Geologic Time

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Geologic Time

• Historical Geology

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• In 1869, John Wesley Powell recognized the
  history of Earths past on display within the
  rocks of the Grand Canyon.

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• Rocks record geological events and changing life
  forms of the past. However, erosion has removed
  a lot of Earths history. It is not enough to
  rely on fossils to tell Earths story.
• Earth is much older than must humans can imagine.
  But, its surface and interior have been changed
  and influenced by the same geological processes
  that continue today.

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Brief history of Geology

• In the mid 1600s Archbishop James Usher, using
  the Bible as a source, determined that Earth was
  more than 5000 years old. He determined that
  Earth had been created in 4004 BC.
• This was accepted as fact by most historians and
  scientists

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• In the late 1700s, James Hutton published his
  Theory of the Earth. He had observed rocks in
  Scotland that the Romans had erected to build
  Hadrians Wall. He knew these rocks had been
  laid down around 40 AD. (less than 2000 years
  before) Yet they werent weathered to the extent
  that certain sedimentary rocks along Scotlands
  coast were. Their weathering and uplifting
  implied that the Earth had to be older than 5000
  years.

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• Hutton believed that the forces and processes
  that we observe today have been at work for a
  long time.
• This is called the principle of
  uniformitarianism.
• Scientists today understand that the same
  processes may not allways have had the same
  relative importance or operated at precisely the
  same rate.

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• Some important processes are not currently
  observable, but evidence that they occur is well
  established. (meteor strikes)
• We must remember that our lives are very brief
  when compared to the age of the Earth.

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• Relative Dating
• Relative Dating tells us the sequence in which
  events occurred, not how long ago they occurred.

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• Law of Superposition- this law states than in an
  undeformed sequence of sedimentary rocks, each
  bed is older than the one above it and younger
  than the one below it. This rule also applies to
  materials such as lava flows and beds of ash from
  volcanic eruptions.

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• The Principle of Original Horizontality this
  prieciple means that layers of sediment are
  generally deposited in a horizontal position. If
  you see rock layers that are bent, it means that
  they were disturbed after they were laid down.

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• The Principle of Cross Cutting Relationships
  This means that when a fault cuts through or when
  magma intrudes other rocks and crystallizes, we
  can assume that the fault or intrusion is younger
  than the rock affected. If a layer is unbroken
  above a fault, it means that the fault was there
  before that layer of rock.

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• Inclusions these are pieces of one rock unit
  that are contained within another. The rock next
  to the one containing the inclusion must have
  been there first in order to provide the rock
  fragments. So the rock unit containing the rock
  fragments is the younger of the two.

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• Unconformities A break in the rock record. It
  represents a long period during which deposition
  stopped, erosion removed previously formed rocks,
  and then deposition resumed. Unconformities
  represent significant geologic events.

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• Angular unconformity this indicates that during
  the pause in deposition, a period of deformation
  (folding or tilting) and erosion occurred.

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• Disconformity Two sedimentary rock layers that
  re separated by an erosional surface. These are
  more common than angular unconformities

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• Nonconformity the erosional surface separates
  older metamorphic or intrusive igneous rocks from
  younger sedimentary rocks.

Nonconformity, comprising bedded sandstones lying
directly on top of coarse-grained granite. The
granite is a plutonic igneous rock which
crystallized at a depth of at least 10
kilometers, indicating the minimum depth of
erosion that must have occurred here. The reddish
colour and the noticeably friable appearance of
the granite are due to weathering that took place
before the sandstone was deposited.
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Unconformity
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• Correlation of Rock Layers Rocks of similar age
  in different regions must be matched up. To do
  this you might be able to match up distinctive or
  uncommon minerals in rock layers that are the
  same in areas that are nearby. In this way, you
  can create a more complete view of the geologic
  history of a region.

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• p 341 shows how the rocks correlate at 3 sites on
  the Colorado Plateau in southern Utah. This
  cannot be used to match rocks that are separated
  by great distances. Fossil correlation comes
  into play in these instances.

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Correlation of Rock Layers
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Fossil Evidence of Past Life

• Fossil Formation the type of fossil is
  determined by the conditions under which an
  organism died and how it was buried.

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• Unaltered Remains Some remains teeth, bones,
  shells, may not have been altered or changed
  hardly at all over time. Finding a fully
  preserved, frozen mammoth.

Baby mammoth
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• Altered Remains petrified fossils. Bone or
  remains are soaked in mineral-rich water,
  penetrating the small cavities and pores of the
  original organism.

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• The minerals precipitate from the water and fill
  the spaces. There are no more cells. The
  molecules of the cell are replaced by molecules
  of minerals, sometimes to the extent that
  microscopic structures are preserved.

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• Mold a shell or other structure is buried in
  sediment then dissolved by underground water.
  The mold reflects only the shape and surface
  features of the organism. There is nothing
  preserved of its internal structure.

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• A cast is created when the hollow space of a mold
  are filled with mineral matter

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• Carbonization this is particularly effective in
  preserving leaves and delicate animal forms.
  Carbonization occurs when an organism is buried
  under fine sediment, pressure squeezes out the
  liquid and gas components of an organism and
  leaves a thin residue of carbon.

