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GEOLOGIC HISTORY

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4) Volcanic Time Markers. volcanic ash. small pieces of igneous rock that are shot into the air during eruptions. volcanic ash is deposited rapidly over a large area. ... – PowerPoint PPT presentation

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Title: GEOLOGIC HISTORY


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GEOLOGIC HISTORY
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RELATIVE AGE DATING
  • places events in a sequence, but does not
    identify their actual date of occurrence.
  • 1st event
  • 2nd event
  • Last event

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PRINCIPLES FOR RELATIVE AGE DATING
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1 ORIGINAL HORIZONTALITY
  • sedimentary rocks are originally deposited in
    horizontal layers.

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2 SUPERPOSITION
  • in undisturbed sedimentary rocks, the oldest
    layer is usually on the bottom.

youngest
oldest
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3 CROSS-CUTTING RELATIONSHIPS
  • an igneous rock is younger than the rocks it has
    intruded.
  • A fault is younger than the rocks it cuts across.

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RELATIVE AGE DATING(Practice)
  • Determining Which is Older

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Dating Intrusions, Extrusions, and Inclusions.
  • intrusion
  • igneous rock that formed when magma squeezed into
    preexisting rocks.
  • an intrusion is younger than the rock it cuts
    through.

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  • extrusion
  • forms when molten rock (lava) flows onto Earths
    surface and solidifies.
  • an extrusion is younger than the rocks beneath
    it.
  • it is older than the rocks deposited on top of
    it.

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  • inclusion
  • a piece of rock that falls into an intrusion of
    magma.
  • the inclusion is older than the magma intrusion.

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  • contact metamorphism
  • occurs when magma or lava comes in contact with
    older bedrock and alters the minerals

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  • In sedimentary rocks.
  • the sediments are older than the rock itself.
  • Veins
  • forms when a watery mineral solution fills a
    crack in the rock.
  • a vein is younger than the rock around it.

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Example 1
  • youngest event (what happened last)
  • deposition of Limestone
  • deposition of Sandstone
  • deposition of Shale
  • oldest event (what happened first)

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Example 8(trial one)
  • youngest event (what happened last)
  • Deposition of Limestone
  • Tilting
  • Deposition of Shale
  • Deposition of Limestone
  • Deposition of Sandstone
  • oldest event (what happened first)

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Unconformity
  • a buried erosional surface.
  • a surface of erosion between rock layers of
    different ages indicating that deposition was not
    continuous.
  • part of the rock record is missing.
  • symbol

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In Most Cases..
  • Erosion occurs.
  • on land.
  • Deposition occurs.
  • in the water.

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Geologic History of an Unconformity.
  • Deposition in a marine environment.
  • Uplift exposed the land to Erosion.
  • The land Subsides below sea level.
  • Deposition begins again.

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When you are describing an unconformity you must
include.
  • Uplift and Erosion
  • Subsidence

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Example 8. (trial two)
  • youngest event (what happened last)
  • Deposition of Limestone
  • Subsidence
  • Uplift and Erosion
  • Tilting
  • Deposition of Shale
  • Deposition of Limestone
  • Deposition of Sandstone
  • oldest event (what happened first)

UNCONFORMITY
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  • Begin Geologic History Lab

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Correlation Techniques.
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correlation
  • the matching of rock layers from one area to
    another.

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1) Walking the outcrop.
  • directly following the individual layers (or rock
    formations) at the Earths surface.
  • Other methods must be used where rock formations
    are separated from one another.

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  • formation
  • a layer or group of layers of rock that have
    similar characteristics.
  • outcrop
  • bedrock that is exposed at the Earths surface.

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2) Similarity of Rock.
  • rocks can be matched on the basis of similarity
    in appearance, color, and mineral composition.
  • this method is useful only across small areas,
    and may be incorrect.

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3) Fossil Evidence.
  • one of the best methods for correlation.
  • fossils
  • any evidence of former life.

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index fossil
  • the remains of animals that lived and died within
    a particular time segment of Earths history.

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Index fossils are useful in correlation because
they
  • are easy to identify.
  • are abundant.
  • are widespread in occurrence.
  • existed for only a brief period of time.

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  • Fossils are found almost exclusively in
    sedimentary rocks.
  • Fossils provide clues to the environments in
    which the organism lived.

