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, geomagnetics)
• 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
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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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CORRELATION

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

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Using Fossils to Correlate Rocks
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Correlating beds using index fossils
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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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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 - flat sediments overly
  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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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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ice ages
Kauai
dinosaurs out
oldest Emperor seamount,80 Ma
dinosaurs in
plants, fish
animals with skeletons
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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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magnetically polarized layers in a volcano
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