1. Geologic Time 1

2. The earth needs a time scale
Which geologic event took place first and
when?
Which rock layer is older, and how is earth
history deciphered?
How do we assign actual years to rock layers?
The earth needs a
time scale
Consider: 1 2 ss
Shale (sh) 3
Limestone (LS) 4 5
Sandstone (ss)

3. Historical aspects about geology Catastrophism vs. Uniformitarianism
There are two schools of thought on the geologic history and processes that formed our earth.
Catastrophism vs. Uniformitarianism 4

4. Catastrophism: (mid-1600’s)
powerful geologic events that shape the earth in a single incident
Volcanic eruptions
Earthquakes
Massive floods
Landsliding 5

5. Catastrophism: (mid-1600’s)
published by Anglican Archbishop, James
Ussher
determined that earth was only a few
thousand years old – created in 4004 BC
suggested that earth landscapes are
fashioned by great catastrophes
an attempt to fit the formation of earth
features into a short amount of time
(6000 years – Biblical philosophy) 6

6. Uniformitarianism – Birth of Modern Geology
“The present is the key to the past.”
Uniformitarianism states:
Physical, chemical, and biological laws that
operate today have also operated in the
geologic past.
Proposed by James Hutton – late 1700’s
argued using the “rock cycle” concept
argued using earth processes that can
be observed
What is required? TIME 7

7. The Uniformitarianism philosophy
Do geologic processes act slowly or rapidly?
How many catastrophic events take place/day?
When was the last major volcanic eruption?
When was the last major earthquake?
How long does it take a river to carve a canyon?
How fast are the continents moving?
Do you consider yourself a catastrophist or a uniformitarianist? 8

8. Taking Uniformitarianism literally – Problem with “U”
Rates and intensities of geologic processes change over time.
Example:
10,000 years ago, large land masses were covered in ice.
Different type of geologic environment than today
Different intensity
Different rates of erosion
Given the concept of Uniformitarianism, would you consider the earth to be very OLD or very YOUNG? 9

9. I my earth science class. Discuss with a friend:
Describe the differences between
catastrophism and uniformitarianism.
2. Provide at least 2 examples each of
3. Identify “problems” with both philosophies.
I will get an A on my exams and quizzes. 10

10. Relative Dating - placing the geologic occurrence
in the proper sequence
Which came first and WHY?
To construct a “relative” time scale, rules were
established (principles of relative dating).
Nicholas Steno ( )
Principle of Original Horizontality
Law of Superposition
Principle of Cross-Cutting Relations
Principle of Inclusions 11

11. 12 The Principle of Original Horizontality:
Let’s unravel some geologic history from observations of various formations
and their contacts.
Nicholas Steno – 1669 proposed the following relative dating principles:
The Principle of Original Horizontality:
Sedimentary rock layers are deposited as horizontal strata.
Any observed non-horizontal strata have been disturbed.
Sediment input C B
basin A 12

12. Original Horizontal Strata
Limestone (ls)
Shale (sh)
Sandstone (ss)
granitic rock 13

13. The Principle of Superposition
In any undisturbed sequence of strata, the oldest stratum is at the bottom of the sequence, and the youngest stratum
is on top.
Unit 1 = old
Unit 5 = young 5 4 3 2 1 14

14. Which strata is older? 5 4 3 2 1
youngest 5 4
oldest 3 2 1 15

15. The principle of Cross-Cutting Relationships
Any geologic feature that cuts across another geologic feature is younger. 5
Unit 1 = older
Unit 6 = youngest 4 3 2 6 1
Which came first:
Unit 5 or Unit 6? 16

16. Which is older, the fault Which is younger, the dike
or volcanic layer?
Which is younger, the dike
or country rock?
fault
dike
Volcanic layer
What type of
fault is this?
Normal
country rock
Determine the relative age of the two dikes. 1 2 17

17. 18 The Principle of Inclusions
A piece of rock (clast) that has become “included”
in another rock body is older than the rock body
it has become part of – why?
Rock body A A A A
Older (Rock A was there first.)
Intrusion of pluton B 18

18. Which “granites” are older and younger?19

19. Which rock body is older?:A ? ? C
Can you identify the inclusions
found in this Sierra Nevada
Mountain batholitic material? 20

20. Superposition 21 Original Horizontality Principle of Inclusions
Youngest
Superposition
Oldest
Principle of
Inclusions
Cross-Cutting Relationship
Which granite is older?
Older A B C
Asp Vn
Younger 21

21. this earth science class.
Discuss with a friend:
4. Explain the concept of relative dating.
5. Draw a diagram, and explain each of the
following dating principles:
Original Horizontality
Superposition
Cross-Cutting Relations
Inclusion Principle
I will get an A on my exams and quizzes. 22

22. Ok – given the principles, what is wrong with
this stack of rock (strata)
youngest 7 6 5 3 2 1
oldest
Missing time – or does time really stop? 23

