Geologic Time Concepts and Principles

1
Chapter 4
Geologic TimeConcepts and Principles
2
Grand Canyon

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

3
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 hundreds of years geologists have been using
  relative dating to establish a relative geologic
  time scale

4
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

5
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

6

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
7
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.

8
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

9
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

10
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

11
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

12

• Illustration of the principles of original
  horizontality

13

• Illustration of the principles of superposition

14
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

15
Relative-Dating Principles

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

16
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.

17
Cross-cutting Relationships

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

• The fault is younger than the beds.

18
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

19
Neptunism

• Werners 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

20
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

21
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
• 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

22
Uniformitarianism

• 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

23
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

24
Uniformitarianism
erosion

• Hutton viewed Earth history as cyclical

uplift
deposition

• 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

25
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

26
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

27
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

28
Atoms

• 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

29
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

30
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.

31
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.

32
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

33
Uranium 238 decay
34
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

35
Uniform Linear Change

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

36
Geometric Radioactive Decay

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

37
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

38
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.

39
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 minerals

40
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

41
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
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

43
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.

44
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 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

45
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.
46
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
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
47
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

48
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

49
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

50
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

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 can
  be correlated from tree to tree
• a procedure called cross-dating
• The tree-ring time scale now extends back 14,000
  years

52
Tree-Ring Dating Method

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

53
Summary

• 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

54
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

55
Summary

• 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

56
Summary

• 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

57
Summary

• 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
  shells
• and is effective back to about 70,000 years