Geologic Time

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Geologic Time
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• Goals for understanding geologic time
• Relative dating vs. Absolute dating
• Relative dating principles how
• geologists date layers of rock
• The dynamics of fossils
• Techniques of radiometric dating
• Why the earth is believed to be 4.6 b.y.
• The Geologic Time Scale
• what problems exist with the
• geologic time scale

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• 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?

Geology needs a Time Scale
Consider
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ss
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Shale (sh)
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Limestone (LS)
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Sandstone (ss)
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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 (1636-1686)
• Principle of Original Horizontality
• Law of Superposition
• Principle of Cross-Cutting Relations
• Principle of Inclusions

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Lets 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
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Original Horizontal Strata
Limestone (ls)
Shale (sh)
Sandstone (ss)
granitic rock
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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
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4
3
2
1
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Which strata is older?
youngest
5 4 3 2 1
5
4
oldest
3
2
1
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• The principle of Cross-Cutting Relationships
• Any geologic feature that cuts across another
  geologic feature is younger.

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Unit 1 older Unit 6 youngest
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3
2
6
1
Which came first Unit 5 or Unit 6?
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Which is older, the fault or volcanic layer?
Which is younger, the dike or country rock?
fault
dike
Volcanic layer
What type of fault is this?
country rock
Normal
Determine the relative age of the two dikes.
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2
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• 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
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Which granites are older and younger?
OLDER
YOUNGER
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Which rock body is older?
B
A
?
?
C
Can you identify the inclusions found in this
Sierra Nevada Mountain batholitic material?
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Original Horizontality
Youngest
Superposition
Oldest
Cross-Cutting Relationship
Principle of Inclusions
Which granite is older?
A
B
C
Asp Vn
Older
Younger
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I
this geology science class.
Discuss with a friend

• Explain the concept of relative dating.
• 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.
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Ok given the principles, what is wrong with
this stack of rock (strata)
youngest
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6
5
3
2
1
oldest
Missing time or does time really stop?
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• The principle of Unconformities
• 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
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disconformity
angular unconformity
Sedimentary rocks
nonconformity
Xln rocks
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I
this geology science class.

• Explain what an unconformity is and
• what it represents
• 2. Diagram pictures that represent the
• three types of unconformities

Discuss with a friend
I will get an A on my exams and quizzes
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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
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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)?
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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
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How do fossils help date rocks?
1200 miles
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7
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Disconformity
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4
3
3
2
2
Which fossils are the youngest and oldest?
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I
this geology science class.

• Give 2 reasons why many organisms are
• are not fossilized.
• Explain the law of fossil succession and
• how this law allows dating of strata.
• How has fossil succession helped geologists
• unravel earth history?

Discuss with a friend
I will get an A on my exams and quizzes.
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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

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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.

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How does radiometric dating work, and where does
the age (number) come from?
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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
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U-235
Pb 207
1 half life 704 million years
U-3
Daughter element
1/2
1/2
1/2
704 m.y.
1.4 b.y.
2.1 b.y.
Parent element
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I
geology science class.

• Specifically define the differences
• between relative and absolute dating
• techniques.
• 2. Define the following absolute dating terms
• parent/daughter elements, half-life
• 3. Explain how the half-life is used to
• calculate an absolute age.

I will get an A on my exams and quizzes.
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• 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

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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

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• 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 earths
• time of formation
• no rocks are represented
• hellish conditions

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• Cenozoic 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

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• Periods based on
• 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

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• Epochs
• 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
• humans time

Age of Reptiles
Amphibians
Age of fish
Invertebrates
How accurate is the Geologic Time Scale?
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I
the geologic Time Scale.

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

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The Geologic Time Scale How much of Earth
history is represented?
Geologic Time Scale
Cenozoic, Mesozoic, Paleozoic Eras
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Precambrian Eon
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• 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

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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 earths 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?

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• 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

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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
• 4.55 billion years
• based on earth rocks and interstellar space
  objects,

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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
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I
what geology can do for me.
Discuss with a friend

• Identify at least 2 problems with the
• accuracy of the Geologic Time Scale.
• 2. Why does the Geologic Time Scale only
• represent about 12 of the earths geologic
• history (assuming the earth is 4.6 b.y. old)?

I will get an A on my exams and quizzes.
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