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Geologic Time

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									  Chapter 4
Geologic Time:
 Concepts and
  Principles
             THE GRAND CANYON




Horizontally bedded sedimentary strata as seen from the North Rim of the Grand Canyon
illustrating the immensity of geologic time. It took hundreds of millions of years for these
strata to be deposited as layers of sediment that were eventually converted into rock. The
geologic history of the Grand Canyon region can be read from these sedimentary layers.
(photo by E.L. Crisp, May 2002)
                Grand Canyon
   When looking down into the Grand Canyon, we are
    really looking at the early history of Earth
                     Grand Canyon
   More than 2 billion years of Earth history are preserved,
       like pages of a book,
       in the rock layers of the Grand Canyon
   Reading this rock book we learn
       that the area underwent episodes of
       mountain building
       advancing and retreating shallow seas, etc.
   We know these things by
       applying the principles of relative dating to the rocks
       and recognizing that present-day processes
       have operated throughout Earth history
     What is deep time or geologic time?
   We are obsessed with time, and organize our
    lives around it.
   Most of us feel we don’t have enough of it.
   Our common time units are
     seconds
     hours
     days
     weeks            Ancient history involves
     months             hundreds of years
     years              thousands of years
                       But geologic time involves
                         millions of years
                         even billions of years
         Concept of Geologic Time
   Geologists use two different frames of reference
     when discussing geologic time
     Relative dating involves placing geologic events
          in a sequential order as determined
          from their position in the geologic record

       It does not tell us how long ago
          a particular event occurred,
          only that one event preceded another

   For over 200 hundred years geologists
     have been using relative dating
     to establish a relative geologic time scale
     BRIEF HISTORY OF GEOLOGY
    AND DISCUSSION OF GEOLOGIC
           TIME CONCEPTS
       There was very little advancement in geology until the middle
    of the eighteenth century. This dark time (prior to mid-1700's)
    for all scientific and original thought was mostly due to a strict
    interpretation of the Book of Genesis in the Bible. Geologic
    time was considered to be but a few thousand years (and some
    people today still adhere to a young Earth based on a literal
    interpretation of the Bible, here is an interesting link {
    Radiometric Dating: A Christian Perspective by Roger C. Wiens
    -- A resource paper of the American Scientific Affiliation and the
    Affiliation of Christian Geologists } written by a christian who is
    a scientist and gives support for the reality of radiometric dating
    of rocks and the 4.6 billion year age for Earth).
   Fossils were regarded as creatures engulfed by the Biblical Flood,
    freaks of nature, inventions of the devil, or figured stones.
    BRIEF HISTORY OF GEOLOGY
               (cont.)
       In 1650, James Ussher (1581-1665), Archbishop of Armagh,
    Ireland, calculated, using genealogies described in Genesis, that
    Earth was created on October 22, 4004 B. C. Thus, Earth is
    only about 6000 years old. (INTERESTING NOTE: Leonardi
    da Vinci (1452-1519) estimated that it took 200,000 years just to
    deposit the sediments in the Po River Valley in Italy.)
   During the late 1700s and into the early 1800s, many naturalists
    believed that Earth history consisted of a series of catastrophic
    upheavals that had shaped the geologic features of the earth.
    Those who believed in this concept of catastrophic earth history
    became known as CATASTROPHISTS. Baron Georges
    Cuvier (1769-1832) is credited as the first to propose this
    concept to explain the rock record. Cuvier proposed that the
    physical and biological history of Earth is explained by a series of
    sudden widespread catastrophes. Each catastrophe killed life
    forms in a portion of the area affected, new life forms were
    created (by Divine Power) or migrated in from elsewhere.
    HISTORY OF GEOLOGY (cont.)
    JAMES HUTTON (1727-1797), a Scottish medical doctor
    (and often referred to as the FATHER OF GEOLOGY),
    proposed a concept in the late 1700s now referred to as
    UNIFORMITARIANISM. Hutton never practiced
    medicine, but was very interested in the processes which
    formed and shaped the earth.
   By careful observations, he proposed that the physical,
    chemical, and biological laws of nature operated the same
    way in the past as they do today – thus, “the present is the
    key to the past” and we can interpret the rock record as
    resulting from the same laws of nature that operate today.
    This is the concept of uniformitarianism.
MODERN GEOLOGIC PHILOSOPHY
    One of Hutton's greatest contributions to geology was his
     concept of UNIFORMITARIANISM.
    This concept, meaning "the present is the key to the past",
     states that by studying geologic processes in operation
     today we can safely assume that such processes operated in
     the past and thus we can interpret rocks as a response to
     geologic processes.
    With modification, this concept is still the basis for modern
     geologic thought.
    We now realize that, although the processes themselves
     probably have not changed with time, the rates of some
     geologic processes may have varied drastically from time to
     time.
    However, the basics laws of nature are still the same today
     as they were in the past.
    So, by using this principle and others we have constructed a
     relative time scale.
Relative Geologic Time Scale
                The relative geologic
                 time scale has a
                 sequence of
                  eons
                  eras

