The Rock Record

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Unit 3

• Chapter 8
• The Rock Record

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Section 1 Determining Relative Age

• The age of the Earth is approximately 4.6 billion
  years old. This idea came from James Hutton, an
  18th century Scottish Physician and Farmer. He
  is the Father of Modern Geology. He came up
  with his ideas from observing changes taking
  place on his farm. He also stated The Present
  is the Key to the Past.

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Uniformitarianism

• 1. Geologic processes now operating were also
  active in the past.
• a. Volcanoes
• b. Erosion
• c. Deposition
• d. Earthquakes
• Present day geologic features were formed by
  these processes over long periods of time.
• Before Huttons time people (science) thought all
  of the worlds features were created by
  catastrophic events.

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Earths Age

• Before Hutton most people thought the age of the
  Earth was about 6000 years old. They also
  thought that all geologic features were created
  the same way and over a long period of time. He
  concluded that the Earth had to be older for the
  changes that he found, to occur.
• Relative Age
• Relative dating is using rock layers to place
  events in the sequence in which they occurred.
  This does not identify the actual date, but a
  date close to when the event occurred.

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LAW OF SUPERPOSITION

• The oldest rocks in a rock layer will be on the
  bottom, the newest rocks are on the top.
  Sedimentary rocks forms in horizontal layers. If
  the rock layers are overturned (flipped over)
  this is not true.

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Principals of Original Horizontality

• Sedimentary rocks generally form in horizontal
  layers. The principal states that left
  undisturbed, the layers will remain horizontal.
  If the layers are not horizontal, some tectonic
  force (action) had to occur. Scientists would
  have to retrace what happen first in order to
  apply the Law of Super Position.
• Graded bedding
• Large particles usually on the bottom are smaller
  are on top.
• Cross-beds
• When sand is deposited, it usually forms beds at
  an angle because the particles slide down.
• Ripple Marks
• Small waves formed on the surface of sand that
  later becomes a rock. Can be used to show which
  layer was on top.

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Unconformity

• Is a gap in the geologic rock record sequence it
  is a layer of rock that is missing in the strata
  sequence. Shows that either the deposition
  stopped or part of the layer was removed by
  erosion.
• Ex. Erosion of rock layers
• Nonconformity
• When sedimentary layers are deposited on igneous
  and metamorphic rock layers that have been
  uplifted and parts eroded away.
• Angular Unconformity
• Rocks that have tilted during uplift, or folded,
  then eroded. When erosion stops, deposition once
  again occurs.
• Disconformities
• Layers that have been uplifted (not tilted still
  horizontal) and then eroded away. Then new
  layers are deposited. This makes gaps in the
  layering.

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Cross Cutting Relationships

• They are igneous layers that are intruded into
  sedimentary rocks must be younger than the rocks
  they flowed through.

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Section 2 Determining Absolute Age

• Absolute Age (time)
• The actual age is the actual time an event
  happened like when school gets out at 245pm.

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Absolute Dating Methods

• If we know the absolute time between two separate
  events we can determine the relative time of all
  the events that occurred between them. Also we
  can determine how long it took for the processes
  to form the Earths features.
• Rates of Erosion
• An estimate of absolute age based on known
  erosional rates of geologic features. This can
  only be used for features that were formed within
  the past 10-20 thousand years. It can be used
  for Niagara Falls, but not the Grand Canyon.
• Rates of Deposition
• This can be used to estimate absolute age based
  on the deposition rates of common rocks such as
  limestone, shale and sandstone. Generally the
  rate is about 30cm in 1000years. If flooding
  occurs, this formula can not be used.
• Varve Counting
• This is like counting the rings on a tree.
  Certain layers of sediment occur annually and can
  be counted. In areas with glacial lakes, fine
  sediments are deposited in the winter where
  larger more course sediments are deposited in the
  summers. Geologists look for the color variation
  and count the layers.

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Radiometric Dating

• Radioactive decay is the spontaneous emission of
  energy from an atom.
• It measures the amount of a parent and daughter
  isotope within a rock or mineral. By using a
  known decay rate they can determine the age of
  something.
• Each time a particle is emitted or captured from
  a parent isotope, the isotopes atomic number
  changes and becomes a different element (daughter
  isotope). Radiation is released until a stable
  isotope is formed.

