Geologic Time by ewghwehws


									  Chapter 2
Geologic Time:
 Concepts and
             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
   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
         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
           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).
   Fossils were regarded as creatures engulfed by the Biblical Flood,
    freaks of nature, inventions of the devil, or figured stones.
       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.
    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.
    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
    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
     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 (Steno, 1669)
     Nicolas Steno (1638-1686)
     In an undisturbed succession of sedimentary rock
     the oldest layer is at the bottom
     and the youngest layer is at the top

   This method is used for determining the relative
       of rock layers (strata) and the fossils they contain
                                     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 (1669)
     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 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)
           THE MORRISON

Horizontal beds of the Morrison Formation near Cleveland, Utah.
               THE MORRISON

The Morrison Formation at Dinosaur National Monument, Utah. Note that the
   beds are strongly dipping here.

 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 (1669)
     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
                                     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)

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

   This is a small reverse fault in Allegheny Group rocks along Schultz Road in Pleasants
    County, West Virginia. A dashed line represents the fault that is probably associated
    with the formation of the Burning Springs Anticline. Light colored mudstones are
    adjacent to the asphalt road. The Lower Freeport continuous coal is near the center of
    the photo with the Upper Freeport sandstone at the top.
        Relative-Dating Principles

   Other principles of relative dating
     Principle of inclusions
     Principle of fossil succession
    that inclusions of one kind of rock in another
    are always representative of the older rock
    material. For example, if a granitic magma has
    intruded into a sandstone and chunks of
    sandstone have been incorporated into the rising
    magma, as cooling occurs there will be
    inclusions of sandstone in the granite and the
    inclusions will represent the older rock.
   Correlation is the matching up of rocks in one area to
    those in another area.
   There are two types of correlation of rock units.
       Physical Correlation: correlation of rock units based on
        physical characteristics of the rocks or position in a sequence
        of rocks. Assumes that the rock units were once continuous.
       Time-rock Correlation: correlation of rock units that are time
        equivalent (rock units in different areas that are of the same
   What are fossils?
       Any remains or evidence of activity of a once living
        organism (usually restricted to prehistoric time).
   Scientists who study fossils are called
    paleontologists (not archeologists!!!).
   Two major types of fossils: Body fossils and
    trace fossils.
              Fossils: evidence of past life

