Chapter 9 formerly Chapt' 8 Geologic Time

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Chapter 9 (formerly Chapt. 8) Geologic Time

• As part of our human need to organize things,
  we need to establish a sequence of events (a
  relative time line) to understand the Who, What,
  When, Where, How.
• Each rock tells a story
• Igneous rocks Crystal size tells the cooling
  history, composition gives insight into origins.
• Clastic sediments Textures tell us of source
  area and depositional conditions.

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Chemical sediments Tell us of environ-mental
(esp. water) conditions in the depositional
basin. Metamorphic rocks Tell us of heat/
pressure conditions and type of parent rock
(sometimes). When analyzing recent human history,
we have the advantage of written dates of events,
i.e., we are in possession of a numerical date.
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Before we had radiometric age dating, we had
to depend on relative dating, i.e., the
establishment of a sequence of events to describe
history of rock exposures, etc.. Remember
Charles Lyell Nicolas Steno.
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From the exposures of the Castner Marble, what
history can we re-assemble?
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In the Franklin Mts., there are two roadcut
exposures of the Castner Marble
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• From the calcite composition, preserved layering,
  nearby algal fossils, we presume that the
  Castner Marble was originally a flat-lying
  limestone (Original Horizontality). From the
  clean outcrop (Slide 4) a series of submarine
  basalts overlie the Castner (Superposition). The
  basalts are overlain by 100s of feet of
  sandstone.
• The granites intruded the entire stack of
  sediments. Rhyolitic ashflows erupted at the
  surface

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Inclusions in the breccia dike may provide
insight as to what lies below
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Is igneous body A a sill or a lava flow?
How would you tell? What features would you look
for?
Sill Lava Flow
B
A
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Interruptions in deposition are called
Unconformities. The types include
Angular Unconformity rocks below tilted or
folded. Nonconformity layers deposited on
igneous/metamorphic rocks.
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Paraconformity Parallel above below, little
evidence of gap. Disconformity Parallel above
below with irregular eroded surface.
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• Correlation allows us to relate events within a
  given region, by verifying the similar ages of
  rock strata and sequences.
• Correlation is done by Physical Criteria and
  Fossil Criteria
• Physical Criteria Similarities in lithology,
  distinctive marker beds, sequences of
  contrasting beds. Figure 9.9 shows how rock
  exposures in the Grand Canyon, Zion National
  Park, and Bryce Canyon National Park are tied
  together producing the Colorado Plateau
  sequence.

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When horizontal facies changes result in
sedimentary rock changes, fossils may need to be
employed for correlation purposes. William Smith
noticed succession of fossils variations in
fossil shape are not random, but rather
successive. Smith also noticed what we now call
Guide or Index Fossils, fossils that were
geographically widespread, but were limited to
short span of geologic time.
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• If a single guide fossil is not available, using
  the overlapping ranges of different fossils may
  be used to verify the age.

Fossils can also offer evidence as to the
environ-mental conditions during their lives or
the movement of their shells after death.
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• Prior to discovery of radioactive decay, James
  Hutton, Charles Darwin, John Wesley Powell, et
  al, inferred that the Earth was of immense age.
  Early estimates were based on how long it would
  take to deposit given sedimentary layers, how
  long would it take to reach the present salinity
  of the ocean, etc..
• Absolute Time Radiometric age-dating measuring
  parent product/daughter product ratios known
  decay rates.

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• Atomic structures shell of electrons orbiting
  nucleus of protons and neutrons.
• Atomic number - of protons determines
  position on the periodic table.
• Atomic mass protons neutrons. When
  there are extra neutrons present in the
  nucleus, these are isotopes. Atomic mass changes
  by 1 for each extra neutron, atomic number does
  not change. Isotopes may have different
  character-istics. In some isotopes, the nuclei
  are unstable and will break down over time.

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Alpha particles 2 protons 2 neutrons Beta
particle 1 electron from nucleus Atomic
number Nucleus captures electron Atomic number
Figure 9.13, p. 286 U238 to Th234 Alpha
emission, Atomic number drops 92 90 Th234 to
Pa234 to U234 Beta emissions, Atomic rises
back to 92 Parent element Uranium Daughter
element Thorium
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• Each decay has a known rate. With this known
  rate and the quantity of parent element vs. the
  daughter element measured, we can estimate the
  length of time of decay. The clock starts when
  the mineral solidifies from the magma.
• Half-Life the amount of time needed for half of
  a given amount of parent product to decay to
  daughter product. Decay occurs on an exponential
  curve (Figure 9.15, pg. 289).

