William Smith, 1815

1
The Map That Changed the World

• William Smith, 1815
• Geologic map of England, Wales, part of Scotland

2
The Map That Changed the World
3
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

4
Geologic Time Scale

• Large divisions based on?
• Paleozoic Era
• Mesozoic Era
• Cenozoic Era

5
Geologic Time Scale

• Large divisions based on characteristics of
  fossils
• Paleozoic Era early life dominated by
  invertebrate animals
• Mesozoic Era middle life
• Cenozoic Era recent life

6
How was the timescale created?
7
How was the timescale created?

• Mapping in 1800s using the principles of
• Superposition
• Original Horizontality
• Original Lateral Continuity
• Cross-cutting relationships
• Also Fossil Correlation

8
Relative-Dating Principles

• Superposition
• Oldest on bottom, youngest on top

• Original Horizontality
• Sediment originally deposited in flat parallel
  layers

Chattanooga Shale, TN
9
Relative-Dating Principles

• Lateral continuity
• sediment extends laterally in all directions
  until it thins and pinches out or terminates
  against the edges of the depositional basin
• Cross-cutting relationships
• an igneous intrusion or a fault must be younger
  than the rocks it intrudes or displaces

10
Cross-cutting Relationships

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

North shore of Lake Superior, Ontario Canada
11
Cross-cutting Relationships

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

Templin Highway, Castaic, California
12
Unconformities

• What is an unconformity?

13
Unconformities

• What is an unconformity?
• A surface of erosion or non-deposition
• Recognizable surface in the rock record

14
Example of an Unconformity

• Tilted sandstone and siltstone below,
  conglomerate above

www.geology.sdsu.edu/visualgeology/geology101/eros
ion6.htm
15
Back to Steno
www.gly.uga.edu/railsback/1121Steno.jpg
16
Why are layers tilted?

• Deformation of rocks
• Occurs after they are deposited
• Important factor in relative dating
• Folding
• Anticlines, synclines
• Rock bends, but does not break
• Faulting
• Normal, reverse, transform
• Rock breaks

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Folding
www.hill.anorak.org.uk/dhtml/glgchap5.html
18
Faulting
www.stmarys.ca/academic/science/geology/structural
/faults.html
19
Relative Dating

• Principles
• Unconformities
• Deformation

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Relative Dating Cross-section
http//facweb.bhc.edu/academics/science/harwoodr/G
eol101/labs/dating/
21
Relative Dating Cross-section
Key E Erosion G L C H Tilting Erosion M D J A Ero
sion N K B Tilting Erosion F Erosion
http//facweb.bhc.edu/academics/science/harwoodr/G
eol101/labs/dating/
22
Relative Dating Cross-section
http//facweb.bhc.edu/academics/science/harwoodr/G
eol101/labs/Dating2/Index.htm
23
Relative Dating Cross-section
Key H F D G I Erosion C Erosion A B M L K Erosion
J
http//facweb.bhc.edu/academics/science/harwoodr/G
eol101/labs/Dating2/Index.htm
24
Grand Canyon history revealed
25
Grand Canyon

• More than 1 billion years of history are
  preserved in the rock layers of the Grand Canyon
• Reading this rock book shows
• periods of mountain building
• advancing and retreating shallow seas
• evolution of faunas
• Determine these things by
• applying the principles of relative dating to the
  rocks
• and recognizing that present-day processes have
  operated throughout Earth history -
  Uniformitarianism

26
Absolute Dating

• Radiometric dating is the most common method of
  obtaining absolute ages
• calculated from the natural rates of decay of
  various natural radioactive elements present in
  trace amounts in some rocks
• Other methods?
• Tree ring counting
• Varves
• Ice cores

27
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

28
Changes in the Concept of Geologic Time

• The concept and measurement of geologic time has
  changed through human history
• James Ussher (1581-1665) in Ireland
• calculated the age of Earth based on recorded
  history and genealogies in Genesis
• he 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

29
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
• used melted iron balls of various diameters
• he estimated Earth was 75,000 years old

30
Changes in the Concept of Geologic Time

• Others used 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
• Or 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

31
History of Historical Geology

• Neptunism
• proposed in 1787 by Abraham Werner (1749-1817)
• all rocks, including granite and basalt, were
  precipitated in an orderly sequence from a
  primeval, worldwide ocean
• Werner was an excellent mineralogist, but is best
  remembered for his incorrect interpretation of
  Earth history

32
History of Historical Geology

• 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, he last one
  was the biblical flood

33
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

34
Uniformitarianism

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

35
Unconformity at Siccar Point
36
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

37
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

38
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

39
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

40
Absolute-Dating Methods

• Understanding absolute dating requires knowledge
  of atoms and isotopes we have it!
• Atomic mass number
• number of protons number of neutrons
• Isotopes different numbers of neutrons
• 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

41
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

42
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

43
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

44
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

45
Uranium 238 decay
46
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

47
Half-Lives

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

48
Geometric Radioactive Decay

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

49
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

50
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, how old is
  the rock?

• 25 means it is 2 half-lives old.

• the rock is 57 x 2 114 million years old.

51
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
• they fit differently into the crystal structure
  of certain minerals
• Geologists can use the crystals containing the
  parents atoms to date the time of crystallization

52
Igneous Crystallization

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

53
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

54
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

55
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.
56
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)

Most of these are useful for dating older rocks
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
57
Mass Spectrometer
www.mines.unr.edu/isotope/gallery.html
58
How do we know the Earth cant be older than
about 6-7 b.y.?

• Moderate half-life isotopes (1 b.y.)
• If Earth was gt 6-7 b.y. old, there wouldnt be
  many parents left

59
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

60
Carbon 14

• Carbon 14 is constantly forming in the upper
  atmosphere
• when a high-energy neutron, a type of cosmic ray
  , strikes a nitrogen 14 atom it may be absorbed
  by the nucleus and eject a proton changing it to
  carbon 14
• The 14C formation rate
• is fairly constant
• and has been calibrated against tree rings

61
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

62
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!

63
Tree-Ring Dating Method

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

64
Summary

• 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

65
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

66
Summary

• Half-life is the length of time it takes for
  one-half of the radioactive parent isotope to
  decay to a stable daughter isotope of a different
  element
• The most accurate radiometric dates are obtained
  from long-lived radioactive isotope/daughter
  pairs
• in igneous rocks

67
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