to 2
        million years ago) are evidence of major intervals of
        Himalayan uplift.

				
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