Plate tectonics

Martin S. (United Kingdom)
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  1. crustal plate
    a rigid layer of the Earth's crust that is believed to drift slowly
    It’s been argued that on the early
    Earth, crustal plates, floating on a heat-softened
    layer of material beneath, would have
    simply been too thick and buoyant to get
    dragged beneath each other as they are today.
  2. tectonics
    the branch of geology studying the folding and faulting of the earth's crust
    And it is
    in the Archaean that scientists can look for the
    beginnings of plate tectonics.
  3. plate tectonics
    the movement or study of the movement of Earth's crust
    And it is
    in the Archaean that scientists can look for the
    beginnings of plate tectonics.
  4. Hadean eon
    the first geologic eon, from about 4.6 to 4.0 billion years ago
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    — to the Hadean eon, which stretches from
    about 3.8 billion years ago to the planet’s birth
    — can provide hints about what Earth’s surface
    environment was like back
  5. subduction
    movement of the edge of one tectonic plate under another
    Three components, most agreed,
    were key: there must be rigid plates at the surface
    of the Earth; those plates must move apart
    through ocean spreading, with new crust
    being made where the sea floor pulls apart;
    and the plates must on occasion dive beneath
  6. archaean
    of or relating to the earliest known rocks formed during the Precambrian Eon
    Most geologists
    have never seen a komatiite; they are
    found almost exclusively among rocks of the
    Archaean era, which are more than 2.5 billion
    years old and thus very rare.
  7. hafnium
    a grey tetravalent metallic element that resembles zirconium chemically and is found in zirconium minerals; used in filaments for its ready emission of electrons
    The evidence
    comes in the form of hafnium isotope ratios in
    the zircon crystals, which preserve signals of
    the lighter minerals typical of continental crust.
  8. Hadean
    the first geologic eon, from about 4.6 to 4.0 billion years ago
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    — to the Hadean eon, which stretches from
    about 3.8 billion years ago to the planet’s birth
    — can provide hints about what Earth’s surface
    environment was like back
  9. chemical property
    a property used to characterize materials in reactions that change their identity
    Plate tectonics
    is driven by Earth’s heat and constrained
    by the physical and chemical properties of the
    crust and mantle.
  10. crustal
    of or relating to or characteristic of the crust of the earth or moon
    It’s been argued that on the early
    Earth, crustal plates, floating on a heat-softened
    layer of material beneath, would have
    simply been too thick and buoyant to get
    dragged beneath each other as they are today.
  11. geochemistry
    the science that studies the earth's physical composition
    Geochemistry can help clarify the temperature
    and pressure at which Archaean rocks formed.
  12. geophysicist
    a geologist who uses physical principles to study the properties of the earth
    But ice floes on a polar sea move in the same
    way, points out geophysicist Don Anderson of
    the California Institute of Technology in
    Pasadena — and those ice floes aren’t experiencing
    plate tectonics.
  13. abyssal
    relating to ocean depths from 2000 to 5000 meters
    Everything we see today, from the
    abyssal plains of the oceans to the heights of
    the Himalayas, is shaped by plate tectonics.
  14. geologist
    a specialist in the history of the Earth recorded in rocks
    On a sunny, breezy day in the Wind
    River Mountains of Wyoming, a
    group of geologists are peering
    intently at a dark ridge of rock.
  15. oceanic
    relating to or occurring or living in or frequenting the open ocean
    Subduction requires rigid plates, and
    as it involves the destruction of crust, new
    crust must be created elsewhere, presumably
    at oceanic spreading ridges (see graphic); otherwise,
    continental crust would eventually disappear.
  16. glaciation
    the process of covering the earth with glaciers or masses of ice
    And between 780 million and 580
    million years ago, Stern says, there was a series
    of glaciations, some very extreme — giving rise
    to the term ‘snowball Earth’.
  17. recycle
    use again after processing
    Subduction
    is the process by which one crustal plate
    slips beneath another, to be recycled into the
    mantle.
  18. biosphere
    the regions of the Earth where living organisms exist
    “The biosphere was out
    of control.”
  19. land mass
    a large continuous extent of land
    Palaeomagnetic
    studies can show how land masses
    moved across latitudes.
  20. eon
    the longest unit of geological time
    A. WITZE
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    — to the Hadean eon, which stretches from
    about 3.8 billion years ago to the planet’s birth
    — can provide hints about what Earth’s surface
    environment was like back
  21. silicate
    a salt or ester derived from silicic acid
    “You don’t
    just make a silicate planet and plate tectonics
    starts,” says Robert Stern, a geologist at the
    University of Texas, Dallas.
  22. volcanism
    the phenomena associated with volcanic activity
    For example, explanations for different
    sorts of volcanism in different settings
    also explain why the mineral make-up of continental
    crust and the crust beneath the oceans
    is so different.
  23. outcrop
    the part of a rock formation that appears above the surface
    In the
    Wind River Mountains, the meeting attendees
    continued to argue about plate tectonics as
    they hiked from outcrop to outcrop.
  24. tectonic
    pertaining to the structure or movement of the earth's crust
    It contains some geochemical
    markers that suggest subduction, but they
    could just as easily be explained by hot
    upwellings of rock known as mantle plumes or
    other non-tectonic phenomena.
  25. isotope
    atom with same atomic number, different number of neutrons
    The evidence
    comes in the form of hafnium isotope ratios in
    the zircon crystals, which preserve signals of
    the lighter minerals typical of continental crust.
  26. distill
    undergo condensation
    This melting distills the crust, producing
    material from which light, continental-style
    crust is made.
  27. sea floor
    the bottom of a sea or ocean
    Three components, most agreed,
    were key: there must be rigid plates at the surface
    of the Earth; those plates must move apart
    through ocean spreading, with new crust
    being made where the sea floor pulls apart;
    and the plates must on occasion dive b
  28. inset
    an artifact that is inserted or is to be inserted
    There are geochemical signatures that
    cannot be explained by other factors, and the
    Clues to the past: zircon crystals (inset) in the Jack Hills of Western Australia have been used to argue for an early start to plate tectonics.
  29. innards
    the organs in a body, collectively
    And the greater temperature of the early Earth’s
    innards would probably have made them move
    in very different patterns from those typical of
    today’s tectonics.
