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
Smith's map of Scotland, England, and Wales

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

15. Back to Steno

16. 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

17. Folding

18. Faulting

19. Relative Dating
Principles
Unconformities
Deformation

20. Relative Dating – Cross-section

21. Relative Dating – Cross-section
Key: E
Erosion G L C H
Tilting M D J A N K B F

22. Relative Dating – Cross-section

23. Relative Dating – Cross-section
Key: H F D G I
Erosion C A B M L K J

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 ( ) 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 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

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 <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

31. 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

32. History of Historical Geology
Catastrophism
proposed by Georges Cuvier ( )
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 ( )
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
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

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 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

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 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.

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 <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

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 Lead billion
Uranium Lead million
Thorium Lead billion
Rubidium Strontium billion
Potassium Argon billion

57. Mass Spectrometer

58. 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

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 < 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