                  periods

                  epochs
         Concept of Geologic Time
   The second frame of reference for geologic time
    is absolute dating
     Absolute dating results in specific dates
          for rock units or events
          expressed in years before the present
       It tells us how long ago a particular event occurred
            giving us numerical information about time
   Radiometric dating is the most common method
     of obtaining absolute ages
     Such dates are calculated
          from the natural rates of decay
          of various natural radioactive elements
          present in trace amounts in some rocks
Geologic Time Scale
        The discovery of radioactivity
          near the end of the 19th century
          allowed absolute ages

          to be accurately applied

          to the relative geologic time scale

        The modern geologic time
         scale is a dual scale
          a relative scale
          and an absolute scale
         Changes in the Concept of
             Geologic Time
   During the 1700s and 1800s Earth’s age
       was estimated scientifically
   Georges Louis de Buffon (1707-1788)
     calculated how long Earth took to cool gradually
     from a molten beginning
     using melted iron balls of various diameters.
     Extrapolating their cooling rate
     to an Earth-sized ball,
     he estimated Earth was 75,000 years old
        Changes in the Concept of
            Geologic Time
   Others used different techniques
   Scholars using rates of deposition of various
    sediments
     and total thickness of sedimentary rock in the crust
     produced estimates of <1 million
     to more than 2 billion years.

   John Joly used the amount of salt carried
     by rivers to the ocean
     and the salinity of seawater
     and obtained a minimum age of 90 million years
         Relative-Dating Principles
   Six fundamental geologic principles are used in
    relative dating
   Principle of superposition
     Nicolas Steno (1638-1686)
     In an undisturbed succession of sedimentary rock
      layers,
     the oldest layer is at the bottom
     and the youngest layer is at the top

   This method is used for determining the relative
    age
       of rock layers (strata) and the fossils they contain
          Principle of Superposition
   Illustration of the principles of superposition

                      Superposition: The youngest
                                      rocks are at the top
                                          of the outcrop




       and the oldest rocks are at the bottom
                                     THE GRAND CANYON



                                                                                   Kaibab Limestone
                                                                       Toroweap Formation

                                                          Coconino Sandstone

                                                                 Hermit Shale



                                                             Supai Group




Horizontally bedded sedimentary strata as seen from the North Rim of the Grand Canyon illustrating the immensity
of geologic time. It took hundreds of millions of years for these strata to be deposited as layers of sediment that were
eventually converted into rock. The geologic history of the Grand Canyon region can be read from these
sedimentary layers. (photo by E.L. Crisp, May 2002)
        Relative-Dating Principles

   Principle of original horizontality
     Nicolas Steno
     Sediment is deposited
           in essentially horizontal layers
     Therefore, a sequence of sedimentary rock layers
     that is steeply inclined from horizontal
     must have been tilted
     after deposition and lithification
           THE MORRISON
            FORMATION




Horizontal beds of the Morrison Formation near Cleveland, Utah.
               THE MORRISON
              FORMATION(again)




The Morrison Formation at Dinosaur National Monument, Utah. Note that the
   beds are strongly dipping here.
DEFORMATION OF ONCE HORIZONTAL
      SEDIMENTARY STRATA




 Sidling Hill Syncline on I-68 near Cumberland, Maryland (Photo
 by E. L. Crisp, August, 2005)
        Relative-Dating Principles
   Principle of lateral continuity
     Nicolas Steno
     Sediment extends laterally in all direction
     until it thins and pinches out
     or terminates against the edges
     of the depositional basin

