1. The Rock and Fossil Record
Chapter 6
Earth’s 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 Uniformitarianism 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 ( )
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 Hutton’s theories sparked a scientific debate
In Hutton’s 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
erosion
deposition
uplift
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
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 Lyell’s book Principles of Geology before his famous 1831 voyage on HMS Beagle
Despite being friends, Lyell did not embrace Darwin’s theories of natural selection.
Much later, Lyell finally accepted Darwin’s theories.

14. Modern Geology
Modern scientists like Stephen Gould have challenged Lyell’s uniformitarianism.
Today scientists beleive that catastrophes do at times play an important role in shaping Earth’s 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 Nonconformities
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

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 Dating: any method of measuring the age of an event or object in years Most common:based 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
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

44. Why are layers tilted? Deformation of rocks Folding Faulting
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

46. Faulting

47. The Map That Changed the World
Smith's map of Scotland, England, and Wales

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 ( ) 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 Earth’s age was estimated scientifically
Georges Louis de Buffon ( ) 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 <1 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 ( )
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 Kelvin’s argument for the age of Earth
Radioactivity is the spontaneous decay of an atom’s 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 1:3  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 <0.5% 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 Lead billion
Uranium Lead million
Thorium Lead billion
Rubidium Strontium billion
Potassium Argon billion

76. Mass Spectrometer

77. How do we know the Earth can’t be older than about 6-7 b.y.?
Moderate half-life isotopes (1 b.y.)
If Earth was > 6-7 b.y. old, there wouldn’t 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 < 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