   Types of fossils
    •   Indirect evidence includes – Trace Fossils
         •   Tracks
         •   Burrows
         •   Coprolites – fossil dung and stomach contents
         •   Gastroliths – stomach stones used to grind food by
             some extinct reptiles
A dinosaur footprint
         The Formation of Body Fossils
       The usual prerequisites for fossilization to form body fossils
    is the possession of hard parts (bones, teeth, mineralized
    exoskeleton, etc.) and the rapid burial of the hard parts by
    sediment (this reduces the amount of oxygen present to very
    low levels and slows decomposition of the hard parts). Usually
    soft parts of an organism rot rapidly. Only rarely are soft parts
    preserved (such as skin impressions for dinosaurs), but under
    some conditions they are preserved and give paleontologists
    valuable information that is usually not present in the rock
    record. After burial some sort of mineralization typically
    occurs. Unaltered remains are very rare.
                    Altered Remains
   Permineralization: Mineral matter from percolating ground
    waters is added to pores and cavities in bones, shell, teeth,
    etc. In this type of preseravation the original material is still
    present with new mineral matter added to the void spaces. Many
    dinosaur bones are preserved by this method.
   Replacement: Sometimes original hard parts (bone in the case
    of dinosaurs) is replaced (sometimes referred to as petrified,
    which means turned to stone) with new mineral matter of a
    different composition than the original mineral matter (often at a
    molecular level, so the microstructure of the original mineral
    matter is preserved). Silica (as microcrystalline quartz, SiO2),
    iron oxide (hematite, Fe2O3), and calcium carbonate (calcite,
    CaCO3) are common replacement minerals (they are also
    common permineralizing agents). Many dinosaur bones are both
    permineralized and partially replaced.
                    Altered Remains
   Recrystallization: The recrystalliztion of fossils is another
    common type of preservation in which the original mineral
    present simply recrystallizes (the original crystals grow larger and
    fill most of the void space). This is more common in
    invertebrate fossils (such as bivalves {clams}, brachiopods,
    gastropods, etc.) than in vertebrate fossils. This form of
    preservation usually destroys or partially obscures the original
    microstructure of the skeletal material. An example would be
    the recrystallization of a clam shell originally composed of the
    mineral aragonite (a metastable form of calcium carbonate) to
    calcite (the more stable form of calcium carbonate at low
                       Altered Remains
   Carbonization: Sometimes soft parts and/or hard parts of the body of an
    organism are compressed by burial before decomposition is complete such
    that the volatile substances (such as oxygen, nitrogen, carbon dioxide, water,
    etc.) are squeezed out leaving behind a film of fairly pure carbon. This is
    particulary common in the preservation plant fossils (such as ferns and leaves
    [Look at the fossil leaves and insects from the Green River Formation of
    Utah that are present in the Geology Lab at WVUP, these are preserved
    by carbonization]) and some invertebrates, but also occurs sometimes for
    vertebrates (for example, fifty million year old fossil fish of the Eocene Green
    River Formation of Wyoming, Colorado, and Utah).
   Molds and Casts: Sometimes the hard parts (bone or other material) (and
    sometimes even soft tissue) of organisms are buried by sediment and even
    may remain until the sediment is lithified (by compaction and cementation),
    but are later dissolved by acidic ground waters percolating through the pores
    of the rock (or decomposed by other processes). This will leave an
    impression of the external morphology of the original material that was
    buried. This is called an external mold. If later the mold is filled in with
    mineral matter or sediment, a cast is formed which mimics the external
    morphology of the original material. Sometimes internal cavities of skeletons
    (from both invertebrates and vertebrates) may be filled with sediment or
    mineral matter resulting in a mold of the internal morphology of the cavity
    that was filled, this is called an internal mold. Internal molds are quite
    common for some invertebrates (such as for clams and gastropods).
Natural casts of shelled invertebrates
   Although rocks may be correlated based on physical correlation
    and superposition, this can only be done in a limited area where
    beds can be traced from one area to another. Also if we are
    correlating over a large area (from region to region, or continent
    to continent), it is unlikely that we can use physical correlation
    because rock types will change.
   To correlate over large regions and to correlate age-equivalent
    strata, geologists must use fossils. The use of fossils to correlate
    sedimentary strata is based on the work of William Smith (1812),
    the first to accurately state and use the Principle of Fossil
   The Principle of Fossil Succession states the assemblages of
    fossils succeed themselves in a definite and determinable order
    and the age of sedimentary strata can be determined by their
    contained fossils.
   To use the Principle of Fossil Succession, geologists and
    paleontologists use Index Fossils (Guide Fossils).

   The Principle of Fossil Succession is based on
    the following:
     Life has varied through time. Of course this implies
      that evolutionary change has occurred over time.
     Because biologic diversity has varied over time, fossil
      assemblages are different in successivly younger
     The relative ages of fossil assemblages can be
      determined by superposition.
              INDEX FOSSILS
   Index fossils are used to correlate age-
    equivalent strata via the Principle of Fossil
   Index fossils have the following characteristics:
     Short geologic time range.
     Wide geographic distribution
     Abundant
     Easily recognizable
Some More History of
   Geologic Time

   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

   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
     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
     which accounted for significant and rapid changes in
     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
   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
     Unconformity at Siccar Point
   Hutton explained that
     the tilted, lower rocks
     resulted from severe upheavals that formed
     these were then worn away
     and covered by younger flat-lying rocks

       the erosional surface
       represents a gap in the rock record
   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.
Uranium 238 decay

   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

   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
     at a constant
Geometric Radioactive Decay

                    In radioactive
                    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
                                 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
    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
   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
     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
       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
     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
     Splitting nuclei into protons and
     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
   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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