Understanding Geologic Time

1
Understanding Geologic Time
The map that changed the world by William Smith
(1815) links fossils rock patterns
3Dapproach
2
Grand Canyon history revealed
3
Grand Canyon

• Preserves more than 1 billion years of history
• This rock book shows
• mountain building
• advancing and retreating seas
• evolution of faunas
• Determine these things by
• applying the principles of relative dating to the
  rocks
• Uniformitarianism

4
Concepts of Geologic Time

• Two frames of reference
• Relative dating describes sequential order
• Absolute dating timing of events in years
  before present

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Relative Geologic Time Scale

• The relative geologic time scale has a sequence
  of
• eons
• eras
• periods
• epochs
• but no numbers indicating how long ago each of
  these times occurred, just the order of
  occurrence

6
Absolute Dating

• - specific dates for rock units or events
• expressed in years before the present
• gives us numerical information about events

7
Absolute Dating
Radiometric dating is the most common method of
obtaining absolute ages calculated from the
rates of decay of various natural radioactive
elements present in trace amounts in some rocks
Other methods
tree ring counting varves (layers year
sediment accumulations) ice (count layers of
ice for annual scale)
8
Geologic Time Scale

• Radioactivity (late 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

9
Changes in the Concept of Geologic Time

• James Ussher (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.
• widely accepted

http//star.arm.ac.uk/history/USSHER.GIF
10
Changes in the Concept of Geologic Time

• Georges Louis de Buffon (1707-1788) calculated
  how long Earth took to cool gradually from a
  molten beginning
• used melted iron balls of various diameters
• he estimated Earth was 75,000 years old
• considered an "old Earth!"

http//www.nceas.ucsb.edu/alroy/lefa/Buffon.jpg
11
Changes in the Concept of Geologic Time

• Rates of deposition of various sediments and
  thickness of sedimentary rock in the crust
• gave estimates of lt1 million
• to more than 2 billion years
• 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

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Relative-Dating

• Six fundamental geologic principles
• 1) Superposition
• 2) Original horizontality
• 3) Lateral continuity
• 4) Cross-cutting relationships
• 5) Inclusions
• 6) Fossil succession

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Relative-Dating Principles

• 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

http//www.science.siu.edu/zoology/king/304/biogrp
hy.htm
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Relative-Dating Principles

• Principle of original horizontality
• Nicolas Steno
• sediment is deposited in essentially horizontal
  layers
• a sequence of sedimentary rock layers that is
  steeply inclined from horizontal must have been
  tilted after deposition and lithification

• Principle of lateral continuity
• Sediment extends laterally in all directions
  until it thins and pinches out or terminates
  against the edges of a basin (also Steno)

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Relative-Dating Principles

• Horizontality
• sediments were originally deposited horizontally
  in a marine environment
• Superposition
• old to young

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Relative-Dating Principles

• 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

http//www.physicalgeography.net/fundamentals/10c.
html
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Cross-cutting Relationships

• A dark-colored dike has intruded into older light
  colored granite the dike is younger than the
  granite

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Cross-cutting Relationships

• A small fault displaces tilted beds the fault is
  younger than the beds.

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Relative-Dating Principles

• Principle of inclusions
• discussed later in the term
• Principle of fossil succession
• discussed later in the term

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History of Historical Geology

• 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

http//de.wikipedia.org/wiki/Abraham_Gottlob_Werne
r
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History of Historical Geology

• Catastrophism
• proposed by Georges Cuvier (1769-1832)
• dominated European geologic thinking
• the history of Earth resulted from a series of
  sudden widespread catastrophes which 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

http//search.eb.com/dinosaurs/dinosaurs/ocuvier00
1p1.html
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History of Historical Geology

• Neptunism and Catastrophism were eventually
  abandoned
• they were not supported by field evidence
• basalt was shown to be of igneous origin
• volcanic rocks interbedded with sedimentary
• 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

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Uniformitarianism

• Present-day processes have operated throughout
  geologic time
• Developed by James Hutton
• Advocated by Charles Lyell
• (1797-1875)
• term uniformitarianism was
• coined by William Whewell in 1832

http//www.stephenjaygould.org/people/charles_lyel
l.html
http//cepa.newschool.edu/het/profiles/whewell.htm
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Unconformity at Siccar Point
Hutton applied the principle of
uniformitarianism when interpreting rocks We
now call what he observed an unconformity .
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Uniformitarianism

