1. Geologic Time—Concepts and Principles
Chapter 4
Geologic Time—Concepts and Principles
-How do we tell time in geology?...relative dating, absolute dating
Absolute age dating: 4 methods:
1. Radiometric age dating: parents…daughters….isotopes…
2. Carbon 14 age dating:
3. Tree Rings:
4. Fission track dating
-Basic principles: horizontality, superposition, cross cutting relationships
-unconformities- 3 types….

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. Grand Canyon More than 1 billion years of history are preserved,
like pages of a book,
in the rock layers of the Grand Canyon
Reading this rock book we learn
that the area underwent episodes of
mountain building
advancing and retreating shallow seas
We know these things by
applying the principles of relative dating to the rocks
and recognizing that present-day processes
have operated throughout Earth history

4. What is time? We are obsessed with time, using
clocks
calendars
appointment books
Mostly we don’t have enough of it.
Our common time units are
seconds
hours
days
weeks
months
years
Ancient history involves
hundreds of years
thousands of years
But geologic time involves
millions of years
even billions of years

5. 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

6. 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

7. 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
It tells us how long ago a particular event occurred
giving us numerical information about time
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

8. 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

9. 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
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.

10. 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

11. 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

12. 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

13. Principle of Superposition
Illustration of the principles of superposition
and original horizontality
Superposition: The youngest
rocks are at the top
of the outcrop
and the oldest rocks are at the bottom

14. 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

15. Principle of Horizontality
Illustration of the principles of superposition
and original horizontality
Horizontality: These sediments were originally
deposited horizontally
in a marine environment
This outcrop is Chattanooga Shale, Tennessee

16. Principle of lateral continuity
In 1669, Nicolas Steno proposed
his principle of lateral continuity,
meaning that layers of sediment extend outward
in all directions until they terminate
Terminations may be abrupt
at the edge of a depositional basin
where eroded
where truncated by faults

17. Gradual Terminations or they may be gradual where a rock unit
becomes progressively thinner
until it pinches out
or where it splits into
thinner units
each of which pinches out,
called intertonging
where a rock unit changes
by lateral gradation
as its composition and/or texture
becomes increasingly different

18. Principle of cross-cutting relationships
James Hutton ( )
An igneous intrusion or a fault
must be younger than the rocks
it intrudes or displaces
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.

19. Cross-cutting Relationships
Templin Highway, Castaic, California
A small fault displaces tilted beds.
The fault is younger than the beds.

20. Principle of Inclusions
According to the principle of inclusions,
which also helps to determine relative ages,
inclusions or fragments in a rock
are older than the
rock itself
Light-colored granite
in northern Wisconsin
showing basalt inclusions (dark)
Which rock is older?
Basalt, because the granite includes it

21. Relative-Dating Principles
Principle of fossil succession
discussed later in the term

22. Early Geologic Concepts/Thoughts: 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

23. 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

24. Catastrophism 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
The last one was the biblical flood

25. Neptunism and Catastrophism Were Eventually abandoned
These hypotheses were abandoned because
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

26. Uniformitarianism Principle of uniformitarianism
Present-day processes have operated throughout geologic time.
Developed by James Hutton, advocated by Charles Lyell ( )
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

27. Angular Unconformity at Siccar Point
Hutton explained that
the tilted, lower rocks
resulted from severe upheavals that formed mountains
these were then worn away
and covered by younger flat-lying rocks
the erosional surface
represents a gap in the rock record= Hiatus

28. 3 Types of Unconformities
Unconformities of regional extent
may change from one type to another
They may not represent the same amount
of geologic time everywhere

29. A Disconformity A disconformity between sedimentary rocks
in California, with conglomerate deposited upon
an erosion surface in the underlying rocks

30. A Nonconformity A nonconformity in South Dakota separating
Precambrian metamorphic rocks from
the overlying Cambrian-aged Deadwood Formation

31. Uniformitarianism Hutton viewed Earth history as cyclical
erosion
erosion
Hutton viewed Earth history as cyclical
deposition
uplift
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

32. 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- too Young!!

33. Absolute-Dating Methods: 1. Radiometric Age Dating
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

34. Atoms Understanding absolute dating requires
knowledge of atoms and isotopes
All matter is made up of atoms
The nucleus of an atom is composed of
protons – particles with a positive electrical charge
neutrons – electrically neutral particles
with electrons – negatively charged particles – encircling the nucleus
The number of protons (= the atomic number)
helps determine the atom’s chemical properties
and the element to which it belongs

35. Isotopes Atomic mass number The different forms of an element’s atoms
= 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

36. 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:
In alpha decay, two protons and two neutrons
(alpha particle) are emitted from the nucleus.

37. 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.

38. 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

39. Uranium 238 decay

40. Half-Lives: key to understanding
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

41. 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,
so has a curved graph

42. Geometric Radioactive Decay
In radioactive decay,
during each equal time unit
one half-life,
the proportion of parent atoms
decreases by 1/2

43. 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

44. Determining Age For example: how old is the rock?
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.

45. 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
Because they fit, some radioactive parents
are included in the crystal structure
of certain minerals

46. 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

47. 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.

48. 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

49. 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

50. 2. 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
and the amount of uranium
with older samples having more tracks
This method is useful for samples between 1.5 and 0.04 million years old

51. 3. 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

52. 4. 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

53. Tree-Ring Dating Method
In cross-dating, tree-ring patterns are used from different trees, with overlapping life spans

54. 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

55. 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

56. Summary The principles of superposition, Radioactivity was discovered
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

57. Summary Geologists determine how many half-lives
of a radioactive parent isotope
have elapsed since the sample crystallized
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

58. 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

59. 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

60. Sedimentary Facies On a continental shelf, sand may accumulate
in the high-energy nearshore environment
while mud and carbonate deposition takes place
at the same time
in offshore low-energy environments

61. Uniform Linear Change In this example of uniform linear change,
water is dripping into a glass
at a constant rate

62. Carbon 14 Carbon 14 is constantly forming When a high-energy neutron
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

63. Carbon 14 The carbon 14 becomes While the organism lives
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

64. 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

65. 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.
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.

66. 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

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