   Principle of cross-cutting relationships
     James Hutton (1726-1797)
     An igneous intrusion or a fault
     must be younger than the rocks
     it intrudes or displaces
Cross-cutting
Relationships
  North shore of Lake
   Superior, Ontario
   Canada
 A dark-colored dike
   has intruded into
   older light colored
   granite.
 The dike is younger
  than the granite.
CROSS-CUTTING RELATIONSHIPS




An basalt dike cutting through granite. The basalt dike is younger than the
granite. (Photo taken on Cadillac Mountain, Bar Harbor, Maine by E. L. Crisp,
August, 2005).
Cross-cutting Relationships
                   Templin
                    Highway,
                    Castaic,
                    California
                   A small fault
                    displaces
                    tilted beds.
                   The fault is
                    younger than the
                    beds.
        Relative-Dating Principles

   Other principles of relative dating
     Principle of inclusions
     Principle of fossil succession

   are discussed later in the next chapter
                     Neptunism

   Neptunism
     All rocks, including granite and basalt,
     were precipitated in an orderly sequence

     from a primeval, worldwide ocean.

     proposed in 1787 by Abraham Werner (1749-1817)

   Werner was an excellent mineralogist,
     but is best remembered
     for his incorrect interpretation of Earth history
                           Neptunism

   Werner’s geologic column was widely accepted
       Alluvial rocks
            unconsolidated sediments, youngest
       Secondary rocks
            rocks such as sandstones, limestones, coal, basalt
       Transition rocks
            chemical and detrital rocks, some fossiliferous rocks
       Primitive rocks
            oldest including igneous and metamorphic
                 Catastrophism
   Catastrophism
     concept proposed by Georges Cuvier (1769-1832)
     dominated European geologic thinking

   The physical and biological history of Earth
     resulted from a series of sudden widespread
      catastrophes
     which accounted for significant and rapid changes in
      Earth
     and exterminated existing life in the affected area

   Six major catastrophes occurred,
     corresponding to the six days of biblical creation
     The last one was the biblical deluge
    Neptunism and Catastrophism
   These hypotheses were abandoned because
       they were not supported by field evidence
   Basalt was shown to be of igneous origin
   Volcanic rocks interbedded with sedimentary
     and primitive rocks showed that igneous activity
     had occurred throughout geologic time

   More than 6 catastrophes were needed
       to explain field observations
   The principle of uniformitarianism
       became the guiding philosophy of geology
               Uniformitarianism
   Principle of uniformitarianism
     Present-day processes have operated throughout
      geologic time.
     Developed by James Hutton (1726-1797), advocated by
      Charles Lyell (1797-1875)
   William Whewell coined the term
    uniformitarianism‖ in 1832
   Hutton applied the principle of uniformitarianism
       when interpreting rocks at Siccar Point, Scotland
   We now call what Hutton observed an unconformity,
       but he properly interpreted its formation
SICCAR POINT, SCOTLAND
     Unconformity at Siccar Point
   Hutton explained that
     the tilted, lower rocks
     resulted from severe upheavals that formed
      mountains
     these were then worn away
     and covered by younger flat-lying rocks




       the erosional surface
       represents a gap in the rock record
                Uniformitarianism
   Hutton viewed Earth                 erosion
    history as cyclical
                                 deposition       uplift

   He also understood
       that geologic processes
        operate over a vast amount of time
   Modern view of uniformitarianism
     Today, geologists assume that the principles or laws
      of nature are constant
     but the rates and intensities of change have varied
      through time
     Some geologists prefer the term ―actualism‖
              Crisis in Geology

   Lord Kelvin (1824-1907)
     knew about high temperatures inside of deep mines
     and reasoned that Earth
     was losing heat from its interior

   Assuming Earth was once molten, he used
     the melting temperature of rocks
     the size of Earth
     and the rate of heat loss
     to calculate the age of Earth as
     between 400 and 20 million years
                 Crisis in Geology
   This age was too young
     for the geologic processes envisioned
     by other geologists at that time,
     leading to a crisis in geology

   Kelvin did not know about radioactivity
       as a heat source within the Earth
         Absolute-Dating Methods
   The discovery of radioactivity
     destroyed Kelvin’s argument for the age of Earth
     and provided a clock to measure Earth’s age