  30. geology
    science of the history of the earth as recorded in rocks
    Plate tectonics is the grand unified theory of
    geology.
  31. re-examine
    question after cross-examination by opposing counsel
    An excursion is already planned for
    next year, to re-examine the evidence for plate
    tectonics in the western Pilbara.
  32. magma
    molten rock in the earth's crust
    But then along came Julian Pearce of Cardiff
    University in Wales, who argued that each of
    the geochemical markers in the western Pilbara
    can be explained by other phenomena,
    such as magmas with an unusual amount of
    water in them, or crustal materia
  33. floe
    a flat mass of ice drifting at sea
    But ice floes on a polar sea move in the same
    way, points out geophysicist Don Anderson of
    the California Institute of Technology in
    Pasadena — and those ice floes aren’t experiencing
    plate tectonics.
  34. metamorphic
    having undergone transformation by pressure, heat, etc.
    Metamorphic
    rocks called blueschists, produced by
    squeezing the basalt from which oceanic crust
    is made at high pressures but not very high
    temperatures, are being made in today’s subduction
    zones; none, Stern says, has been
    found that is more than
  35. suture
    a seam used in surgery
    He argues that there’s
    plenty of evidence for plate tectonics stretching
    back at least 3.1 billion years2 — including
    geochemical work, seismic images of the
    ‘sutures’ where colliding continents join and,
    indeed, a few ancient ophiolites.
  36. planet
    a celestial body that revolves around the sun
    It is in the Archaean that the
    first earthly ecosystems are found, with their
    clues to life’s earliest days on the planet.
  37. unify
    join or combine
    Plate tectonics is the grand unified theory of
    geology.
  38. ground forces
    a permanent organization of the military land forces of a nation or state
    Ground forces
    Such differences of interpretation make the
    problem of solving when plate tectonics began
    extremely difficult.
  39. rebuttal
    the speech act of refuting by offering a contrary argument
    An age gone by
    After reading Stern’s arguments, Alfred
    Kröner of the University of Mainz in Germany
    fired off a rebuttal.
  40. collide
    crash together with violent impact
    He argues that there’s
    plenty of evidence for plate tectonics stretching
    back at least 3.1 billion years2 — including
    geochemical work, seismic images of the
    ‘sutures’ where colliding continents join and,
    indeed, a few ancient ophiolites.
  41. ecosystem
    organisms interacting with their physical environment
    It is in the Archaean that the
    first earthly ecosystems are found, with their
    clues to life’s earliest days on the planet.
  42. simulation
    the act of imitating the behavior of some situation
    But new simulations, using more
    sophisticated calculations, suggest that the
    crust may have been thinner than once
    thought3 — as thin as 4 kilometres or less —
    which would be thinner than today’s crust.
  43. mountainside
    the side or slope of a mountain
    Today, they jut out of a mountainside
    like the spiny tail of a sleeping dragon.
  44. mineral
    a solid inorganic substance occurring in nature
    For example, explanations for different
    sorts of volcanism in different settings
    also explain why the mineral make-up of continental
    crust and the crust beneath the oceans
    is so different.
  45. seismic
    subject to or caused by an earthquake or earth vibration
    He argues that there’s
    plenty of evidence for plate tectonics stretching
    back at least 3.1 billion years2 — including
    geochemical work, seismic images of the
    ‘sutures’ where colliding continents join and,
    indeed, a few ancient ophiolites.
  46. scrutinize
    examine carefully for accuracy
    But as the other geologists chip off fresh layers
    and scrutinize them through hand lenses,
    murmurs of dissent start to grow.
  47. diagnostic
    concerned with identifying the nature or cause of something
    Of the three, it seems subduction is closest
    to being diagnostic of plate tectonics.
  48. constrain
    hold back
    Plate tectonics
    is driven by Earth’s heat and constrained
    by the physical and chemical properties of the
    crust and mantle.
  49. interact
    do something together or with others
    A. J. VAN DER VELDEN M. HARRISON
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    131
    rocks show features that hint that plates had
    interacted along their edges.
  50. chemically
    with respect to chemistry
    The scene brings home the difficulties of trying
    to study the early Earth — there aren’t many
    old rocks to look at, and those that are around
    are often so altered, chemically and physically,
    as to be nearly indecipherable.
  51. continent
    one of the large landmasses of the earth
    As
    far back as there has been complex life — and
    perhaps even before — continents have come
    together and moved apart in a dance that has
    altered climates and geographies, opening up
    new possibilities for life and sometimes closing
    down old ones.
  52. geological
    of or relating to the study of Earth and its structure
    Stern points to the geological record of three
    types of rock.
  53. interpret
    make sense of; assign a meaning to
    Crashing continents
    Scant and difficult-to-interpret evidence presents
    one set of problems; slippery definitions
    present another.
  54. buoyant
    tending to float on a liquid or rise in air or gas
    It’s been argued that on the early
    Earth, crustal plates, floating on a heat-softened
    layer of material beneath, would have
    simply been too thick and buoyant to get
    dragged beneath each other as they are today.
  55. clarify
    make clear by removing impurities or solids, as by heating
    Geochemistry can help clarify the temperature
    and pressure at which Archaean rocks formed.
  56. structural
    relating to the composition of something
    And structural geology
    can identify features that, in today’s world
    at least, seem to be indicative of plate tectonics.
  57. suggest
    make a proposal; declare a plan for something
    Geoff Davies, a
    modeller at Australian National University in
    Canberra, presented work suggesting that one
    of the biggest stumbling blocks to an early start
    may have been removed.
  58. ultra
    far beyond the norm
    And rocks from ‘ultra-high-pressure terranes’
    of the sort produced where one plate rides over
    another are at most 630 million years old.
  59. overdue
    not paid at the scheduled time
    “It was overdue,” says Kröner.
  60. melt
    reduce or cause to be reduced from a solid to a liquid state
    In his
    model, hot upwellings of rock known as
    mantle plumes partly melt the crust above
    them.
  61. feature
    a prominent attribute or aspect of something
    Fragments of zircon crystals dated even earlier
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    Plate tectonics has created oceans and pushed up mountain ranges.
  62. scientist
    a person with advanced knowledge of empirical fields
    And it is
    in the Archaean that scientists can look for the
    beginnings of plate tectonics.