23. 24 The principle of Unconformities Igneous or metamorphic rock
rock surface that represents a period of erosion or non- deposition
referred to as “missing time”
three major types of unconformities:
disconformity
angular unconformity
non-conformity
disconformity – unconformity in non-disturbed
sedimentary layers
angular unconformity – uncon. lies between angled
strata and overlying
horizontal strata
non-conformity – sedimentary strata
overlies crystalline
rocks (ig and met)
Unconformity
Igneous or metamorphic rock 24

24. 25 disconformity angular unconformity nonconformity Sedimentary rocks
Xln rocks 25

25. this earth science class.
6. Explain what an unconformity is and
what it represents.
7. Diagram pictures that represent the
three types of unconformities.
Discuss with a friend
I will get an A on my exams and quizzes 26

26. How do we get a fossil? – preservation of past life
Fossils – evidence of past life or “time pieces,”
the remains or traces of prehistoric life
Paleontology – study of fossils
How do we get a fossil? – preservation of past life
2 conditions must exist for preservation
rapid burial
possession of hard parts
Prehistoric bug
Rapid burial of
sediment covers
the bug – fossil
Bug dies
Bug soft parts are
eaten or dissolve 27

27. How do fossils help scientists relatively date layers
Fossils – evidence of past life or “time pieces,”
the remains or traces of prehistoric life
Preservation of fossils
Small percentage of fossils preserved
throughout geologic time – WHY?
Most organisms composed of soft parts.
Organisms with hard parts and within
a sedimentary environment are favored.
Very rare to see vast array of other life
forms
How do fossils help scientists relatively date layers
of rock (strata)? 28

28. William Smith – Principle of Fossil Succession
Fossil organisms succeed one another in a
definite and determinable order, and therefore
any time period can be recognized by its fossil
content.
“Fossils are arranged according to their age by
using the law of superposition.”
Fossil succession:
allows geologists to age date wide geographical
areas
documents the evolution of life
Age of mammals
Age of reptiles
Age of fish
Youngest
Oldest 29

29. How do fossils help date rocks?
1200 miles 7 7 6 6
Disconformity 5 4 3 3 2 2
Which fossils are the youngest
and oldest? 1 30

30. 31

31. this earth science class.
8. Give 2 reasons why many organisms are
are not fossilized.
9. Explain the law of fossil succession and
how this law allows dating of strata.
10. How has fossil succession helped
geologists unravel earth history?
Discuss with a friend:
I will get an A on my exams and quizzes.

32. OK – We have relative dating and fossils – How do
we get “absolute” ages on the rocks (numbers)?
Radiometric dating – applying a number
radioactive atoms (isotopes) decay at a
constant rate over time 33

33. The time of decay can be measured.
Radioactive decay of an
unstable isotope atom + + + +
Decay process + + +
Pb206 (lead) + +
U238 (Uranium) + +
The time of decay can be measured.
Isotope decay does not vary under various
weathering conditions.
Isotopes decay at a fixed rate.
One isotope will decay into another isotope. 34

34. Stable Daughter Product Currently Accepted Half-Life Values
How does radiometric dating work, and where does the
age (number) come from? 35
Terms:
Parent element: the “beginning” element that contains 100%
of radioactive particles
Daughter element: the element that the parent element decays
into (or turns into over time)
Half life: the time required for ½ of the parent to decay into
the daughter element
Parent Isotope
Stable Daughter Product
Currently Accepted Half-Life Values
Uranium-238
Lead-206
4.5 billion years
Uranium-235
Lead-207
704 million years
Thorium-232
Lead-208
14.0 billion years
Rubidium-87
Strontium-87
48.8 billion years
Potassium-40
Argon-40
1.25 billion years
Samarium-147
Neodymium-143
106 billion years

35. U-235 Pb 207 704 m.y. 1.4 b.y. 2.1 b.y. 36 U-3 Daughter element 1/2
1 half life = 704 million years
Daughter
element
1/2
1/4
1/8
704 m.y.
1.4 b.y.
2.1 b.y.
Parent
element 36

36. this earth science class.
11. Specifically define the differences
between relative and absolute dating
techniques.
12. Define the following absolute dating
terms parent/daughter elements, half-life
13. Explain how the half-life is used to
calculate an absolute age.
I will get an A on my exams and quizzes. 37

37. What is the importance of radiometric dating?
produced thousands of dates for earth
events
rocks have been dated at more than 3 b.y.
granite in South Africa dated at 3.2 b.y.
granite contains inclusions of quartzite
quartzite inclusions must be older
Acasta gneiss in Northern Canada – 4.0 b.y.
Earth believed to be 4.55 (4.6) b.y. old
Radiometric dating:
vindicated the ideas of Hutton, Darwin, and others
consistent with relative dating techniques
allowed “absolute” dating on the Geologic Time Scale 38

