Introduction to Earth History

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Introduction to Earth History

• CHAPTER 1

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Physical versus Historical Geology

• Physical geology origin, classification, and
  composition of Earth materials and internal and
  surficial processes (modification of Earth
  materials)
• Historical geology Earths evolution, changes
  in lands and seas, mountain building and
  destruction, fossil succession through time

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Scientific Method in Geology

• Questions
• Collection of data (observations)
• Development of hypothesis (explanation)
• Further testing of hypothesis

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Introduction to Plate Tectonics

• Grand unifying concept of Earths outer layer
• Tectonics large-scale deformation
• Plate large slab of Earths lithosphere
• Asthenosphereplastic layer on which plates move
• Plate boundaries
• Convergent (closing)
• Divergent (spreading)
• Transform (sliding)

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Founders of Historical Geology

• European
• Nicolaus Steno
• Abraham Gottlob Werner
• James Hutton
• William Smith
• Charles Lyell
• Charles Darwin

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Stenos Laws

• Principle of Superposition
• In an undisturbed succession of sedimentary rock
  layers, the oldest layer is at the bottom and the
  youngest layer is at the top
• Principle of Original Horizontality
• 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
• Principle of Lateral Continuity
• Sediment extends laterally in all direction until
  it thins and pinches out or terminates against
  the edges of the depositional basin

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Where would you find the youngest rocks? The
oldest?
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How were these rocks deposited?
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Illustration of original lateral continuity.
Cross-section A shows a sandstone stratum
deposited within a low-lying area or sedimentary
basin that received sediment eroded from
surrounding uplands. Cross-section B shows the
same area after erosion has exposed the sandstone
on hillsides.
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James Hutton

• William Whewell created the term
  Uniformitarianism
• The past history of our globe must be explained
  by what can be seen to be happening now.
• Uniformitarianism vs Actualism

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Angular unconformity at Siccar Point, eastern
Scotland. (A) It was here that James Hutton first
realized the historical significance of an
unconformity. The drawings (B) indicate the
sequence of events documented in this famous
exposure.
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Principle of Biologic Succession

• William Smith --correlating rock strata based on
  types of fossils each contained.
• Did not know why life forms for each age were
  unique to that time period, but this allowed
  correlation of units around the world to be put
  in chronological order.

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Lyells Principles

• Principle of cross-cutting relationships
• An igneous intrusion or a fault must be younger
  than the rocks it intrudes or displaces
• Principle of inclusions
• A rock that contains fragments or inclusions of
  another rock must be younger than the rock from
  which the fragments or inclusions came.

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An example of how the sequence of geologic
events can be determined from cross-cutting
relationships and superposition. From first to
last, the sequence indicated in the cross-section
is first deposition of D, then faulting to
produce fault B, then intrusion of igneous rock
mass C, and finally erosion followed by
deposition of E. Strata labeled D are oldest, and
strata labeled E are youngest.
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(A) Granite inclusions in sandstone indicate
that granite is the older unit.(B) Inclusions of
sandstone in granite indicate that sandstone is
the older unit.
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Darwin

• Natural Selection
• 1809-1882

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Time and Geology

• Relative vs absolute age dating
• Chronologic order vs exact age date

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Time and Geology

• Divisions of stratigraphy (chronostratigraphic
  units) correspond to chronologic units
• System (Period)
• Series (Epoch)
• Stage (Age)

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Time and Geology

• Development of geochronologic nomenclature
• Cambrian System Cambria (Roman name for Wales)
• Silurian and Ordovician Systems Silures and
  Ordovices were ancient Celtic Tribes
• Devonian System Devonshire, England
• Carboniferous System British coal measures
• Permian System Perm Province, Russia
• Triassic System set of three formations in
  Germany
• Jurassic System Jura Mountains, Franco-Swiss
  border
• Cretaceous System Latin for Chalk (creta)
• Tertiary System "montes tertiarii" of Italian
  Alps
• Quaternary System soft sediments of northern
  France

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Quantitative Geologic Time

• Early estimates
• Solar and lunar cycles calibrated with Old
  Testament 6,000 years
• Evolutionary rate calculation beginning of
  Cenozoic was 80 million years ago (Lyell, 1839)
• Depositional rate calculations age ranges 1
  million to 1 billion years (1850s)
• Salinity of sea water calculations 90 million
  years since seas developed (Joly, 1901)
• Rate of cooling calculation 24 to 40 million
  years since formation (Kelvin, 1890s)

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Quantitative Geologic Time

• Modern radioisotopic methods and concepts
• Employs rate of natural, spontaneous breakdown of
  nuclear structure of atoms radioactivity
• Parent nuclide daughter product particle
  expelled
• Rate of nuclear decay is constant
• Crystallization of minerals locks in an original
  quantity of radioactive atoms
• Radioactive isotopes each has a unique rate and
  mode of decay

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Quantitative Geologic Time

• Modern radioisotopic methods and concepts (cont)
• Radiometric dating of a crystal possible because
  daughter products are retained
• Original quantity of parent determined by
  counting daughter products (P D O)
• Mass Spectrometer device used to measure minute
  amounts of isotopes

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Figure 1-20 (p. 23)Radioactive decay series of
uranium-238 (238U) to lead-206 (206Pb).
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Figure 1-25 (p. 26)Rate of radioactive decay of
uranium-238 to lead-206.
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Figure 1-23 (p. 25)Igneous rocks that have
provided absolute radiogenic ages can often be
used to date sedimentary layers. (A) The shale is
bracketed by two lava flows. (B) The shale lies
above the older flow and is intruded by a younger
igneous body.
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Quantitative Geologic Time

• Principal geologic timekeepers useful decay
  processes
• Carbon14 method half-life 5,730 years
• Rb/Sr isochron plots
• U/Pb method using Uranium235 or U238
• U/Th method using Uranium 234
• K/ Ar method using electron capture
• Nuclear fission-track counting in crystals

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Uranium dating

• U-Pb
• U-238 Half Life4.5 Billion years
  DaughterLead-206
• Uranium-235 704 Million Daughter Lead-207
• Source Zircon, U minerals

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K-Ar dating

• Potassium-40 Half Life 1251 Million years
  Daughter Argon-40 (and calcium-40)
• Sources micas, volcanic rocks, feldspar

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Rb-Sr dating

• Rubidium-87 Half Life 48.8 billion years
  Daughter Strontium-87
• Source is similar to K/Ar sources

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C-14

• Carbon-14 Half Life 5730 years Daughter
  Nitrogen-14
• Source organic material
• Does not abide by typical P/D calculation of age

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Nuclear Fission Tracks

• Counting etched paths in minerals-- destruction
  from radioactive decay
• Discovered in the 1960s
• Useful for a wide variety of time-scales

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Age of the Earth

• Oldest known Earth material is 4.2 billion years
  (zircon crystals from western Australia)
• Age of meteorites U/Pb and Rb/Sr dating yields
  4.6 billion years
• Moon rocks U/Pb and Rb/Sr dates are 4.6 billion
  years maximum
• http//www.pbs.org/wgbh/evolution/library/03/3/l_0
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