  63. evidence
    knowledge on which to base belief
    On other Earth-like planets there’s no evidence
    for today’s plate tectonics.
  64. viable
    capable of life or normal growth and development
    “Maybe plate tectonics on the early Earth was
    viable after all,” says Davies.
  65. detach
    cause to become separated
    The material left behind as the melt is
    creamed off — denser than it was before the
    distillation — then detaches itself from the
    crust and sinks back into the mantle.
  66. lava
    rock that in its molten form issues from volcanos
    This rock, says Kevin Chamberlain, a geologist
    from the University of Wyoming in
    Laramie, could be a special kind — a lava
    called a komatiite.
  67. endorse
    approve of
    Michael Brown, a geologist at the university
    of Maryland in College Park, doesn’t endorse
    Stern’s late start.
  68. incorporate
    make into a whole or make part of a whole
    The zircons are crystals
    that formed in the Hadean and later became
    incorporated into younger rocks.
  69. graphic
    written or drawn or engraved
    Three components, most agreed,
    were key: there must be rigid plates at the surface
    of the Earth; those plates must move apart
    through ocean spreading, with new crust
    being made where the sea floor pulls apart;
    and the plates must on occasion dive beneath
  70. unified
    formed or joined into a whole
    Plate tectonics is the grand unified theory of
    geology.
  71. planetary
    of or relating to a celestial body that orbits around a star
    A dramatic
    shift, such as the introduction of plate
    tectonics, must have had huge planetary consequences.
  72. cluster
    a grouping of a number of similar things
    The geologists clustered around the possible
    komatiites in the Wyoming hills had gathered
    to discuss these questions.
  73. data
    a collection of facts from which conclusions may be drawn
    To help them decide, the scientists brought
    to the table data from an array of disciplines.
  74. intermittent
    stopping and starting at irregular intervals
    An ‘intermittent approach’ would be a wonderful
    way to reconcile things — but it takes
    geology even further from the comforting
    realm of uniformitarianism, into a world
    where the most basic principles come and go
    in fits and starts.
  75. rigid
    incapable of or resistant to bending
    Three components, most agreed,
    were key: there must be rigid plates at the surface
    of the Earth; those plates must move apart
    through ocean spreading, with new crust
    being made where the sea floor pulls apart;
    and the plates must on occasion dive b
  76. researcher
    a scientist devoted to systematic investigation
    The various
    researchers are hoping to settle the matter with
    a field trip.
  77. volcanic
    relating to eruptions of gas and lava from the earth's crust
    Some
    2.7 billion years ago, these rocks were alive with
    volcanic fire.
  78. definition
    a brief explanation of the meaning of a word or phrase
    Crashing continents
    Scant and difficult-to-interpret evidence presents
    one set of problems; slippery definitions
    present another.
  79. convince
    make realize the truth or validity of something
    And
    on the Wyoming hillside few of them are convinced
    that they are seeing the rock textures
    typically found in komatiites.
  80. temperature
    the degree of hotness or coldness of a body or environment
    And the greater temperature of the early Earth’s
    innards would probably have made them move
    in very different patterns from those typical of
    today’s tectonics.
  81. interpreted
    understood in a certain way; made sense of
    Wilde argues that the zircons
    should be interpreted more conservatively
    — that the evidence points to there being some
    continental crust, but not plate tectonics and its
    associated recycling, by 4.4 billion years ago6.
  82. university
    an institution of higher learning that grants degrees
    This rock, says Kevin Chamberlain, a geologist
    from the University of Wyoming in
    Laramie, could be a special kind — a lava
    called a komatiite.
  83. clue
    evidence that helps to solve a problem
    It is in the Archaean that the
    first earthly ecosystems are found, with their
    clues to life’s earliest days on the planet.
  84. convincing
    causing one to believe the truth of something
    But several attendees
    at the meeting said they found Zhao’s data
    convincing.
  85. indicative
    pointing out or revealing clearly
    And structural geology
    can identify features that, in today’s world
    at least, seem to be indicative of plate tectonics.
  86. onset
    the beginning or early stages
    The specific aim of the meeting
    was to try to fix a date for the onset of plate
    tectonics: the earliest possibility is pretty
    much straight after the planet formed, about
    4.5 billion years ago; the latest is just
    1 billion years ago.
  87. dramatic
    characteristic of a stage performance
    A dramatic use of this argument is that
    made by Stern.
  88. identify
    recognize as being
    And structural geology
    can identify features that, in today’s world
    at least, seem to be indicative of plate tectonics.
  89. constituent
    one of the individual parts making up a composite entity
    Plate tectonics has lots of constituent
    parts.
  90. stumble
    miss a step and fall or nearly fall
    Geoff Davies, a
    modeller at Australian National University in
    Canberra, presented work suggesting that one
    of the biggest stumbling blocks to an early start
    may have been removed.
  91. conference
    a prearranged meeting for consultation or discussion
    Their visit to
    the mountain ridge had been organized as
    part of a June conference held in Lander,
    Wyoming.
  92. component
    one of the individual parts making up a larger entity
    Three components, most agreed,
    were key: there must be rigid plates at the surface
    of the Earth; those plates must move apart
    through ocean spreading, with new crust
    being made where the sea floor pulls apart;
    and the plates must on occasion dive b
  93. typical
    exhibiting the qualities that identify a group or kind
    And the greater temperature of the early Earth’s
    innards would probably have made them move
    in very different patterns from those typical of
    today’s tectonics.
  94. layer
    a single thickness of some substance or material
    But as the other geologists chip off fresh layers
    and scrutinize them through hand lenses,
    murmurs of dissent start to grow.
  95. dissent
    a difference of opinion
    But as the other geologists chip off fresh layers
    and scrutinize them through hand lenses,
    murmurs of dissent start to grow.
  96. incidental
    minor or casual or subordinate in significance or nature
    Working out which attributes are essential
    to the theory, and which incidental, is not easy.
  97. eruption
    the sudden occurrence of a violent discharge
    On the basis that dramatic effects
    require dramatic causes, he argues that the
    introduction of plate tectonics, and with it an
    increase in planet-cooling volcanic eruptions,
    might have precipitated the great glaciations.