• Hutton viewed Earth history as cyclical

• He also understood that geologic processes
  operate over a vast amount of time
• Modern view of uniformitarianism
• geologists assume that the principles or laws of
  nature are constant
• but the rates and intensities of change have
  varied through time

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Crisis in Geology

• Lord Kelvin (1824-1907)
• knew about high temperatures inside of deep mines
  and reasoned that Earth is 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

http//www.energyquest.ca.gov/scientists/kelvin.ht
ml
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Crisis in Geology

• This age was too young for the geologic processes
  envisioned by other geologists at that time
• Kelvin did not know about radioactivity as a heat
  source within the Earth

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Absolute-Dating Methods

• The discovery of radioactivity
• destroyed Kelvins argument for the age of Earth
• 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

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Absolute-Dating Methods

• Understanding absolute dating requires knowledge
  of atoms and isotopes
• Atomic mass number
• number of protons number of neutrons
• Isotopes different numbers of neutrons, same
  number of protons
• Different isotopes 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

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Radioactive Decay

• Radioactive decay is the process whereby an
  unstable atomic nucleus spontaneously changes
  into an atomic nucleus of a different element
• Three types of radioactive decay
• alpha decay, two protons and two neutrons (alpha
  particle) are emitted from the nucleus

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Radioactive Decay

• beta decay, a neutron emits a fast moving
  electron (beta particle) and becomes a proton

• electron capture decay, a proton captures an
  electron and converts to a neutron

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Radioactive Decay

• Some isotopes undergo only one decay step before
  they become stable.
• rubidium 87 decays to strontium 87 by a single
  beta emission
• potassium 40 decays to argon 40 by a single
  electron capture

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Radioactive Decay

• Other isotopes undergo several decay steps
• 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

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Age Dating with Half-Lives

• 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

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Half-Lives

• The length of half-lives for different isotopes
  of different elements can vary from
• lt 1/1000000000 of a second
• up to 49 billion years
• Radioactive decay
• is geometric not linear
• a curved graph

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Uniform Linear Change

• In this example of uniform linear change, water
  is dripping into a glass at a constant rate

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Geometric Radioactive Decay
During each half-life, the proportion of parent
atoms decreases by 1/2
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Determining Age

• By measuring the parent/daughter ratio and
  knowing the half-life of the parent, 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

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Determining Age

• For example
• If a rock has a parent/daughter ratio of 13 ,
  the remaining parent proportion is 25
• 25 2 half lives

• If half life is 57 milliion years then the rock
  is 57 million years x 2
• 114 million years old

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What Materials Can Be Dated?

• Most radiometric dates are obtained from igneous
  rocks
• As magma cools and crystallizes, radioactive
  parent atoms separate from daughter atoms
• Parent and daughter fit differently into the
  crystal structure of certain minerals
• Geologists can use the crystals containing the
  parent atoms to date the time of crystallization

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Igneous Crystallization

• Crystallization of magma separates parent atoms
  from previously formed daughters
• This resets the radiometric clock to zero
• Then the parents gradually decay

42
Sedimentary Rocks

• Generally, sedimentary rocks cannot be
  radiometrically dated
• the date obtained would correspond to the time of
  crystallization of the mineral, not the time that
  it was deposited as a sedimentary particle

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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 (to other parts of the rock) 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.
Dating the whole rock yields a date of 700
million years time of crystallization.
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Sources of Uncertainty

• 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
• it partially resets the radiometric clock and the
  age will be too young
• If parents escape, the date will be too old
• Most reliable dates use multiple methods

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Sources of Uncertainty

• Dating techniques are always improving.
• Presently measurement error is typically lt0.5
  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

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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
• 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
billion
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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
• and the amount of uranium
• with older samples having more tracks

• This method is useful for samples between 1.5 and
  0.04 million years old

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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 lt 50,000 years old

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Carbon 14

• Carbon 14 is constantly forming in the upper
  atmosphere
• The 14C formation rate
• is fairly constant
• and has been calibrated against tree rings

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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
• when it dies the carbon 14 begins to decay
  without being replenished
• Thus, carbon 14 dating measures the time of death

51
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!

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Tree-Ring Dating Method

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