When looking down into the Grand Canyon, we are really looking all the way back to the early history of Earth

1
Grand Canyon

• When looking down into the Grand Canyon, we are
  really looking all the way back to the early
  history of Earth

Chapter 4 Geologic TimeConcepts and Principles
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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

3
Concept of Geologic Time

• Relative Dating putting rock layers and events
  in order relative to when they occurred.
• Absolute dating which results in specific
  numerical dates for rock units or events
• Such dates are calculated from the natural rates
  of decay of various natural radioactive elements
  present in trace amounts in some rocks

4

Geologic Time Scale -- today

• The discovery of radioactivity near the end of
  the 1800s allowed absolute ages to be accurately
  applied to the relative geologic time scale
• The most recent geologic time scale model is a
  dual scale
• a relative scale and an absolute scale

Fig. 4-1, p. 62
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Changes in the Concept of Geologic Time

• Attempts to give an age to the earth
• James Usher (1581-1665) in Ireland
• calculated the age of Earth based on genealogies
  in Genesis
• Announced that Earth was created on
• October 22, 4004 B.C.
• A century later it was still considered heresy to
  say Earth was more than about 6000 years old.

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Changes in the Concept of Geologic Time

• Georges Louis de Buffon (1707-1788)
• calculated how long Earth took to cool gradually
  from molten iron balls
• Earth about 75,000 years

Others calculated the rate of sediment
deposition Also the rate of salt build-up in the
oceans from the continental rivers Ages in
millions to billions of yearsIn 1953, the
dating of meteorites was accomplished. Age
4.5 by
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Relative-Dating Principles

• Six fundamental geologic principles are used
  today in relative dating
• 1. 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
• (note Steno lived contemporaneously with Usher)
• This method is used for determining the relative
  age of rock layers (strata) and the fossils they
  contain

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• Illustration of the principles of superposition

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Relative-Dating Principles

• 2. 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

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• Illustration of the principles of original
  horizontality

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Principle of Lateral ContinuityNicholas
StenoSediment extends laterally in all
direction until it thins and pinches out or
terminates against the edges of the depositional
basin
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Principle of Cross-Cutting Relationships James
Hutton (1726-1797An igneous intrusion or a fault
event must be younger than the rocks it intrudes
or cuts across
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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.

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Cross-cutting Relationships

• Templin Highway, Castaic, California
• A small fault displaces tilted beds.

• The fault is younger than the beds.

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Principle of inclusionsThat which is included is
_________(older? Younger?)
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Principle of inclusions
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• Principle of
• fossil succession

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Catastrophism

• 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
• (also relatively modern, and built on Ushers
  Biblical age of the Earth)

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Uniformitarianism

• Principle of uniformitarianism
• Present-day processes have operated throughout
  geologic time. This includes the physical,
  chemical and biological processes
• 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

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Unconformity at Siccar Point 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
21
Uniformitarianism
erosion

• Hutton viewed Earth history as cyclical

uplift
deposition

• Old Earth 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

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Sequence of Events
Key to Rock Types
23
Unconformities 3 Types
1
2
3
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Using Radioactive Decay to obtain numerical age

• Understanding absolute dating requires knowledge
  of atoms and isotopes
• The nucleus of an atom is composed of
• protons particles with a positive electrical
  charge
• 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

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Isotopes

• 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
  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 -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.

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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.

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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

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Uranium 238 decay
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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
• Can vary from less than 1/billionth of a second
  to 49 billion years
• Is geometric not linear, so has 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

32
Geometric Radioactive Decay

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

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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.

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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 previously
  formed daughter atoms
• Some radioactive parents are included in the
  crystal structure of certain mineralsDating of
  sedimentary rocks RARE dating the mineral
  glauconite, because it forms in certain marine
  environments as a reaction with clay during the
  formation of the sedimentary rock

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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.

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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
  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.
• The most reliable dates use multiple methods.

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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
  information.
• 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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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.
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.
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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
• Such isotopes include
• Parents Daughters Half-Life (years)

Uranium 238 Lead 206 4.5 billion Uranium
235 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
• the amount of uranium

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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 70,000
  years old
• It is not useful for most rocks,
• but is useful for archaeology
• and young geologic materials

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Carbon 14

• Carbon 14 is constantly forming in the upper
  atmosphere
• 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

43
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

44
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 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

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Summary

• 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
  geology
• 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