   Radioactivity is the spontaneous decay
       of an atom’s nucleus to a more stable form
   The heat from radioactivity
       helps explain why the Earth is still warm inside
   Radioactivity provides geologists
     with a powerful tool to measure
     absolute ages of rocks and past geologic events
                Atoms: A Review
   Understanding absolute dating requires
       knowledge of atoms and isotopes
   All matter is made up of atoms
   The nucleus of an atom is composed of
    protons – particles with a positive electrical charge
    
   neutrons – electrically neutral particles

 with electrons – negatively charged particles –
    outside the nucleus
   The number of protons (= the atomic number)
     helps determine the atom’s chemical properties
     and the element to which it belongs
               Isotopes: A Review
   Atomic mass number
    = number of protons + number of neutrons
   The different forms of an element’s atoms
     with varying numbers of neutrons
     are called isotopes
   Different isotopes of the same element
     have different atomic mass numbers
     but behave the same chemically
   Most isotopes are stable,
       but some are unstable
   Geologists use decay rates of unstable isotopes
       to determine absolute ages of rocks
             Radioactive Decay
   Radioactive decay is the process whereby
     an unstable atomic nucleus spontaneously transforms
     into an atomic nucleus of a different element
   Three types of radioactive decay:
     In alpha decay, two protons and two neutrons
     (alpha particle) are emitted from the nucleus.
           Radioactive Decay
   In beta decay, a neutron emits a fast moving electron
    (beta particle) and becomes a proton.




   In electron capture decay, a proton captures an
    electron and converts to a neutron.
                Radioactive Decay
   Some isotopes undergo only one decay step
    before they become stable.
       Examples:
          rubidium 87 decays to strontium 87 by a single beta
           emission
          potassium 40 decays to argon 40 by a single electron
           capture
   But other isotopes undergo several decay steps
       Examples:
          uranium 235 decays to lead 207 by 7 alpha steps and 6 beta
           steps
          uranium 238 decays to lead 206 by 8 alpha steps and 6 beta
           steps
Uranium 238 decay
                      Half-Lives

   The half-life of a radioactive isotope
     is the time it takes for
     one half of the atoms

     of the original unstable parent isotope

     to decay to atoms

     of a new more stable daughter isotope

   The half-life of a specific radioactive isotope
       is constant and can be precisely measured
                     Half-Lives

   The length of half-lives for different isotopes
     of different elements
     can vary from
     less than one billionth of a second
     to 49 billion years!

   Radioactive decay
     is geometric (or exponential), NOT linear,
     and produces a curved graph
          Uniform Linear Change



   In this example
     of uniform
      linear change,
     water is
      dripping into a
      glass
     at a constant
      rate
Geometric Radioactive Decay



                    In radioactive
                     decay,
                    during each
                     equal time unit
                          half-life
                    the proportion
                     of parent atoms
                    decreases by 1/2
                   Determining Age

   By measuring the parent/daughter ratio
       and knowing the half-life of the parent
            which has been determined in the laboratory
     geologists can calculate the age of a sample
     containing the radioactive element

   The parent/daughter ratio
       is usually determined by a mass spectrometer
          an instrument that measures the proportions
          of atoms with different masses
               Determining Age
   Example:
     If a rock has a parent/daughter ratio of 1:3
     or a ratio of (parent)/(parent + daughter) = 1:4
      or 25%,
     and the half-life is 57 million years,

                               how old is the rock?
                                 25% means it is 2 half-lives
                                  old.
                                 the rock is 57my x 2 =114
                                  million years old.
    What Materials Can Be Dated?
   Most radiometric dates are obtained
       from igneous rocks
   As magma cools and crystallizes,
     radioactive parent atoms separate
     from previously formed daughter atoms

   Because they are the right size
     some radioactive parents
     are included in the crystal structure of cooling
      minerals
    What Materials Can Be Dated?

