The Rock and Fossil Record

1
The Rock and Fossil Record

• Chapter 6
• Earths Story and Those Who First Listened
• Relative Dating Which Came First?
• Absolute Dating A Measure of Time
• Looking at Fossils
• Time Marches On

2
Terms You Must Know

• Fossil
• Trace fossil
• Mold
• Cast
• Index fossil
• Geologic time scale
• Eon
• Era
• Period
• Epoch
• extinction

• Uniformitarianism
• Catastrophism
• Paleontology
• Relative dating
• Superposition
• Geologic column
• Unconformity
• Absolute dating
• Isotope
• Radioactive decay
• Radiometric dating
• Half-life

3
People To Know

• James Hutton
• Charles Lyell
• Georges Cuvier

4
Uniformitarianism

• Developed by James Hutton, advocated by Charles
  Lyell (1797-1875)
• James Hutton wrote Theory of the Earth
• Hutton stated that present-day processes, such as
  erosion and deposition, have operated throughout
  geologic time
• Uniformitarianism is a principle that states that
  geologic processes that occurred in the past can
  be explained by current geologic processes
• 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

5
Siccar Point

• Huttons theories sparked a scientific debate
• In Huttons time, people believed that the Earth
  was only a few thousand years old.
• What Hutton proposed could not happen in just a
  few thousand years
• He formed his theories by observing the geologic
  processes at Siccar Point
• Deposition and folding were observed

6
Unconformity

• Hutton first described the evidence for this
  break in the rock record. He was impressed by a
  placed called Siccar Point in Scotland where some
  of the rocks were laid down horizontally, but the
  rocks underneath them were at an angle. The only
  explanation Hutton felt was plausible was that
  the underlying rocks had originally been laid
  down horizontally, then were tilted and eroded
  and more rocks were then deposited on top of
  them. Hutton realized that this scenario
  represented a large gap in time between the first
  set of rocks being laid down, then tilted, and
  the second set of rocks deposited. These gaps in
  the rock record are called unconformities.

7
Unconformity at Siccar Point
8
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

9
Grand Canyon history revealed
10
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 plant and animal species
• 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

11
Catastrophism

• 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

12
Uniformitarianism vs Catastrophism

• Catastrophism remained the guiding principle
  until the work of Charles Lyell
• 1830-1833 Lyell published Principles of Geology,
  in which he reintroduced uniformitarianism.
• He believed that geologic change happened at the
  same rate in the past as it happens in the
  present-- gradually!

13
Lyell and Darwin

• Charles Darwin and Charles Lyell were good
  friends.
• Darwin accepted and supported uniformitarianism
• Darwin had read Lyells book Principles of
  Geology before his famous 1831 voyage on HMS
  Beagle
• Despite being friends, Lyell did not embrace
  Darwins theories of natural selection.
• Much later, Lyell finally accepted Darwins
  theories.

14
Modern Geology

• Modern scientists like Stephen Gould have
  challenged Lyells uniformitarianism.
• Today scientists beleive that catastrophes do at
  times play an important role in shaping Earths
  history.
• Most geologic change is gradual and uniform but
  catastrophes have caused geologic change.
• Ex. Craters formed due to asteroids and comets

15
Paleontology

• Paleontology is thee scientific study of fossils-
  plant and animal.
• Fossils are the remains of organisms preserved by
  geologic processes.
• Vertebrate paleontologists study fossils of
  animals that have backbones.
• Invertebrate paleontologists study fossils of
  animals that do not have backbones.
• Paleobotanists study fossils of plants

16
Relative Dating

• "Relative Dating" This phrase may conjure up odd
  jokes and images of kissing cousins to some, but
  to geologists the phrase refers to distinguishing
  the age relationships between contiguous rock
  layers.

17
Relative-Dating Principles
Relative dating is any method of determining
whether an event or object is older or younger
than other events or objects.

• Superposition
• Oldest on bottom, youngest on top

Chattanooga Shale, TN
18
Relative Dating

• Relative dating
• Can only be used when the rock layers have been
  preserved in their original sequence top layer
  being the youngest layer
• Helps scientists determine whether one fossil is
  older than the other

19
Disturbing Forces

• Not all rock formations are arranged with the
  oldest layers on the bottom.
• Natural forces can fold, tilt, break, or remove
  parts of the rock layer
• Geologists use a geologic column to help them
• Relative dating assumes that if rock layers are
  not horizontal, then something must have
  disturbed them after they formed.

20
Unconformities

• What is unconformity?
• Unconformity is a break in the geologic record
  created when rock layers are eroded or when
  sediment is not deposited for a long period of
  time.
• A surface of erosion or non-deposition
• Recognizable surface in the rock record

21
Unconformities

• Unconformities involve time gaps, typically on
  the order of tens of millions of years or more. A
  time gap refers to missing time (as in taking a
  vacation). A time gap may be due to a time of
  "nondeposition", meaning that no sediments were
  deposited for an interval of time. More likely,
  unconformities indicate a time when uplift and
  erosion have occurred such that layers deposited
  at an earlier time have been stripped away.
  Typically, unconformities involve
• Major sea level changes
• Major tectonic events

22
Unconformity

• When a geologists finds an unconformity, they
  must question whether the missing layer was
  never present or whether is was somehow removed
• Nondeposition- stopage of deposition when a
  supply of sediment is cut off
• Erosion can create unconformities.