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Potassium-Argon dating (K40 Ar40) useful because
of presence of potassium in many igneous
minerals. To be useful, daughter element must be
stable. Other useful isotopes are shown in Table
9.1 Sources of error mineral must remain a
closed system since crystallization. If amount
of parent or daughter element changes, this gives
distorted estimate. Argon is gas, may escape
during reheating (metamorphism).
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• Carbon 14 (C14) is useful for dating of events
  lt75,000 yrs. Newer information suggests, though,
  that atmospheric carbon 14 may vary due to solar
  activity.
• Dendrochronology use of tree-ring data to
  estimate ages. Width and density of ring
  suggests climatic conditions (amount of rainfall,
  temperature, length of growing season).
  Reference chronologies established for given
  areas. Wood fragments with a few rings may be
  matched against reference datum.

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• Fig. 9.14 (pg. 287) shows decay of U238 through
  13 intermediate, unstable daughter products until
  stable daughter product Pb206 is reached.
• Radiometric ages are best available science
  estimates. Oldest granite South Africa dated
  at 3.2 billion years, contains quartzite
  inclusions (older rock). Quartzite is likely the
  product of metamorphism of sandstone, which was
  derived from the weathering of still older rock.

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Geologic Time Scale Using principles of
Superposition, Cross-cutting relationships,
Inclusions, Fossil Succession, etc., 18th century
European scientists constructed the geologic time
scale shown on Fig. 9.17, pg. 292. Period name
origins are listed on Table 9.2, pg.
296. Largest time units Eons Precambrian
beginning (5.4 b.y.) to 540 million years
ago. Phanerozoic 540 million years ago to
present.
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• Next smaller units Eras
• Precambrian Hadean (5.4 to 3.8 b.y.) Archaean
  (3.8 to 2.5 b.y.) and Proterozoic (2.5 b.y. to
  540 m.y.)
• Phanerozoic Paleozoic (540 to 248 m.y.)
  Mesozoic (248 to 65 m.y.) Cenozoic (65 m.y. to
  present)
• Eras divided into Periods, Periods are divided
  into Epochs.
• We live in the Holocene (Recent) Epoch last
  12,000 yrs.

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Precambrian Time Scale. Oldest rocks about 3.8
b.y.. Existance of older rocks is suggested by
zircons in old metamorphics
545 m.y.
Proterozoic Era
2.5 b.y.
Archaean Era
Precambrian
4.0 b.y.
Hadean Era
4.6 b.y.
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Georgia Valley Ridge sedimentary rocks
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Mesozoic Era Age of the Dinosaurs.
66 m.y.
Cretaceous Period
144 m.y.
Jurassic Period
Mesozoic Era
208 m.y.
Triassic Period
245 m.y.
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Cenozoic Era Time Scale the last 66 m.y.
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Time units are divided by profound changes in
life forms and in some areas, a significant
change in sedimentation. Precambrian/Phanerozoic
boundary (beginning of Paleozoic Era, Cambrian
Period), marked by first widespread occurrence of
hard-shelled creatures. Paleozoic/Mesozoic
boundary (end of Permian Period) marked by mass
extinctions, especially of marine organisms
(estimated 90 of marine species).
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Mesozoic/Cenozoic boundary (end of Cretaceous
Period), widespread extinctions, e.g., dinosaurs,
ammonites, et al. Difficulties in Radiometric Age
Dating In metamorphic rocks, which minerals
have had their clocks reset is determined by
the temperature, pressure, and water conditions.
Some minerals, e.g., zircons (hard, stable) in a
gneiss may have come from several sources.
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When clastic sedimentary rocks contain
minerals with radioactive isotopes, most often,
the isotopes show the age of the original igneous
source rock, or perhaps a metamorphic source
rock, depending on the circumstances. To
estimate ages of sedimentary rocks, geologists
need to be able to tie sedimentary unit to
datable igneous rocks. Volcanic ash beds, small
intrusions (dikes sills), i.e., igneous events
that do not disrupt sedimentation.
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Volcanic ash beds - chronological snap-shots.
In Ga. Valley and Ridge Province, Ordovician
limestones may contain 4 or more volcanic ash
beds. Figure 9.18 illustrates how igneous dike
(66 m.y.) cuts Mancos Shale and Mesaverde
Formation. Top of dike has been truncated (cut
off) by erosion prior to the deposition of the
Wasatch Formation.
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Major Philosophies on Origin of Life "Naturalism"
        Naturalistic Evolution (as presented in
textbook) Evolution occurs randomly, as
mutations "Creationism"         Creation
Scientists         Young-earth creationism
        New-earth creationism         Flood
geology "Theistic evolution"         Old-earth
creationism         Evolution with a guiding
hand         Intelligent Design Theory
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