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Geologic Time Concepts and Principles


Relative dating involves placing geologic events in a sequential order as ... The second frame of reference for geologic time is absolute dating ... – PowerPoint PPT presentation

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Title: Geologic Time Concepts and Principles

Chapter 4
Geologic TimeConcepts and Principles
Grand Canyon
  • When looking down into the Grand Canyon, we are
    really looking all the way back to the early
    history of Earth

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
  • For hundreds of years geologists have been using
    relative dating to establish a relative geologic
    time scale

Relative Geologic Time Scale
  • The relative geologic time scale has a sequence
  • eons
  • eras
  • periods
  • epochs
  • but no numbers indicating how long ago each of
    these times occurred

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
  • 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 1800s allowed absolute ages to be accurately
    applied to the relative geologic time scale
  • The geologic time scale is a dual scale
  • a relative scale
  • and an absolute scale

Fig. 4-1, p. 62
Changes in the Concept of Geologic Time
  • The concept and measurement of geologic time has
    changed through human history
  • Early Christian theologians conceived of time as
    linear rather than circular
  • James Usher (1581-1665) in Ireland
  • calculated the age of Earth based on recorded
    history and genealogies in Genesis
  • Announced that Earth was created on October 22,
    4004 B.C.
  • A century later it was considered heresy to say
    Earth was more than about 6000 years old.

Changes in the Concept of Geologic Time
  • During the 1700s and 1800s Earths 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
  • Using rates of deposition of various sediments
    and thickness of sedimentary rock in the crust
    gave estimates of 1 million to more than 2
    billion years.
  • Using the amount of salt carried by rivers to the
    ocean and the salinity of seawater John Joly in
    1899 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

Relative-Dating Principles
  • Principle of original horizontality
  • Nicolas Steno
  • Sediment is deposited in essentially horizontal
  • Therefore, a sequence of sedimentary rock layers
    that is steeply inclined from horizontal must
    have been tilted after deposition and

  • Illustration of the principles of original

  • Illustration of the principles of superposition

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

Relative-Dating Principles
  • Principle of inclusions
  • discussed later in the term
  • Principle of fossil succession
  • discussed later in the term

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
  • Templin Highway, Castaic, California
  • A small fault displaces tilted beds.
  • The fault is younger than the beds.

  • 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

  • Werners geologic column was widely accepted
  • Alluvial rocks
  • unconsolidated sediments, youngest
  • Secondary rocks
  • rocks such as sandstones, limestones, coal,
  • Transition rocks
  • chemical and detrital rocks, some fossiliferous
  • Primitive rocks
  • oldest including igneous and metamorphic

  • 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 flood

Neptunism and Catastrophism Were Eventually
  • 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, advocated by Charles
    Lyell (1797-1875)
  • Hutton applied the principle of uniformitarianism
    when interpreting rocks at Siccar Point Scotland
  • We now call what he observed an unconformity but
    he properly interpreted its formation
  • Term uniformitarianism was coined by William
    Whewell in 1832

Unconformity at Siccar Point
  • Hutton explained that
  • the tilted, lower rocks resulted from severe
    upheavals that formed mountains
  • The mountains were then worn away and covered by
    younger flat-lying rocks
  • the erosional surface represents a gap in the
    rock record

  • Hutton viewed Earth history as cyclical

  • 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

Crisis in Geology
  • Lord Kelvin (1824-1907)
  • knew about high temperatures inside of deep mines
    and reasoned that Earth is losing heat from its
  • 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
  • For the geologic processes envisioned by other
    geologists at that time, this age was too young!
  • What was the flaw in Kelvins calculation?
  • Kelvin did not know about radioactivity as a heat
    source within the Earth

Absolute-Dating Methods
  • The discovery of radioactivity destroyed Kelvins
    argument for the age of Earth and provided a
    clock to measure Earths age
  • Radioactivity is the spontaneous decay of an
    atoms 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

  • Understanding absolute dating requires knowledge
    of atoms and isotopes
  • The nucleus of an atom is composed of
  • protons particles with a positive electrical
  • neutrons electrically neutral particles
  • electrons the negatively charged particles
    encircling the nucleus
  • atomic number
  • Equal to the number of protons
  • helps determine the atoms chemical properties
    and the element to which it belongs

  • Atomic mass number number of protons number
    of neutrons
  • The different forms of an elements atoms with
    varying numbers of neutrons are called isotopes
  • Different isotopes of the same element have
    different atomic mass numbers but behave the same
  • 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 -the process whereby an
    unstable atomic nucleus spontaneously changes
    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
  • 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
  • Can vary from less than 1/billionth of a second
    to 49 billion years
  • Is geometric not linear, so has 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, one 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

Determining Age
  • For example
  • If a rock has a parent/daughter ratio of 13
  • a parent proportion of 25,
  • and the half-live is 57 million years,
  • 25 means it is 2 half-lives old.
  • the rock is 57 x 2 114 million years old.