38. Relative dating + Absolute dating
Lets make a Geologic Time Scale
Relative dating + Absolute dating
The Geologic Time Scale:
It combines both relative and absolute dating
Created during the nineteenth century in Western Europe
and Great Britain
Sub-divides the 4.6 billion-year-history of the earth
Eons
Eras
Periods
Epochs 39

39. Precambrian Building the Geologic Time Scale 40 Phanerozoic
“visible life”
fossil record becomes more
detailed
animals have hard shells
and skeletons
Building the
Geologic Time Scale
Proterozoic
Multi-celled, soft body
organisms
“early life”
Precambrian
Archean
Single cell life developed
most “ancient” rocks found
preserved rocks at the base
of the Archean
Hadean
represents the earth’s
time of formation
no rocks are represented
“hellish” conditions

40. 41 Cenozoic Era Mesozoic Era Paleozoic Era birds and mammals
flourished
appearance of man
Mesozoic Era
marks the rise in dinosaurs
dominant vertebrates
first flowering plants
first shrew-like
mammals
Paleozoic Era
known as ancient life
life progressed from marine
invertebrates to fish,
amphibians, and reptiles

41. 42 Periods based on: Cretaceous, Jurassic, Triassic Cambrian period
fossil types
massive extinctions
geographical locations
characteristics of strata
Cretaceous, Jurassic, Triassic
age of reptiles
dinosaurs dominant
massive dinosaur extinction
at 65 m.y. –Cretaceous
“Jurassic Park”
Cambrian period
animals with hard shells
diversification of life
“the Cambrian explosion”

42. 43 Epochs Age of Reptiles Age of fish Invertebrates
not defined by extinction
events, but % of fossils
still living
plants and animals found
in the Pliocene epoch
have living species today
Eocene-few species
surviving today
Holocene
human’s time
Age of
Reptiles
Amphibians
Age of fish
Invertebrates
How accurate is the
Geologic Time Scale?

43. the Geologic Time Scale.44 I
the Geologic Time Scale.
14. You should be able to draw the Geologic
Time Scale and label it with the following:
Eons, Eras, Periods, and Cenozoic/
Tertiary epochs.
15. List major characteristics of each
period.
16. How did the strength of both absolute and
relative dating techniques contribute the
development of the geologic time scale?

44. Cenozoic, Mesozoic, Paleozoic
The Geologic Time Scale – How much of Earth history is
represented?
Geologic Time Scale
Cenozoic, Mesozoic, Paleozoic
Eras
12%
88%
Precambrian Eon 45

45. Difficulties in dating the Geologic Time Scale
Not all rocks can be dated radiometrically.
all minerals must contain 100% parent atoms.
Sedimentary rocks can only rarely be dated.
some parent atoms come from pre-existing rocks
that have been weathered and transported.
sedimentary rocks are dated in proximity of
igneous bodies.
Metamorphic rocks are challenging.
some minerals do not necessarily represent the
time when the rock was formed 46

46. How is the age of the Earth determined?
Why is it difficult to determine the age
of the earth? (think rock cycle)
The external and internal forces constantly
recycle earth material, obliterating rock clues to
the earth’s past.
What evidence suggests a 4.6 b.y. old earth?
Precambrian rocks (Acasta gneiss, northern
Canada) date at 4.0 billion years.
Mineral grain found in sedimentary rock
(Australia) dates at 4.4 billion years.
What does the mineral grain in a
sedimentary rock indicate about the
4.4 b.y. age relationship? 47

47. Acasta gneiss, northern Canada
known as the Acasta gneiss complex
dated at the Hadean Eon (4.0 billion years old)
part of the Canadian Slave craton 48

48. Evidence from space to age date the earth-- moon dust and meteorites:
moon dust from Apollo astronauts dated at
4.55 billion years
the Allende Meteorite:
a carbonaceous chondrite meteorite that was found in
Chihuahua, Mexico, 1969
contains unaltered material from the formation of
the solar system
composed of tiny amounts of carbon that form the
compounds of amino acids (essential for life)
age dating of this and other meteorites is around
based on earth rocks and interstellar space objects,
earth is believed to be around 4.6 billion years old. 49

49. Allende Meteorite, Chihuahua, Mexico, Feb. 8, 1969
unaltered material from our
solar system
contains carbon (3 parts/1000)
some carbon compounds in
the form of amino acids
dark areas – olivine with
trace amounts of iron and
carbon
calcium and aluminum oxide
compounds
first matter to form during
solar system formation
older than earth
carbonaceous chondrite 50

50. I what ES can do for me. Discuss with a friend:
17. Identify at least 2 problems with the
accuracy of the Geologic Time Scale.
18. Why does the Geologic Time Scale only
represent about 12% of the earth’s geologic
history (assuming the earth is 4.6 b.y. old)?
I will get an A on my exams and quizzes. 51