23
3 Types of Unconformity

• Disconformities
• Most common
• Part of a sequence of parallel rock layers is
  missing
• Nonconformities
• Angular unconformities

24
disconformity

• Disconformities are much harder to recognize in
  the field, because often there is no angular
  relationship between sets of layers.
  Disconformities are usually recognized by
  correlating from one area to another and finding
  that some strata is missing in one of the areas.

25
nonconfomity

• Nonconformities occur where rocks that formed
  deep in the Earth, such as intrusive igneous
  rocks or metamorphic rocks, are overlain by
  sedimentary rocks formed at the Earth's surface.
  The nonconformity can only occur if all of the
  rocks overlying the metamorphic or intrusive
  igneous rocks have been removed by erosion.

26
Nonconfomity

• Notice that there is an "intersection" of a
  vertical rock butting up against a horizontal
  sock. In an unconformity, it is two of the same
  type of rock (e.g. sedimentary sedimentary). A
  nonconformity is two layers of different types
  (e.g. igneous sedimentary).

27
Angular unconformity

• Angular unconformities are easy to recognize in
  the field because of the angular relationship of
  layers that were originally deposited
  horizontally.

28
Example of an Unconformity

• Tilted sandstone and siltstone below,
  conglomerate above

www.geology.sdsu.edu/visualgeology/geology101/eros
ion6.htm
29
Dating rocks

• Relative dating
• Using a set of principles to put rocks in their
  proper sequences of formation
• Absolute dating
• Using radioactive decay to determine the exact
  age of rocks

30
Absolute Datingany method of measuring the age
of an event or object in years Most commonbased
on Radioactive Decay
Parent
daughter
Why does it work? 1. The decay rate is CONSTANT,
independent of external conditions in the
earth. 2. The daughter/Parent ratio can be
precisely measured.
31
Radioactive Decay

• The process in which a radioactive isotope tends
  to break down into a stable isotope of the same
  element or another element.
• Sounds great but what is an isotope?
• An isotope is an atom that has the same number of
  protons (atomic ) as other atoms of the same
  element do but that has a different number of
  neutrons (and thus a different atomic mass)

32
Isotopes

• Most isotopes are stable, meaning they stay in
  their original form
• But some are unstable
• Unstable isotopes are radioactive
• Radioactive decay is the process in which a
  radioactive isotope tends to bread down into a
  stable isotope of the same or another element

33
How does it work?What does this have to do with
the age of rocks?

• Unstable isotope is called parent isotope
• The stable isotope produced by radioactive decay
  is the daughter isotope.
• Decay is constant
• The more daughter isotope- the older the rock!

34
Radiometric dating

• A method of determining the age of an object by
  estimating the relative percentages of a
  radioactive (parent) isotope and a stable
  (daughter) isotope
• Ratio or parent material to daughter material

35

• Absolute dating
• Helps scientists determine actual age of fossils
• Rocks near fossils contain radioactive elements
  unstable elements that break down into different
  elements
• Half-life of a radioactive element is the time it
  takes for half of the atoms in a sample to decay
• Scientists compare the amount of radioactive
  element in a sample to the amount of the element
  into which it breaks down
• Scientists use this info to calculate the age of
  the rock, which then tells the age of the fossil

36
Half-life the time required for one-half of the
radioactive nuclei in a sample to decay
37
Dating with carbon-14 (radiocarbon dating)

• Carbon is normally found in three forms stable
  C-12, stable C-14 radioactive C-14
• All combine with oxygen to form CO2
• Half-life of only 5730 years
• Used to date very recent events
• Carbon-14 is produced in the upper atmosphere
• Useful tool for anthropologists, archeologists,
  and geologists who study very recent Earth history

38
Carbon-14

• Carbon-14 is continuously created in the
  atmosphere by cosmic radiation.
• There is one atom of radioactive C-14 for every
  trillion atoms of C-12 in the atmosphere
• Plants absorb C-14 directly through their leaves
  in the form of carbon dioxide
• Animals take in C-14 indirectly when they eat
  plants
• Although C-14 disintegrates at a constant rate,
  it is continuously renewed as long as an organism
  remains alive.
• When an organism dies, it stops absorbing new
  C-14 and its radiocarbon clock is set.

39
Types of radiometric dating

• Potassium-argon method
• K-40 half-life 1.3 billion years
• Decays into argon and calcium
• Used to date rocks older than 100,00 years old
• Uranium-lead method
• U-238 half-life 4.5 billion years
• Decays into lead-206
• Used for rocks more than 10 million years old
• Rubidium-strontium method
• Rb-87 half-life 49 billion years
• Decays into strontium-87
• Used for rocks more than 10 million years old
• Carbon-14 method

40
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

41
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
42
Cross-cutting Relationships

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

Templin Highway, Castaic, California
43
Back to Steno
www.gly.uga.edu/railsback/1121Steno.jpg
44
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

45
Folding
www.hill.anorak.org.uk/dhtml/glgchap5.html
46
Faulting
www.stmarys.ca/academic/science/geology/structural
/faults.html
47
The Map That Changed the World
48
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

49
Geologic Time Scale

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

50
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

51
How was the timescale created?
52
How was the timescale created?

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

53
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

54
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

55
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

56
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

57
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

58
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

59
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

60
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

61
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

62
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

63
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

64
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

65
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

66
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

67
Geometric Radioactive Decay

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

68
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

69
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.

70
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

71
Igneous Crystallization

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

72
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

73
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

74
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.
75
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
76
Mass Spectrometer
www.mines.unr.edu/isotope/gallery.html
77
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

78
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

79
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

80
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

81
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!

82
Tree-Ring Dating Method

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

83
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

84
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

85
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

86
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