   The daughter atoms are different elements
     with different sizes
     and, therefore, do not generally fit

     into the same minerals as the parents

   Geologists can use the crystals containing
     the parent atoms
     to date the time of crystallization
           Igneous Crystallization
   Crystallization of magma separates parent atoms
       from previously formed daughters
   This resets the radiometric clock to zero.
   Then the parents gradually decay.
             Sedimentary Rocks
   Generally, sedimentary rocks can NOT be
    radiometrically dated
     The date obtained would correspond to the time of
      crystallization of the mineral,
     when it formed in an igneous or metamorphic rock,
     and NOT the time that it was deposited as a
      sedimentary particle
   Exception: The mineral glauconite can be dated
     because it forms in certain marine environments as a
      reaction with clay minerals
     during the formation of the sedimentary rock
             Sources of Uncertainty
   In glauconite, potassium 40 decays to argon 40
       Because argon is a gas,
       it can easily escape from a mineral
   A closed system is needed for an accurate date!
       Neither parent nor daughter atoms
       can have been added or removed
       from the sample since crystallization
   If leakage of daughters has occurred,
       this partially resets the radiometric clock
       and the age of the rock will show to be too young
   If parents escape, the date obtained will be too old.
   The most reliable dates use multiple methods.
          Sources of Uncertainty
   During metamorphism, some of the daughter or
    parent atoms may escape
     leading to a date that is inaccurate.
     However, if all of the daughters are forced out during
      metamorphism,
     then the date obtained would be the time of
      metamorphism—a useful piece of information.
   Dating techniques are always improving.
     Presently measurement error is typically <0.5% of
      the age, and in some cases, better than 0.1%
     A date of 540 million might have an error of ±2.7
      million years, or as low as ±0.54 million
Dating Metamorphism
          a. A mineral has just
             crystallized from magma.
          b. As time passes, parent
             atoms decay to daughters.
          c. Metamorphism drives the
             daughters out of the
             mineral as it recrystallizes.

          d. Dating the mineral today
             yields a date of 350 million
             years = time of
             metamorphism, provided
             the system remains closed
             during that time.
       Long-Lived Radioactive
     Isotope Pairs Used in Dating
   The isotopes used in radiometric dating
     need to be sufficiently long-lived
     so the amount of parent material left is measurable

 Such isotopes include:
Parents      Daughters            Half-Life (years)
Uranium 238      Lead 206         4.5 billion
                                                  Most of these
Uranium 234      Lead 207         704 million     are useful for
Thorium 232      Lead 208         14 billion      dating older
Rubidium 87      Strontium 87      48.8 billion   rocks
Potassium 40     Argon 40         1.3 billion
             Fission Track Dating
   Atomic particles in uranium
       will damage crystal structure as uranium decays
   The damage can be seen as fission tracks
       under a microscope after etching the mineral
   The age of the
    sample is related to             This method is
     the number of                   useful for samples
      fission tracks                  between 40,000
     and the amount of               years and 1.5
      uranium                         million years old
     with older samples
      having more tracks
     Radiocarbon Dating Method
   Carbon is found in all forms of life
   It has 3 isotopes
     carbon 12 and 13 are stable, but carbon 14 is not
     Carbon 14 has a half-life of 5730 years
     Carbon 14 dating uses the carbon 14/carbon 12 ratio
           of material that was once living
   The short half-life of carbon 14
     makes it suitable for dating material
     < 70,000 years old
   It is not useful for most rocks,
     but is useful for archaeology
     and young geologic materials
                       Carbon 14
   Carbon 14 is constantly forming
       in the upper atmosphere
   When cosmic rays
     strike atoms of upper atmospheric
      gases,
     Splitting nuclei into protons and
      neutons
     When a neutron strikes a nitrogen
      14 atom
     it may be absorbed
     by the nucleus and eject a proton
     changing it to carbon 14
                      Carbon 14
   The carbon 14 becomes
     part of the natural carbon cycle
     and becomes incorporated into
      organisms
   While the organism lives
     it continues to take in carbon 14,
     but when it dies
     the carbon 14 begins to decay
     without being replenished

   Thus, carbon 14 dating
       measures the time of death
         Tree-Ring Dating Method

   The age of a tree can be determined
     by counting the annual growth rings
     in lower part of the stem (trunk)

   The width of the rings are related to climate
     and can be correlated from tree to tree
     a procedure called cross-dating

   The tree-ring time scale
       now extends back 14,000 years
        Tree-Ring Dating Method




   In cross-dating, tree-ring patterns are used from
    different trees, with overlapping life spans

								
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