  98. precipitate
    bring about abruptly
    On the basis that dramatic effects
    require dramatic causes, he argues that the
    introduction of plate tectonics, and with it an
    increase in planet-cooling volcanic eruptions,
    might have precipitated the great glaciations.
  99. consensus
    agreement in the judgment reached by a group as a whole
    But a
    week of communing at the conference and
    under the high mountain sun brought them
    toward a consensus of sorts.
  100. dynamic
    characterized by action or forcefulness of personality
    Dynamic planet
    The nature of that special something cuts to
    the discipline’s philosophical heart.
  101. commune
    share or interact intimately with
    But a
    week of communing at the conference and
    under the high mountain sun brought them
    toward a consensus of sorts.
  102. alter
    cause to change; make different
    The scene brings home the difficulties of trying
    to study the early Earth — there aren’t many
    old rocks to look at, and those that are around
    are often so altered, chemically and physically,
    as to be nearly indecipherable.
  103. recompense
    make payment to
    But as if to recompense
    those who study them, such ancient
    rocks, particularly of Archaean age, offer geologists
    great rewards.
  104. phenomenon
    any state or process known through the senses
    It contains some geochemical
    markers that suggest subduction, but they
    could just as easily be explained by hot
    upwellings of rock known as mantle plumes or
    other non-tectonic phenomena.
  105. minority
    being or relating to the smaller in number of two parts
    “Sure, there will be a minority point of view.”
  106. Timothy
    a disciple of Saint Paul who became the leader of the Christian community at Ephesus
    Timothy Kusky of St Louis University in
    Missouri, who led the original study, says that
    Zhao took samples from a part of the rock
    already known to be much younger than the
    main part of the ophiolite.
  107. environment
    the totality of surrounding conditions
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    — to the Hadean eon, which stretches from
    about 3.8 billion years ago to the planet’s birth
    — can provide hints about what Earth’s surface
    environment was like back
  108. process
    a particular course of action intended to achieve a result
    How can you
    understand ancient rocks when you do not
    know what processes put them there?
  109. interpretation
    the act of expressing something in an artistic performance
    Measuring one spot on a crystal, as
    opposed to another, can yield very different
    hafnium values that lead to very different interpretations,
    he says.
  110. sophisticated
    having worldly knowledge and refinement
    But new simulations, using more
    sophisticated calculations, suggest that the
    crust may have been thinner than once
    thought3 — as thin as 4 kilometres or less —
    which would be thinner than today’s crust.
  111. polar
    of or existing within the Arctic or Antarctic Circles
    But ice floes on a polar sea move in the same
    way, points out geophysicist Don Anderson of
    the California Institute of Technology in
    Pasadena — and those ice floes aren’t experiencing
    plate tectonics.
  112. volcano
    a fissure in the earth's crust through which gases erupt
    In 2001, a
    team reported that an ophiolite from Dongwanzi,
    China, was 2.5 billion years old — mak-
    Subducting oceanic plate
    Passive continental
    margin (no subduction)
    Mantle
    upwelling
    Oceanic
    spreading ridge
    Continental
    plate
    Volcano
    THE DRIVING FO
  113. incorporated
    formed or united into a whole
    The zircons are crystals
    that formed in the Hadean and later became
    incorporated into younger rocks.
  114. valid
    well grounded in logic or truth or having legal force
    In many cases, data can be
    interpreted in several completely different
    ways — all of which may seem valid.
  115. constrained
    lacking spontaneity; not natural
    Plate tectonics
    is driven by Earth’s heat and constrained
    by the physical and chemical properties of the
    crust and mantle.
  116. technology
    the practical application of science to commerce or industry
    But ice floes on a polar sea move in the same
    way, points out geophysicist Don Anderson of
    the California Institute of Technology in
    Pasadena — and those ice floes aren’t experiencing
    plate tectonics.
  117. remnant
    a small part remaining after the main part no longer exists
    — Robert Stern
    Nature © 2006 PublishingGroup
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    130
    ing it by far the oldest such subduction remnant
    yet discovered4.
  118. distinctive
    of a feature that helps to identify a person or thing
    Ophiolites are distinctive sections
    of the ocean crust that gets mashed up,
    often through subduction, on the edges of
    continents.
  119. reconcile
    come to terms
    An ‘intermittent approach’ would be a wonderful
    way to reconcile things — but it takes
    geology even further from the comforting
    realm of uniformitarianism, into a world
    where the most basic principles come and go
    in fits and starts.
  120. altered
    changed in form or character without becoming something else
    The scene brings home the difficulties of trying
    to study the early Earth — there aren’t many
    old rocks to look at, and those that are around
    are often so altered, chemically and physically,
    as to be nearly indecipherable.
  121. create
    bring into existence
    Fragments of zircon crystals dated even earlier
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    128
    Plate tectonics has created oceans and pushed up mountain ranges.
  122. diverse
    distinctly dissimilar or unlike
    Time to talk: Earth scientists gathered at a meeting in Wyoming to present diverse data on the early Earth.
  123. ancient
    belonging to times long past
    But as if to recompense
    those who study them, such ancient
    rocks, particularly of Archaean age, offer geologists
    great rewards.
  124. theory
    a belief that can guide behavior
    Plate tectonics is the grand unified theory of
    geology.
  125. approach
    move towards
    But how can that approach hold up when
    a science from a world where plate tectonics
    explains more or less everything is applied to a
    world that may have lacked it?
  126. style
    how something is done or how it happens
    Hugh
    Smithies of the Geological Survey of Western
    Australia says that the eastern part of the Pilbara,
    between 3.5 billion and 3.2 billion years
    old, “shows no clear evidence for modern-style
    plate tectonics”.
  127. discern
    perceive, recognize, or detect
    But in all these approaches, as with the komatiites,
    age makes the picture hard to discern.
  128. hint
    an indirect suggestion
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    — to the Hadean eon, which stretches from
    about 3.8 billion years ago to the planet’s birth
    — can provide hints about what Earth’s surface
    environment was like back
  129. colleague
    an associate that one works with
    Last year in Science5, geochemist Mark Harrison
    of the University of California, Los Angeles,
    and colleagues used the Jack Hills zircons to
    argue that continental crust was present 4.4 billion
    to 4.5 billion years ago.
