1. Geologic Time— Concepts and Principles
Chapter 4
Geologic Time— Concepts 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

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 ( ) 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 Earth’s age was estimated scientifically
Georges Louis de Buffon ( )
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 ( )
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 ( )
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

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 ( )
Werner was an excellent mineralogist, but is best remembered for his incorrect interpretation of Earth history

19. Neptunism Werner’s 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 ( )
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 Hutton viewed Earth history as cyclical erosion
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 Kelvin’s calculation?
Kelvin did not know about radioactivity as a heat source within the Earth

27. Absolute-Dating Methods
The discovery of radioactivity destroyed Kelvin’s argument for the age of Earth and provided a clock to measure Earth’s age
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

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 atom’s 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 element’s 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
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 1:3
= 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 metamorphism—a useful piece of information.
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

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 Lead billion
Uranium Lead million
Thorium Lead billion
Rubidium Strontium billion
Potassium Argon 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 < 70,000 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