William E. Ferguson

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Geologic Time
William E. Ferguson
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Geologic Time

• A major difference between geologists and most
  other scientists is their attitude about time.
• A "long" time may not be important unless it is gt
  1 million years.

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Amount of Time Required for Some Geologic
Processes and Events
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Some geologic processes can be documented using
historical records(brown area is new land from
1887-1988)
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Uniformitarianism
The present is the key to the past.
James Hutton

• Natural laws do not change
• however, rates and intensity of
• processes may.

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Two ways to date geologic events

• 1 RELATIVE DATING (relative position of fossils,
  structure)
• 2 ABSOLUTE DATING (isotopic, tree rings, varves,
  etc.)

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RELATIVE GEOLOGIC TIME

• Steno Laws (1669) developed to arrange rock units
  in time-order
• Principle of Superposition
• Principle of Original Horizontality
• Law of Cross -Cutting Relationships
• Law of Inclusions

Laws apply to both sedimentary and volcanic rocks.
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Principle of Superposition
In a sequence of undisturbed layered rocks, the
oldest rocks are on the bottom.
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Principle of Superposition
Youngest rocks
Oldest rocks
Jim Steinberg/Photo Researchers
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Principle of Original Horizontality
Layered strata are deposited horizontal or nearly
horizontal or nearly parallel to the Earths
surface.
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Principles of original horizontality and
superposition
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Law of Cross-Cutting Relationships

• A rock or feature is younger than any rock or
  feature it cuts across.

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Law of Cross-cutting Relationships
Fig. 9.9
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LAW OF INCLUSIONS

• Included rocks are older than surrounding rocks.

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PRINCIPLE OF FAUNAL SUCCESSION

• Principle of Faunal Succession - groups of fossil
  plants animals have followed one another in a
  definite discernable order so certain fossil
  assemblages characterize a specific time.
• INDEX FOSSILS - fossils used to correlate a
  specific time period
• Based on distinct preservable parts, lived a
  short time , in a specific environment with wide
  distribution - MICROFOSSILS

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Ammonite Fossils
Petrified Wood
Chip Clark
Tom Bean
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Using Fossils to Correlate Rocks
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Correlating beds using index fossils
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Unconformity

• A buried surface of erosion
• Separates much older, eroded strata from younger
  ones
• Hiatus - the time gap or the time lost in the
  record

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Unconformitites - 3 kinds

• Disconformity - undeformed beds
• Nonconformity - sedimentary over igneous or
  metamorphic rx.
• Angular Unconformity - overlying tilted beds

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Formation of a Disconformity
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South rim of the Grand Canyon
250 million years old
Paleozoic Strata
550 million years old
1.7 billion years old
Precambrian
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South rim of the Grand Canyon
250 million years old
550 million years old
1.7 billion years old
Nonconformity
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Nonconformity in the Grand Canyon
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Nonconformity in the Grand Canyon
Tapeats Sandstone (550 million years old)
Vishnu Schist (1700 million years old)
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Angular unconformity, Grand Canyon
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The Great Unconformity of the Grand Canyon
Geoscience Features Picture Libraryc
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Formation of an Angular Unconformity
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Reconstructing Relative Sequence of Events
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CORRELATION

• Process used to tie separated strata together
• Based on matching physical features such as
• Physical continuity - trace of rock unit
• Similar rock types - marker beds, coal seams,
  rare minerals, odd color

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South rim of the Grand Canyon
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Generalized Stratigraphic Section of Rocks
Exposed in the Grand Canyon
after Beus Moral (1990)
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Some of the Geologic Units Exposed in the Grand
Canyon
Michael Collier
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The Geologic Time Scale

• Divisions in the worldwide stratigraphic column
  based on variations in preserved fossils
• Built using a combination of stratigraphic
  relationships, cross-cutting relationships, and
  absolute (isotopic) ages

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The Geologic Column and Time Scale
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Absolute geochronology

• Adds numbers to the stratigraphic column based on
  fossils.
• Based on the regular radioactive decay of some
  chemical elements.

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Isotopic dating

• Radioactive elements (parents) decay to
  nonradioactive (stable) elements (daughters).
• The rate at which this decay occurs is constant
  and knowable.
• Therefore, if we know the rate of decay and the
  amount present of parent and daughter, we can
  calculate how long this reaction has been
  proceeding.

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Isotopes

• Different forms of the same
• element containing the same
• number of protons, but varying
• numbers of neutrons.
• i.e.
• 235U, 238U 87Sr, 86Sr 14C, 12C

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Naturally Occurring Isotopes of Carbon
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Beta Decay
Electron Capture
Alpha Decay
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Production and Decay of Radiocarbon
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Radioactive Decay of Rubidium to Strontium
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Half-life
The half-life of a radioactive isotope is defined
as the time required for half of it to decay.
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Proportion of Parent Atoms Remaining as a
Function of Time
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Geologically Useful Decay Schemes
Parent Daughter Half-life (years)
235U 207Pb 4.5 x 109 238U 206Pb 0.71 x
109 40K 40Ar 1.25 x 109 87Rb 87Sr 47 x
109 14C 14N 5730
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PROBLEMS

• NEED A CLOSED SYSTEM!!!
• MINERAL MAY LEAK PARENT OR DAUGHTER
• MINERAL MAY BE CONTAMINATED WITH EITHER PARENT OR
  DAUGHTER

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Another Clock Paleomagnetism