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archaeopteryx
fish
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• Preservation in amber some very delicate
  organisms, such as insects, are preserved in the
  hardened resin of ancient trees.

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• Indirect evidence Trace fossils
• Footprints
• Burrows
• Coprolites fossilized dung
• Gastroliths stomach stones used in the
  grinding of food in the gizzards of some ancient
  reptiles

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Insect in amber
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Burrows
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Gastroliths
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coprolite
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coprolite
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• Conditions favoring preservation two conditions
  are important for preservation rapid burial and
  the possession of hard parts.

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• Fossils and Correlation William Smith
  demonstrated in the late 18th century that rock
  layers contain a distinct assortment of fossils
  that did not occur in the layers above or below
  it. Hw also discovered that sedimentary rock
  layers in distant areas could be identified and
  correlated by the distinct fossils they contained

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• The principle of fossil succession states that
  the fossil organisms succeed one another in a
  definite and determinable order. Any time period
  can be recognized by its fossil content.

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• So we can pinpoint an Age of Trilobites, Age of
  Fishes, Age of Coal Swamps, Age of reptiles, Age
  of mammals. These Ages correspond to particular
  time periods. They are in the same order of
  dominant organisms on every continent

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• Geologists pay particular attention to index
  fossils. These are fossils that are widespread
  geographically and are limited to a short span of
  geologic time and occur in large numbers.

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• Interpreting environments geologists can
  conclude that a region was once covered by a
  shallow sea when the remains of certain clam
  shells are found in the limestone of that region.
  Heavy shells imply an environment of pounding
  waves. Corals require warm shallow seas,
  implying a tropical climate like Florida.

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• Dating with Radioactivity
• Remember that an atom contains protons and
  neutrons in their nucleus and electrons in orbit
  around the nucleus.
• The atomic number corresponds with the number of
  protons in the nucleus.
• Different elements have different numbers of
  protons in their nuclei.

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• An atoms mass number is the number of protons
  plus the number of neutrons in the nucleus.
• The number of neutrons can vary and these
  variants, or isotopes, have different mass
  numbers.
• In some isotopes, the forces that hold the
  protons and neutrons together are not
  sufficiently strong to maintain the nucleus. In
  this case, nuclei are unstable and spontaneously
  break apart.

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• This is a process called radioactivity or
  radioactive decay.
• This process continues until a stable or
  non-radioactive isotope is formed.

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• One well documented decay series is uranium -238.
• U-238 decays over time to form lead-206.
• Half life is the amount of time necessary for one
  half of the nuclei in a sample to decay to its
  stable isotope.
• If the half life of a radioactive isotope is
  known and the parent/daughter ratio can be
  measured, the age of the sample can be calculated.

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• Radiometric dating
• The procedure is used to calculate the age of
  rocks, or other materials, using radioactive
  isotopes. The rate of radioactive decay is not
  influenced or varied by anything physical in the
  environment. It has been occurring in the rock
  since it was formed.

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• If U-238 becomes incorporated in crystallizing
  magma, there is no lead. The clock starts at
  this point. As the uranium decays, atoms of the
  daughter product are formed and measurable amount
  of lead eventually accumulate.
• This method is accurate only if the mineral
  remained in a closed system during the entire
  period since its formation.

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• If more radioactive elements or daughter elements
  are added at a later date, it is not possible to
  calculate a correct date.
• U-238 has 13 intermediate daughter products
  before Pb-206 is produced

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Carbon 14 dating-

• C-14 is a radioactive isotope of carbon. It is
  produced continuously in the upper atmosphere.
  It becomes incorporated in the CO2 in the
  atmosphere, and, into the carbon cycle of Earth
  processes.

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• All living organisms contain some C-14. The C-14
  to C-12 ratio is constant as long as they are
  alive. They will continue to incorporate C-14 as
  long as they are living. As soon as they die,
  C-14 is no longer incorporated, and the C-14 to
  C-12 ratio changes as the C-14 decays.

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• C-14 dating is a valuable tool for scientists who
  study recent Earth history.
• Radiometric dating have become very important
  tools for geologists.
• Rocks on Earth have been dated to be as much as 4
  billion years old. Meteorites have been dated at
  4.6 billion years.
• Radiometric dating has supported the ideas of
  James Hutton, Charles Darwin and others.

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• Difficulties with the Geologic Time Scale
• Not all rocks can be dated by radiometric
  methods. For a radiometric date to be useful,
  all minerals in the rock must have formed at
  about the same time.

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• Radioactive isotopes can be used to determine
  when minerals in an igneous rock crystallized and
  when pressure and heat made new minerals in a
  metamorphic rock.
• Sedimentary rocks may contain particles that
  contain radioactive isotopes but these particles
  are not the same age as the rock in which they
  occur.

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• A metamorphic rock may contain minerals that tell
  when the rock was metamorphosed, not when the
  rock first formed.

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• To date sedimentary rock, geologists relate their
  age to datable igneous masses. Using the
  principle of superposition you can tell a
  sedimentary bed below an ash layer is older than
  the ash. A volcanic dike cutting through a
  sedimentary layer is younger than the sedimentary
  rock. If the ash is dated, and the dike is dated
  you can get an idea of the age of the sedimentary
  rock.