What Materials Can Be Dated?
  • Most radiometric dates are obtained from igneous
  • As magma cools and crystallizes,
  • radioactive parent atoms separate from previously
    formed daughter atoms
  • Some radioactive parents are included in the
    crystal structure of certain 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
    parents 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.

Not Sedimentary Rocks
  • Generally, sedimentary rocks cannot be
    radiometrically dated because the date obtained
    would correspond to the time of crystallization
    of the mineral, when it formed in an igneous or
    metamorphic rock,not the time that it was
    deposited as a sedimentary particle
  • Exception dating the mineral glauconite, because
    it forms in certain marine environments as a
    reaction with clay 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
  • that is, neither parent nor daughter atoms can
    have been added or removed from the sample since
  • If leakage of daughters has occurred
  • it partially resets the radiometric clock and the
    age will be too young
  • If parents escape, the date will be too old.
  • The most reliable dates use multiple methods.

Sources of Uncertainty
  • During metamorphism, some of the daughter atoms
    may escape
  • leading to a date that is too young.
  • However, if all of the daughters are forced out
    during metamorphism, then the date obtained would
    be the time of metamorphisma useful piece of
  • Dating techniques are always improving.
  • Presently measurement error is typically of the age, and even 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
c. Metamorphism drives the daughters out of the
mineral as it recrystallizes.
Dating the whole rock yields a date of 700
million years time of crystallization.
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
  • 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 Uranium
234 Lead 207 704 million Thorium 232
Lead 208 14 billion Rubidium 87 Strontium
87 48.8 billion Potassium 40 Argon 40 1.3
Fission Track Dating
  • Uranium in a crystal will damage the crystal
    structure as it decays
  • The damage can be seen as fission tracks under a
    microscope after etching the mineral
  • The age of the sample is related to
  • the number of fission tracks
  • the amount of uranium

Radiocarbon Dating Method
  • Carbon is found in all 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 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
  • When a high-energy neutrona type of cosmic ray
    strikes a nitrogen 14 atomit may be absorbed by
    the nucleus and eject a proton changing it to
    carbon 14
  • The 14C formation rate
  • is fairly constant
  • has been calibrated against tree rings

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
  • The width of the rings are related to climate can
    be correlated from tree to tree
  • a procedure called cross-dating
  • The tree-ring time scale now extends back 14,000

Tree-Ring Dating Method
  • In cross-dating, tree-ring patterns are used from
    different trees, with overlapping life spans

  • Early Christian theologians viewed time as linear
    and decided that Earth was very young (about 6000
    years old)
  • A variety of ages for Earth were estimated during
    the 18th and 19th centuries using scientific
    evidence, ages now known to be too young
  • Neptunism and catastrophism were popular during
    the 17th, 18th and early 19th centuries because
    of their consistency with scripture, but were not
    supported by evidence

  • James Hutton viewed Earth history as cyclical and
    very long
  • His observations were instrumental in
    establishing the principle of uniformitarianism
  • Charles Lyell articulated uniformitarianism in a
    way that soon made it the guiding doctrine of
  • Uniformitarianism holds that
  • the laws of nature have been constant through
    time and that the same processes operating today
    have operated in the past, although not
    necessarily at the same rates

  • The principles of superposition, original
    horizontality, lateral continuity and
    cross-cutting relationships are basic for
    determining relative geologic ages and for
    interpreting Earth history
  • Radioactivity was discovered during the late 19th
    century and lead to radiometric dating, which
    allowed geologists to determine absolute ages for
    geologic events

  • The most accurate radiometric dates are obtained
    from long-lived radioactive isotope/daughter
    pairs in igneous rocks
  • Common pairs include
  • uranium 238 lead 206
  • uranium 235 lead 207
  • thorium 232 lead 208
  • rubidium87 strontium 87
  • potassium 40 argon 40

  • The most reliable radiometric ages are obtained
    using two different pairs in the same rock
  • Carbon 14 dating can be used
  • only for organic matter such as wood, bones, and
  • and is effective back to about 70,000 years
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