• Earths magnetic field reverses every half
  million years
• Reversals are recorded in rocks that are forming
  at that time - seafloor
• Time scale calibrated by both relative absolute
  time methods

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Earths Magnetic Field
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Lavas record magnetic reversals
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Calculating Relative Plate Motion
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1871
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1968
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Paleontology

• The study of life in the past based on fossilized
  plants and animals.
• Fossil Evidence of past life
• Fossils preserved in sedimentary rocks are used
  to determine 1) Relative age 2) Environment
  of deposition

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Many methods have been used to determine the age
of the Earth

• 1) Bible In 1664, Archbishop Usher of Dublin
  used chronology of the Book of Genesis to
  calculate that the world began on Oct. 26, 4004
  B.C.
• 2) Salt in the Ocean (ca. 1899) Assuming the
  oceans began as fresh water, the rate at which
  rivers are transporting salts to the oceans would
  lead to present salinity in 100 m.y.

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Many methods have been used to determine the age
of the Earth

• 3) Sediment Thickness Assuming the rate of
  deposition is the same today as in the past, the
  thickest sedimentary sequences (e.g., Grand
  Canyon) would have been deposited in 100 m.y.
• 4) Kelvins Calculation (1870) Lord Kelvin
  calculated that the present geothermal gradient
  of 30C/km would result in an initially molten
  earth cooled for 30 100 m.y.

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Flawed assumptions

• Bible is not a science text or history book
• Salt is precipitated in sedimentary formations
• Both erosion and non-deposition are major parts
  of the sedimentary record
• Radioactivity provides another heat source

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The heat inside the Earth

• The discovery of radioactivity at the turn of the
  century by Bequerel, Curie, and Rutherford not
  only provided the source of the heat to override
  Kelvins calculations but provided the basis for
  all later quantitative estimates of the ages of
  rocks.

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Oldest rocks on Earth

• Slave Province, Northern Canada
• Zircons in a metamorphosed granite dated at 3.96
  Ga by the U-Pb method
• Yilgarn block, Western Australia
• Detrital zircons in a sandstone dated at 4.10 Ga
  by U-Pb method.
• Several other regions dated at 3.8 Ga by various
  methods including Minnesota, Wyoming, Greenland,
  South Africa, and Antarctica.

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Age of the Earth

• Although the oldest rocks found on Earth are 3.96
  Ga (or even 4.1), we believe that the age of the
  Earth is approximately 4.6 Ga. All rocks of the
  age 4.6 to 4.0 Ga have been destroyed (the rock
  cycle) or are presently covered by younger rocks.

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Age of the Earth

• This is based on the age of rocks brought back
  from the Moon (4.4 Ga), and meteorites (4.6 Ga),
  that are thought to be good representatives of
  the early solar system as well as more
  complicated geochemical modeling. This data
  suggests that the present chemical composition of
  the crust must have evolved for more than 4.5 Ga.

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Double it and add 1

• number of number of number of D/P
• half-lives parents daughters
• 0 64 0 0
• 1 32 32 1
• 2 16 48 3
• 3 8 56 7
• 4 4 60 15
• 5 2 62 31

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The geologic timescale and absolute ages

• Isotopic dating of intebedded volcanic rocks
  allows assignment of an absolute age for fossil
  transitions

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The big assumption

• The half-lives of radioactive isotopes are the
  same as they were billions of years ago.

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Test of the assumption

• Meteorites and Moon rocks (that are thought to
  have had a very simple history since they
  formed), have been dated by up to 10 independent
  isotopic systems all of which have given the same
  answer. However, scientists continue to
  critically evaluate this data.

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Frequently used decay schemeshave half-lives
which vary by a factor of gt 100
parent daughter half life (years)
235U 207Pb 4.5 x 109 238U 206Pb 0.71 x
109 40K 40Ar 1.25 x 109 87Rb 87Sr 47 x
109 147Sm 144Nd 106 x 109
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What if the rates have varied?
What we think happened
rate of decay
?
? time
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What if the rates have varied?
What we know didnt happen
rate of decay
?
? time
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Best initial D 0

• Two ways around this problem
• 1) Choose minerals with no initial daughter.
• 2) Use a method that tells you the initial
  concentration of D and P.

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Minerals with no initial daughter

• 40K decays to 40Ar (a gas)
• Zircon ZrSiO4
• ion radius (Å)
• Zr4 0.92
• U4 1.08
• Pb2 1.37

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Principle of Lateral Continuity
Map view
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Principle of Lateral Continuity
Map view
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Principle of Lateral Continuity
Map view
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Sedimentation of Beds A-D Beneath the Sea
Fig. 9.6
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Uplift and Exposure of D to Erosion
Fig. 9.6
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Continued Erosion Removes D and Exposes C to
Erosion
Fig. 9.6
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Subsidence and Sedimentation of E over C
Unconformity a buried surface of erosion
Fig. 9.6
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Sedimentation of Beds A-D Beneath the Sea
Fig. 9.8
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Deformation and Erosion During Mountain Building
Fig. 9.8
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Erosional Surface Cuts Across Deformed Rocks
Fig. 9.8
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Subsidence and Subsequent Deposition Buries
Erosional Surface
Angular
Unconformity
Fig. 9.8
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Fig. 9.10
Schlumberger Executive Communications
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Sequence A forms during lower sea level
Fig. 9.11a
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Sequence B forms during higher sea level
Fig. 9.11b
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The Geologic Time Scale
Fig. 9.13
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Areas with Potentially Hazardous Amounts of Radon