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Half-life

• At the time a rock is formed it starts to decay
  at a constant rate. Half life is the rate at
  which a radioactive element decays. It is the
  time it takes for half of the radioactive atoms
  in a sample to decay to a stable product.
• The ratio of the amount of radioactive element
  left in the rock to the amount of stable product
  can be used to determine the absolute age of the
  rock.

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Radioactive Isotopes

• a. Potassium-Argon Dating
• - Potassium 40 decays to Argon 40
• - It has a half life of 1.3 billion
  years
• - Potassium is very common, found in feldspar,
  micas and amphiboles.
• - It is common in all rock families
• - It can date rocks a young as 50,000
  years and as old as 4.6 billion.
• b. Rubidium-Strontium Dating
• - Rubidium 87 decays to Strontium 87
• - Its half life is 49 billion years
• - Best for dating extremely old rocks
  from 10 million to 4.6 billion
• - It is common in feldspar micas
  (most all igneous rocks)
• c. Uranium-Lead Dating
• - Uranium 238 decays to Lead 206
• - Its half life is 4.5 billion years
• - It occurs only in trace amounts in
  Zircon (igneous)

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d. Radio Carbon Dating

• It measures the ratio of Carbon 14 to Nitrogen14.
  
• N14 Normal Nitrogen while C14 Radioactive Carbon
• Limitations
• It can only be used on organic objects
• Its half life is 5730 so it can only be used for
  objects 70,000 years old or younger.
• It rarely in Sedimentary or Metamorphic Rocks
• It does not give reliable results for rocks
  younger than
• 10 million years old

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Section 3 Fossil Records

• Fossils
• Evidence of early life preserved in Rock
• Paleontology
• The study of prehistoric times
• Paleontologist
• The scientist that studies the evidence

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Interpreting the Fossil Records

• By looking at fossils we can see how an organism
  lived and how they have either changed or gone
  extinct. This process allows us to glimpse their
  past and how Earth has changed by climate, land
  mass, temp etc.

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Fossilization

• 1. Mummification
• Taking internal organs out and replacing it with
  a substance usually herbs, then wrapping them in
  cloth strips prepared with a solution to preserve
  a body.
• 2. Original Remains
• a. Organism that is preserved in its entirety
• b. Frozen animals and humans (wooly mammoth)
• c. Bone and teeth
• d. Any organism trapped in amber
• 3. Tar Seeps
• Tar from underground comes to the surface and
  traps an animal in it. La Brea Tar Pits, Cal.
• 4. Freezing
• This occurs when an animal or organism is frozen
  quickly, bacteria did not decay the soft tissue.
  These objects are frozen whole. The wooly
  mammoth is an example.
• 5. Petrifaction
• It is when soft organic material is decayed away
  to form a mold and the minerals such as calcite,
  silica and pyrite are deposited to form the
  original shape.

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Types of Fossils

• Carbonaceous films
• It is when carbon compounds making up the tissue
  undergo a chemical change and form a thin film on
  the object.
• 2. Molds and Casts
• Molds are a hollow depression in the rock that
  shows the original shape and surface of the
  fossil.
• Casts form as hard parts (and soft sediments)
  sediment hardens.
• 3. Coprolites
• Fossilized dung or waste material
• 4. Gastrolith
• Use to help dinosaurs digest. They have a
  distinct shape, smooth and round.
• Trace fossils
• They are preserved indirect evidence of
  prehistoric life. Includes impressions left in
  rocks like footprints, tracks, burrows and bite
  marks.

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Index Fossils

• Using Index Fossils
• They are the remains of animals that lived for a
  short period of time and then became extinct.
  Therefore they would be found only in one layer.
• There are 4 characteristics
• Easily recognizable
• Abundant
• Widespread in occurrence
• Existed for only a brief period of time

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Index Fossils and Absolute Age

• Since they are only around for a short period
  scientists can use them to absolute age an area.
  This can be done in separate locations to prove
  they have something in common or even to find oil
  and gas reserves.