  130. stretch
    extend one's limbs or muscles, or the entire body
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    — to the Hadean eon, which stretches from
    about 3.8 billion years ago to the planet’s birth
    — can provide hints about what Earth’s surface
    environment was like back
  131. reliable
    able to be depended on; consistent or steady
    The
    other was that the only reliable evidence for
    subduction on the early planet came from a
    period more recent than that.
  132. ballot
    a document listing the alternatives that is used in voting
    At the end, a closing ballot
    showed that many had begun to push their
    thinking further back into the past; a majority
    of attendees voted for plate tectonics having
    started between 3 billion and 4 billion years ago.
  133. institute
    set up or lay the groundwork for
    But ice floes on a polar sea move in the same
    way, points out geophysicist Don Anderson of
    the California Institute of Technology in
    Pasadena — and those ice floes aren’t experiencing
    plate tectonics.
  134. survey
    determining opinions by interviewing people
    Hugh
    Smithies of the Geological Survey of Western
    Australia says that the eastern part of the Pilbara,
    between 3.5 billion and 3.2 billion years
    old, “shows no clear evidence for modern-style
    plate tectonics”.
  135. presumably
    by reasonable assumption
    Subduction requires rigid plates, and
    as it involves the destruction of crust, new
    crust must be created elsewhere, presumably
    at oceanic spreading ridges (see graphic); otherwise,
    continental crust would eventually disappear.
  136. ratio
    relation with respect to comparative quantity or magnitude
    The evidence
    comes in the form of hafnium isotope ratios in
    the zircon crystals, which preserve signals of
    the lighter minerals typical of continental crust.
  137. passive
    lacking in energy or will
    In 2001, a
    team reported that an ophiolite from Dongwanzi,
    China, was 2.5 billion years old — mak-
    Subducting oceanic plate
    Passive continental
    margin (no subduction)
    Mantle
    upwelling
    Oceanic
    spreading ridge
    Continental
    plate
    Volcano
    THE DRIVING FO
  138. peer
    look searchingly
    On a sunny, breezy day in the Wind
    River Mountains of Wyoming, a
    group of geologists are peering
    intently at a dark ridge of rock.
  139. attribute
    a quality belonging to or characteristic of an entity
    Working out which attributes are essential
    to the theory, and which incidental, is not easy.
  140. section
    one of several parts or pieces that fit with others
    Palaeomagnetic
    and other studies show that sections of Earth’s
    crust moved relative to each other in the
    Archaean, just as modern crustal plates do.
  141. physically
    in accord with physical laws
    The scene brings home the difficulties of trying
    to study the early Earth — there aren’t many
    old rocks to look at, and those that are around
    are often so altered, chemically and physically,
    as to be nearly indecipherable.
  142. involve
    contain as a part
    Subduction requires rigid plates, and
    as it involves the destruction of crust, new
    crust must be created elsewhere, presumably
    at oceanic spreading ridges (see graphic); otherwise,
    continental crust would eventually disappear.
  143. gather
    assemble or get together
    The geologists clustered around the possible
    komatiites in the Wyoming hills had gathered
    to discuss these questions.
  144. publish
    prepare and issue for public distribution or sale
    In a paper published last year,
    he took an extreme position, proposing that
    Earth has been free of plate tectonics for
    almost four-fifths of its life, with the system we
    see today starting up only a billion years ago1.
  145. float
    be on or below a liquid surface and not sink to the bottom
    It’s been argued that on the early
    Earth, crustal plates, floating on a heat-softened
    layer of material beneath, would have
    simply been too thick and buoyant to get
    dragged beneath each other as they are today.
  146. contain
    hold or have within
    It contains some geochemical
    markers that suggest subduction, but they
    could just as easily be explained by hot
    upwellings of rock known as mantle plumes or
    other non-tectonic phenomena.
  147. poll
    the counting of votes (as in an election)
    Meeting organizers polled the attendees
    twice on when they thought plate tectonics
    began.
  148. operate
    perform as expected when applied
    Since the
    early nineteenth century, geology has been
    ruled by the principle of uniformitarianism —
    that the planet operates on unchanging laws,
    and that the present can be used as a key to the
    past.
  149. constituted
    brought about or set up or accepted
    At the conference, that minority pretty
    much constituted Stern.
  150. modern
    ahead of the times
    The problem is that Earth could display one
    or even two of these properties without necessarily
    having a system like that described by
    modern plate tectonics (see ‘A world without
    tectonics’).
  151. science
    a branch of study or knowledge involving the observation, investigation, and discovery of general laws or truths that can be tested systematically
    But how can that approach hold up when
    a science from a world where plate tectonics
    explains more or less everything is applied to a
    world that may have lacked it?
  152. array
    an impressive display or assortment
    To help them decide, the scientists brought
    to the table data from an array of disciplines.
  153. voting
    a choice that is made by counting the number of people in favor of each alternative
    By the end of the
    meeting, he remained the one person voting
    for a start to plate tectonics at 1 billion years
    ago.
  154. present
    happening or existing now
    Since the
    early nineteenth century, geology has been
    ruled by the principle of uniformitarianism —
    that the planet operates on unchanging laws,
    and that the present can be used as a key to the
    past.
  155. era
    a period marked by distinctive character
    Most geologists
    have never seen a komatiite; they are
    found almost exclusively among rocks of the
    Archaean era, which are more than 2.5 billion
    years old and thus very rare.
  156. constitute
    form or compose
    At the conference, that minority pretty
    much constituted Stern.
  157. barrier
    a structure or object that impedes free movement
    One was that
    plate tectonics could not begin until Earth’s
    crust was cool enough, and that barrier was
    only passed about a billion years ago.
  158. philosophical
    relating to the investigation of existence and knowledge
    Dynamic planet
    The nature of that special something cuts to
    the discipline’s philosophical heart.
  159. exclusively
    without any others being included or involved
    Most geologists
    have never seen a komatiite; they are
    found almost exclusively among rocks of the
    Archaean era, which are more than 2.5 billion
    years old and thus very rare.
  160. latitude
    an imaginary line around the Earth parallel to the equator
    Palaeomagnetic
    studies can show how land masses
    moved across latitudes.
  161. non
    negation of a word or group of words
    It contains some geochemical
    markers that suggest subduction, but they
    could just as easily be explained by hot
    upwellings of rock known as mantle plumes or
    other non-tectonic phenomena.