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4) Volcanic Time Markers.
  • volcanic ash
  • small pieces of igneous rock that are shot into
    the air during eruptions.
  • volcanic ash is deposited rapidly over a large
    area.
  • Therefore, it can serve as an age marker in rocks
    that are 1000s of km apart.

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ABSOLUTE AGE DATING
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Absolute (or actual) Age Dating
  • absolute age
  • actual age in years.
  • Age can be determined by.
  • counting lake varves (annual lake sediment).
  • counting tree rings.
  • most common method is using radioactive dating.
  • (measuring the amount of a particular
    radioactive element in a rock).

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Radioactive Decay is
  • the natural and spontaneous breakdown,
  • of the nucleus,
  • of unstable atoms into more stable atoms.
  • Radioactive decay
  • releases energy and subatomic particles.
  • occurs at a constant rate, which can not be
    changed.

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  • The radioactive isotope (Parent material) will
    break down naturally into another element called
    the decay product (Daughter material).
  • The rate of radioactive decay is measured in
    terms of half-lives.

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half-life
  • the amount of time it takes for half of the
    radioactive element to decay.

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The age of a rock can be determined by
comparing.
  • the relative amount of the Undecayed substance
    (radioactive parent material)
  • to the relative amount of Decay product (stable
    daughter material).

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COMPLETE EXAMPLE
  • Color in the bars.
  • determine the relative amount of daughter and
    parent using fractions.
  • use the number of half-lives to determine the age
    of a sample.

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Graph Example
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0
  • 0 half-life parent daughter
  • 1 half-life parent daughter
  • 2 half-lives parent daughter
  • Draw Graph
  • The more daughter material in the sample, the
    older the sample.

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The amount of time for a half-life is different
for each radioactive substance.
  • See ESRT.

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  • Carbon -14 ..decays to
  • half-life
  • Uranium-238.decays to
  • half-life

Nitrogen-14
5700 years.
Lead-206
4,500,000,000 years.
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  • Radioactive substances with short half-lives,
    such as C-14, are good for dating recent organic
    remains.
  • Those with longer half-lives, such as U-238, are
    useful for dating older rocks.

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PRACTICE QUESTIONS
  • ABSOLUTE AGE DATING

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GEOLOGIC HISTORY
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Geologic History from the Rock Record
  • some life forms exist only during specific
    intervals of time.
  • therefore, fossils in rock can be used to
    determine relative age.
  • ex) dinosaurs existed only during the Mesozoic
    Era.

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Geologic Time Scale
  • geologists have used fossil evidence to divide
    4.6 billion years of Earths history into smaller
    units.
  • it is these divisions that make up the Geologic
    Time Scale.
  • see ESRT p.8 9, Geologic History of New York
    State.

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Precambrian
  • represents 88 of Earths history.
  • fossils in these rocks are rare and difficult to
    identify
  • earliest life was small and lacked hard parts.
  • these rocks may have been eroded away.

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Uniformity
  • the present is the key to the past.
  • assumes that the geologic processes occurring
    today also occurred in the past.
  • therefore, we can interpret past events by
    examining what is happening today.

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WORKSHEET
  • THE GEOLOGIC TIME SCALE

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EVOLUTION OF EARTH AND LIFE
  • Fossils
  • a wide variety of life forms have lived over
    time.

Anomalocaris canadensis
Recent stromatolites at Hamelin Pool, Aus
Burgess Shale
Hallucigenia sparsa
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  • most of these life forms are now extinct.
  • Since, the chances of fossilization are low.
  • most forms of past life probably have not been
    identified.
  • we compare fossils to similar life forms that
    exist today.

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Environmental change.
  • fossil evidence
  • corals shallow, warm ocean water.

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  • coal deposits swamps.

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  • salt and gypsum evaporating oceans.

Dead Sea
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  • environments have changed as the Earths
    lithospheric plates have moved.
  • as a landmass moves closer to the equator, its
    climate gets warmer.
  • plate collisions form mountains.

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Evolution and Extinction
  • organic evolution
  • theory that life forms change through time.
  • offspring with favorable variations will survive.
  • offspring with unfavorable variations will become
    extinct.

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Mass Extinctions
  • periods of time when large numbers of species
    went extinct.
  • believed to have been caused by catastrophic
    events.impact events.
  • example) K-T boundary, 65 mya, 70 of all species
    went extinct (including the dinosaurs).
  • extinctions allowed other species (mammals) to
    thrive and evolve.

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