The Geologic Time Scale

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The Geologic Time Scale

• Earths history

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Discovering Earths History

• In the 18th and 19th Centuries scientists knew
  the Earth had a very long, active history.
• At the time they had no way of actually dating
  the Earth.
• A ____________________was developed based only on
  _____________.

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Discovering Earths History

• In 1869, ____________________ down the
  _______________through the Grand Canyon he
  realized the history of the Earth is concealed in
  rocks.
• Rocks record geological events and changing life
  forms of the past.

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Discovering Earths History

• We have learned the Earth is much older than
  anyone previously imagined.
• In the mid-1600s Archbishop James Ussher
  believed the Earth was created in 4004 B.C. His
  theories were widely accepted at the time.
• In the late 1700s James Hutton created
  the___________________________ the thought that
  the forces and processes we observe today are the
  same that have been at work on the Earth the
  beginning of time.

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Discovering Earths History

• Therefore, by studying the processes today, we
  can see what occurred throughout Earths history.

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Relative Dating

• Tells the sequence in which events occurred, not
  how long ago they occurred.
• Nicolaus Steno - geologist
• 1. _________________________ in an undeformed
  sequence of sedimentary rocks, each bed is older
  than the one above it, and younger than the one
  below it
• 2.________________________________________
  layers of sediment are typically deposited in a
  horizontal position.
• If layers are flat, they have not been disturbed,
  if they are bent they have been disturbed by
  tectonic processes.

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Relative Dating

• 3.___________________________When a fault cuts
  through, or when magma intrudes other rocks and
  crystallizes, we can assume the fault or
  intrusion is younger than the rocks affected.
• ____________________ pieces of one rock unit
  that are contained within another.
• Pg. 339

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Relative Dating

• ____________________ represents a long period
  during which deposition stops, erosion removed
  previously formed rocks, and then deposition
  began again.
• 3 types
• 1.
• 2.
• 3.

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Relative Dating

• _________________________ rocks of similar age
  in different regions that must be matched up.
• Used to develop a geologic timeline of the Earth.

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Evidence of Past Life

• ___________________ remains or traces of
  prehistoric life.
• Found in sedimentary rock
• The type of fossil that is formed depends on 2
  things
• 1.
• 2.

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Evidence of Past Life

• __________________________ some remains of
  organisms teeth, bones, and shells hardly
  change over time.
• ________________________ the remains of
  organisms likely to change over time.
• Examples petrified rock, casts, molds,
  carbonization, amber

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Evidence of Past Life

• ______________________ trace fossils
• Tracks, burrows, coprolites, and gastroliths.
• Conditions favoring preservation
• 1.
• 2.

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Evidence of Past Life

• William Smith, in the late 18th Century stated
• Fossils are not randomly distributed throughout
  the rock layers
• ______________________________ fossil organisms
  succeed one another in a definite and
  determinable order. Therefore, any time period
  can be recognized by its fossil content

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Evidence of Past Life

• Order of fossils as identified by geologists
  around the world
• Age of Trilobites
• Age of Fishes
• Age of Coal Swamps
• Age of Reptiles
• Age of Mammals
• This same order is found in the fossil record on
  every continent on Earth.

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Evidence of Past Life

• __________________ widespread geologically,
  limited to a short time span of geologic time,
  and occur in large numbers.
• Fossils are also used to interpret and describe
  ancient environments.
• Ex we know that Kansas used to be covered by a
  shallow sea due to the clam shells found in
  Kansas limestone.
• Ex they can also tell the relative temperature
  of the environment in which they are found.

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Dating with Radioactivity

• Review basic atomic structure
• Atoms have a nucleus which contains the atoms
  protons and neutrons.
• ___________can be found orbiting the nucleus
• In atoms, the number of protons is equal to the
  number of electrons.
• The ____________________________ is the number of
  protons plus the number of neutrons.
• The _____________________________ tells the
  number of protons found in the nucleus.

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Dating with Radioactivity

• _______________ are atoms of the same element
  having the same number of protons and different
  numbers of neutrons.
• Usually these isotopes are stable. Some, however
  are not causing the nucleus to spontaneously
  break apart this is called ____________________
  ____________

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Dating with Radioactivity

• This decay continues until a stable nucleus or
  isotope is formed.
• _____________________ 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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Dating with Radioactivity

• _________________________ - a way to calculate
  the ages of rocks and minerals that contain
  certain radioactive isotopes.
• Each radioactive isotope has been decaying at a
  constant rate since the formation of the rocks in
  which it occurs.
• Five have proven particularly useful in providing
  radiometric dating for ancient rocks
• Uranium-238, Uranium-235, Thorium-232,
  Rubidium-87, and Potassium-40.

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Dating with Radioactivity

• Radiometric dating can only occur for rocks
  contained in a closed system since their
  formation.
• This is due to the fact that all parent and
  daughter isotopes must be kept within the rock so
  measurement can take place
• Any loss or addition of parent or daughter
  isotopes will result in a faulty dating

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Dating with Radioactivity

• Carbon-14 dating
• Carbon-14 is continuously produced in the
  atmosphere it is continuously incorporated into
  living organisms.
• When an organism dies, it no longer takes up
  carbon-14, and it generally decreases as it
  decays.
• By comparing the amount of carbon-12 to the
  amount of carbon-14, radio-carbon dates can be
  determined.

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The Geologic Timescale

• Based on interpretations of the rock record,
  geologists have divided Earths 4.6 billion year
  history into units of time that represent
  specific amounts of time. Taken together, these
  time spans make up the ___________________________
  .

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The Geologic Time Scale

• __________- represent the largest amounts of time
  they are further divided into _________, which
  are divided into _______________, which are
  divided into smaller units of time called
  ______________.

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The Geologic Time Scale

• ______________________ began 540 million years
  ago
• Means visible life
• Rocks of this Eon contain many fossil records
  that document major changes in living organisms
  over time.

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The Geologic Time Scale

• Phanerozoic Eon is divided into 3 eras the
  _______________ (ancient life),
  ___________________ (middle life), and the
  _____________ (recent life).
• The eras were divided according to major
  extinctions that took place at those times.

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The Geologic Time Scale

• The eras are divided into periods that represent
  somewhat less profound change in life forms.
• The Cenozoic is further divided into epochs.

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The Geologic Time Scale

• Precambrian Time fewer life forms were present
  on earth during this time. Their fossils are
  difficult to identify and many times the rock and
  fossil record have been disturbed.
• It represents nearly 88 of Earths history.