  162. shape
    a perceptual structure
    Everything we see today, from the
    abyssal plains of the oceans to the heights of
    the Himalayas, is shaped by plate tectonics.
  163. barren
    completely wanting or lacking
    Today, Earth’s interior is too
    cool to produce this particular rock; 2.7 billion
    years ago, the hot lava would have run like
    water over the barren landscapes.
  164. oppose
    be against
    Measuring one spot on a crystal, as
    opposed to another, can yield very different
    hafnium values that lead to very different interpretations,
    he says.
  165. associate
    bring or come into action
    Wilde argues that the zircons
    should be interpreted more conservatively
    — that the evidence points to there being some
    continental crust, but not plate tectonics and its
    associated recycling, by 4.4 billion years ago6.
  166. margin
    the boundary line or area immediately inside the boundary
    In 2001, a
    team reported that an ophiolite from Dongwanzi,
    China, was 2.5 billion years old — mak-
    Subducting oceanic plate
    Passive continental
    margin (no subduction)
    Mantle
    upwelling
    Oceanic
    spreading ridge
    Continental
    plate
    Volcano
    THE DRIVING FO
  167. shift
    move very slightly
    A dramatic
    shift, such as the introduction of plate
    tectonics, must have had huge planetary consequences.
  168. resolve
    find a solution or answer
    Field trips don’t always resolve things.
  169. require
    have need of
    Subduction requires rigid plates, and
    as it involves the destruction of crust, new
    crust must be created elsewhere, presumably
    at oceanic spreading ridges (see graphic); otherwise,
    continental crust would eventually disappear.
  170. realm
    a domain in which something is dominant
    An ‘intermittent approach’ would be a wonderful
    way to reconcile things — but it takes
    geology even further from the comforting
    realm of uniformitarianism, into a world
    where the most basic principles come and go
    in fits and starts.
  171. dense
    having high compaction or concentration
    The material left behind as the melt is
    creamed off — denser than it was before the
    distillation — then detaches itself from the
    crust and sinks back into the mantle.
  172. alternative
    one of a number of things from which only one can be chosen
    But it is, he says, a
    detailed alternative theory to plate
    tectonics, and one that can be tested
    with further studies.
  173. factor
    anything that contributes causally to a result
    There are geochemical signatures that
    cannot be explained by other factors, and the
    Clues to the past: zircon crystals (inset) in the Jack Hills of Western Australia have been used to argue for an early start to plate tectonics.
  174. chemical
    produced by reactions involving atomic or molecular changes
    Plate tectonics
    is driven by Earth’s heat and constrained
    by the physical and chemical properties of the
    crust and mantle.
  175. property
    something owned
    Plate tectonics
    is driven by Earth’s heat and constrained
    by the physical and chemical properties of the
    crust and mantle.
  176. pattern
    a repeated design, structure, or arrangement
    And the greater temperature of the early Earth’s
    innards would probably have made them move
    in very different patterns from those typical of
    today’s tectonics.
  177. crash
    break violently or noisily
    Crashing continents
    Scant and difficult-to-interpret evidence presents
    one set of problems; slippery definitions
    present another.
  178. expert
    a person with special knowledge who performs skillfully
    But these men
    and women are experts in the truly old.
  179. significance
    the quality of being important
    For a while
    two independent groups have been quietly
    warring over the significance of a pile of
    ancient zircons from the Jack Hills region of
    Western Australia.
  180. majority
    the main part
    At the end, a closing ballot
    showed that many had begun to push their
    thinking further back into the past; a majority
    of attendees voted for plate tectonics having
    started between 3 billion and 4 billion years ago.
  181. sink
    fall or descend to a lower place or level
    The material left behind as the melt is
    creamed off — denser than it was before the
    distillation — then detaches itself from the
    crust and sinks back into the mantle.
  182. plenty
    a full supply
    He argues that there’s
    plenty of evidence for plate tectonics stretching
    back at least 3.1 billion years2 — including
    geochemical work, seismic images of the
    ‘sutures’ where colliding continents join and,
    indeed, a few ancient ophiolites.
  183. computer
    a machine for performing calculations automatically
    In the early 1990s,
    computer models created by Davies and others
    suggested that the crust on the early Earth
    would have been too thick and buoyant to get
    dragged down beneath another plate during
    subduction.
  184. specific
    stated explicitly or in detail
    The specific aim of the meeting
    was to try to fix a date for the onset of plate
    tectonics: the earliest possibility is pretty
    much straight after the planet formed, about
    4.5 billion years ago; the latest is just
    1 billion years ago.
  185. principle
    a basic generalization that is accepted as true
    Since the
    early nineteenth century, geology has been
    ruled by the principle of uniformitarianism —
    that the planet operates on unchanging laws,
    and that the present can be used as a key to the
    past.
  186. complex
    complicated in structure
    As
    far back as there has been complex life — and
    perhaps even before — continents have come
    together and moved apart in a dance that has
    altered climates and geographies, opening up
    new possibilities for life and sometimes closing
    down old ones.
  187. pile
    a collection of objects laid on top of each other
    For a while
    two independent groups have been quietly
    warring over the significance of a pile of
    ancient zircons from the Jack Hills region of
    Western Australia.
  188. interior
    inside and toward a center
    Today, Earth’s interior is too
    cool to produce this particular rock; 2.7 billion
    years ago, the hot lava would have run like
    water over the barren landscapes.
  189. lead
    take somebody somewhere
    The exchange of papers led to the Wyoming
    conference.
  190. thick
    not thin
    It’s been argued that on the early
    Earth, crustal plates, floating on a heat-softened
    layer of material beneath, would have
    simply been too thick and buoyant to get
    dragged beneath each other as they are today.
  191. edge
    a line determining the limits of an area
    Ophiolites are distinctive sections
    of the ocean crust that gets mashed up,
    often through subduction, on the edges of
    continents.
  192. examine
    observe, check out, and look over carefully or inspect
    An excursion is already planned for
    next year, to re-examine the evidence for plate
    tectonics in the western Pilbara.
  193. recent
    of the immediate past or just previous to the present time
    The
    other was that the only reliable evidence for
    subduction on the early planet came from a
    period more recent than that.
  194. preserve
    keep in safety and protect from harm, loss, or destruction
    The evidence
    comes in the form of hafnium isotope ratios in
    the zircon crystals, which preserve signals of
    the lighter minerals typical of continental crust.
  195. surface
    the outer boundary of an artifact or a material layer
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    — to the Hadean eon, which stretches from
    about 3.8 billion years ago to the planet’s birth
    — can provide hints about what Earth’s surface
    environment was like back
  196. display
    something intended to communicate a particular impression
    The problem is that Earth could display one
    or even two of these properties without necessarily
    having a system like that described by
    modern plate tectonics (see ‘A world without
    tectonics’).
  197. wind
    air moving from high pressure to low pressure
    On a sunny, breezy day in the Wind
    River Mountains of Wyoming, a
    group of geologists are peering
    intently at a dark ridge of rock.
  198. institution
    a custom that has been an important feature of some group
    Geophysicist Paul Silver, of the Carnegie
    Institution of Washington, raised the notion
    that plate tectonics may have started and
    stopped several times during Earth’s history.
  199. eventually
    after an unspecified period of time or a long delay
    Subduction requires rigid plates, and
    as it involves the destruction of crust, new
    crust must be created elsewhere, presumably
    at oceanic spreading ridges (see graphic); otherwise,
    continental crust would eventually disappear.
  200. debate
    a discussion with reasons for and against some proposal
    Alexandra Witze joins the geologists debating the issue.
  201. depth
    the extent downward or backward or inward
    Evidence
    for the earlier style, he says, comes from
    minerals that are typical of higher-temperature,
    lower-pressure environments; these suggest
    a hotter Earth where plates did not
    subduct beneath each other to great depths
    and pressures.
  202. describe
    give a statement representing something
    The problem is that Earth could display one
    or even two of these properties without necessarily
    having a system like that described by
    modern plate tectonics (see ‘A world without
    tectonics’).
  203. contrast
    the opposition or dissimilarity of things that are compared
    In contrast, looking at the western part of the
    Pilbara — which is 3.3 billion to 3.0 billion years
    old — Smithies sees plenty of evidence for plate
    tectonics.
  204. settle
    become resolved, fixed, established, or quiet
    The various
    researchers are hoping to settle the matter with
    a field trip.
  205. notion
    a general inclusive concept
    Geophysicist Paul Silver, of the Carnegie
    Institution of Washington, raised the notion
    that plate tectonics may have started and
    stopped several times during Earth’s history.
  206. yield
    give or supply
    Measuring one spot on a crystal, as
    opposed to another, can yield very different
    hafnium values that lead to very different interpretations,
    he says.
  207. remove
    take something away as by lifting, pushing, or taking off
    Geoff Davies, a
    modeller at Australian National University in
    Canberra, presented work suggesting that one
    of the biggest stumbling blocks to an early start
    may have been removed.
  208. signal
    any action or gesture that encodes a message
    The evidence
    comes in the form of hafnium isotope ratios in
    the zircon crystals, which preserve signals of
    the lighter minerals typical of continental crust.
  209. maintain
    keep in a certain state, position, or activity
    “It’s not a simple question,” he maintains.
  210. climate
    the weather in some location averaged over a period of time
    As
    far back as there has been complex life — and
    perhaps even before — continents have come
    together and moved apart in a dance that has
    altered climates and geographies, opening up
    new possibilities for life and sometimes closing
    down old ones.
  211. include
    have as a part; be made up out of
    He argues that there’s
    plenty of evidence for plate tectonics stretching
    back at least 3.1 billion years2 — including
    geochemical work, seismic images of the
    ‘sutures’ where colliding continents join and,
    indeed, a few ancient ophiolites.
  212. version
    something a little different from others of the same type
    In a paper in press in Geology8, Brown
    suggests that there have been two styles of
    plate tectonics: the modern kind that we see
    today, and an earlier version that lasted from
    about 2.7 billion to 700 million years ago.
  213. essential
    basic and fundamental
    Working out which attributes are essential
    to the theory, and which incidental, is not easy.
  214. fresh
    recently made, produced, or harvested
    But as the other geologists chip off fresh layers
    and scrutinize them through hand lenses,
    murmurs of dissent start to grow.
  215. remain
    continue in a place, position, or situation
    Smithies thinks the
    western Pilbara contains the remains of an
    oceanic arc — the sort of line of islands, such as
    the Aleutians of Alaska, that are characteristic
    of some oceanic subduction zones7.
  216. convinced
    having a strong belief or conviction
    And
    on the Wyoming hillside few of them are convinced
    that they are seeing the rock textures
    typically found in komatiites.
  217. image
    a visual representation produced on a surface
    He argues that there’s
    plenty of evidence for plate tectonics stretching
    back at least 3.1 billion years2 — including
    geochemical work, seismic images of the
    ‘sutures’ where colliding continents join and,
    indeed, a few ancient ophiolites.
  218. provide
    give something useful or necessary to
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    — to the Hadean eon, which stretches from
    about 3.8 billion years ago to the planet’s birth
    — can provide hints about what Earth’s surface
    environment was like back
  219. independent
    free from external control and constraint
    For a while
    two independent groups have been quietly
    warring over the significance of a pile of
    ancient zircons from the Jack Hills region of
    Western Australia.
  220. lack
    the state of needing something that is absent or unavailable
    But how can that approach hold up when
    a science from a world where plate tectonics
    explains more or less everything is applied to a
    world that may have lacked it?
  221. rare
    especially good, remarkable, or superlative
    Most geologists
    have never seen a komatiite; they are
    found almost exclusively among rocks of the
    Archaean era, which are more than 2.5 billion
    years old and thus very rare.
  222. consequence
    a phenomenon that is caused by some previous phenomenon
    A dramatic
    shift, such as the introduction of plate
    tectonics, must have had huge planetary consequences.
  223. several
    of an indefinite number more than 2 or 3 but not many
    But several attendees
    at the meeting said they found Zhao’s data
    convincing.
  224. raise
    move upwards
    Geophysicist Paul Silver, of the Carnegie
    Institution of Washington, raised the notion
    that plate tectonics may have started and
    stopped several times during Earth’s history.
  225. region
    the extended spatial location of something
    For a while
    two independent groups have been quietly
    warring over the significance of a pile of
    ancient zircons from the Jack Hills region of
    Western Australia.
  226. range
    a variety of different things or activities
    Fragments of zircon crystals dated even earlier
    NEWS FEATURE NATURE|Vol 442|13 July 2006
    128
    Plate tectonics has created oceans and pushed up mountain ranges.
  227. found
    set up
    Most geologists
    have never seen a komatiite; they are
    found almost exclusively among rocks of the
    Archaean era, which are more than 2.5 billion
    years old and thus very rare.
  228. brave
    possessing or displaying courage
    One of the brave few is Jean
    Bédard, a geologist at the Geological Survey
    of Canada.
  229. physical
    involving the body as distinguished from the mind or spirit
    Plate tectonics
    is driven by Earth’s heat and constrained
    by the physical and chemical properties of the
    crust and mantle.
  230. comfort
    a state of being relaxed and feeling no pain
    An ‘intermittent approach’ would be a wonderful
    way to reconcile things — but it takes
    geology even further from the comforting
    realm of uniformitarianism, into a world
    where the most basic principles come and go
    in fits and starts.
  231. removed
    separate or apart in time, space, or character
    Geoff Davies, a
    modeller at Australian National University in
    Canberra, presented work suggesting that one
    of the biggest stumbling blocks to an early start
    may have been removed.
  232. reading
    written material intended to be read
    An age gone by
    After reading Stern’s arguments, Alfred
    Kröner of the University of Mainz in Germany
    fired off a rebuttal.
  233. Chinese
    of or pertaining to China or its peoples or cultures
    The Chinese ophiolite isn’t the only evidence
    that is getting fresh scrutiny.
  234. issue
    some situation or event that is thought about
    Alexandra Witze joins the geologists debating the issue.
  235. increase
    a process of becoming larger or longer or more numerous
    On the basis that dramatic effects
    require dramatic causes, he argues that the
    introduction of plate tectonics, and with it an
    increase in planet-cooling volcanic eruptions,
    might have precipitated the great glaciations.
  236. measure
    determine the dimensions of something or somebody
    Measuring one spot on a crystal, as
    opposed to another, can yield very different
    hafnium values that lead to very different interpretations,
    he says.
  237. instance
    an item of information that is typical of a class or group
    For instance, another Australian geologist
    presented seemingly convincing evidence that
    plate tectonics had begun by 3.3 billion years
    ago in Western Australia, based on the very
    different histories of two sections of an ancient
    rock formation call
  238. valley
    a long depression in the surface of the land
    Valley, J. W. et al Science 312, 1139a (2006).
  239. record
    anything providing permanent evidence about past events
    Stern points to the geological record of three
    types of rock.
  240. published
    prepared and printed for distribution and sale
    In a paper published last year,
    he took an extreme position, proposing that
    Earth has been free of plate tectonics for
    almost four-fifths of its life, with the system we
    see today starting up only a billion years ago1.
  241. college
    an institution of higher education
    Michael Brown, a geologist at the university
    of Maryland in College Park, doesn’t endorse
    Stern’s late start.
  242. series
    similar things placed in order or one after another
    And between 780 million and 580
    million years ago, Stern says, there was a series
    of glaciations, some very extreme — giving rise
    to the term ‘snowball Earth’.
  243. similar
    having the same or nearly the same characteristics
    The data also suggest, Harrison argues, that
    that crust was being recycled down into the
    mantle by 4.4 billion years ago — perhaps
    though a process similar to plate tectonics.
  244. occasion
    an event that occurs at a critical time
    Three components, most agreed,
    were key: there must be rigid plates at the surface
    of the Earth; those plates must move apart
    through ocean spreading, with new crust
    being made where the sea floor pulls apart;
    and the plates must on occasion dive b
  245. straight
    having no deviations
    The specific aim of the meeting
    was to try to fix a date for the onset of plate
    tectonics: the earliest possibility is pretty
    much straight after the planet formed, about
    4.5 billion years ago; the latest is just
    1 billion years ago.
  246. plain
    simple
    Everything we see today, from the
    abyssal plains of the oceans to the heights of
    the Himalayas, is shaped by plate tectonics.
  247. share
    assets belonging to an individual person or group
    Some of the newly shared data favour an
    early start for plate tectonics.
  248. team
    a cooperative unit
    In 2001, a
    team reported that an ophiolite from Dongwanzi,
    China, was 2.5 billion years old — mak-
    Subducting oceanic plate
    Passive continental
    margin (no subduction)
    Mantle
    upwelling
    Oceanic
    spreading ridge
    Continental
    plate
    Volcano
    THE DRIVING FO
  249. simple
    having few parts; not complex or complicated or involved
    “It’s not a simple question,” he maintains.
  250. century
    a period of 100 years
    Since the
    early nineteenth century, geology has been
    ruled by the principle of uniformitarianism —
    that the planet operates on unchanging laws,
    and that the present can be used as a key to the
    past.
  251. experience
    the content of observation or participation in an event
    But ice floes on a polar sea move in the same
    way, points out geophysicist Don Anderson of
    the California Institute of Technology in
    Pasadena — and those ice floes aren’t experiencing
    plate tectonics.
  252. period
    an amount of time
    The
    other was that the only reliable evidence for
    subduction on the early planet came from a
    period more recent than that.
  253. wild
    wild, free, and not controlled or touched by humans
    “It was a wild time
    of change,” says Stern.
  254. Washington
    1st President of the United States
    Geophysicist Paul Silver, of the Carnegie
    Institution of Washington, raised the notion
    that plate tectonics may have started and
    stopped several times during Earth’s history.
  255. effect
    a phenomenon that is caused by some previous phenomenon
    On the basis that dramatic effects
    require dramatic causes, he argues that the
    introduction of plate tectonics, and with it an
    increase in planet-cooling volcanic eruptions,
    might have precipitated the great glaciations.
  256. view
    the visual percept of a region
    “Sure, there will be a minority point of view.”
  257. clear
    readily apparent to the mind
    Hugh
    Smithies of the Geological Survey of Western
    Australia says that the eastern part of the Pilbara,
    between 3.5 billion and 3.2 billion years
    old, “shows no clear evidence for modern-style
    plate tectonics”.
Created on Sun Nov 06 11:23:21 EST 2011 (updated Sun Nov 06 11:23:42 EST 2011)

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