Geologic Time Notes

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Geologic Time Notes
Geologic Time
,
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Fossils can only be found in rocks.

• sedimentary

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On to the next question...
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Why is this statement false The caveman had
dinosaur for breakfast.?

• man and dinosaurs
• never lived at the same time

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Geologic Time Scale

• The geologic Time Scale is a record of Earths
  history
• The Earth is 4.6 Billion Years Old
• The names of the divisions do not change however
  the years designating the beginning and end of
  these divisions are often reconsidered

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Eons

• The time scale is divided into eons
• An Eon is the longest time unit and is measured
  in billions of years

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Eons

• The history of the Earth is divided into 4 eons
• Hadeon(4.6 Bya-3.8 Bya)
• Archaen(3.8 Bya-2.5 Bya)
• Proterozoic(2.5 Bya-542 mya)
• Phanerozoic(542 mya-Present)

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Precambrian

• The Precambrian includes the
• Hadeon
• Archaen
• Proterozoic eons.
• Includes 90 of Earths History
• The end of the Proterozoic is defined by the
  first appearance of organisms with hard
  parts(shells, skeletons)

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Precambrian

• Up until the end of the Proterozoic all organisms
  had soft bodies
• Many of these organisms resemble
• Sponges
• Snails
• Worms

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Eras

• Eras are the next longest span of time.
• Eras are measured in hundreds of millions of years

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Eras

• The names of the eras of the Phanerozoic are
  based on their age
• Paleo means old
• Meso means middle
• Ceno means recent
• Zoic means life

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Eras

• The Phanerozoic Eon is divided into 3 Eras
• Paleozoic(542 mya-250 mya)
• Mesozoic(250 mya-65 mya)
• Cenozoic(65 mya-Present)

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Paleozoic Era

• During the Paleozoic era the oceans had a wide
  diversity of plants and animals
• Trilobites were the dominant organism in the
  oceans during the early Paleozoic
• All trilobites were extinct by the end of the
  Paleozoic

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Paleozoic Era

• Land plants and land animals first appeared
• At the end of the Paleozoic 90 of marine
  organisms went extinct
• This is considered to be the largest mass
  extinction event in Earths History

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Mesozoic Era

• The Mesozoic era is known for
• The emergence of the dinosaurs
• Reef Building corals
• Predatory reptiles
• Amphibians living on land and in water
• Dinosaur population began to decline towards the
  end and mammals began to evolve

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Mesozoic Era

• Like the Paleozoic the end of the Mesozoic is
  marked by a massive extinction event

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Cenozoic Era

• During the Cenozoic
• Mammals increased in number and diversity
• Human ancestors developed
• Grasses and flowering plants expanded on land
• Ocean life remained relatively unchanged however

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Periods

• Eras are divided into periods
• Periods are usually defined by life forms that
  appeared or went extinct during that time
• Some periods are named for a geographic area in
  which the first rock of that age was discovered

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Periods

• The Mississippian period was named for a
  distinctive limestone that formed along the
  Mississippi River

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Periods

• The Jurassic Period is named for the rocks
  discovered in the Jura Mountains in Europe

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Epochs

• Periods are divided into Epochs which are
  measured in millions of years to tens of millions
  of years
• The fossil record the Cenozoic Era is relatively
  complete
• There has been less time for weathering and
  erosion to destroy the fossil record

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Epochs

• Certain organisms are used to distinguish the
  various epochs
• Marine Fossils are used to mark the Oligocene
• Terrestrial plant fossils are used to mark the
  Eocene

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Relative Dating
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Relative Dating Of Rocks

• Relative Dating estimates the order of past
  geologic events by using basic stratigraphic
  rules
• Relative dating does not give us the ages of past
  events but can tell us the order in which they
  happened

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Relative Dating

• Uniformitarianism states that The Present is the
  Key to the past.
• In other words processes that are occurring today
  were more than likely occurring throughout
  Earths history

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Relative Age
If a sedimentary rock outcrop has not been
overturned, which layer would be the oldest?
What is this principle known as?
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oldest rock layers are on bottom
Principle of Superposition
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Principle of Superposition

• The principle of superposition states that in an
  undisturbed rock sequence the oldest rocks are on
  the bottom and going up they progressively get
  younger

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Using this principle, label the strata below from
oldest to youngest.
youngest
oldest
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On to the next question...
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Faults are always (older, younger) than the
rocks they cut through.
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Which is older F or S How do you know?
F cuts through S S must have been there first
(it's older)
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On to the next question...
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If a geologist finds an igneous sill, how can she
determine if the sill is an intrusion or an
extrusion?
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Intrusions
contact metamorphism on all sides
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extrusions
no contact metamorphism on top
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Is H an intrusion or extrusion?How can you
tell?
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H is an intrusion.Contact metamorphism on top
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On to the next question...
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What is an unconformity?

• a buried erosional surface

How does it complicate the relative dating of
rock layers?

• a part of the rock record
• is missing

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What processes could lead to an unconformity?

• weathering and erosion

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Using the diagram to the left, identify where the
unconformity is located by drawing an arrow and
writing the word unconformity next to it.
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On to the next question...
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What characteristics must fossils have in order
to be good index fossils?

• lived over a large geographic area (large
  horizontal distribution)
• lived for a short period of time (small vertical
  distribution)

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I n the diagram below, a geologist has matched
up rock layers based on index fossils.
In geology, this is referred to as CORRELATION
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Absolute Dating
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Absolute Age
Why are radioactive isotopes useful in
determining the absolute age of a rock?
their half-lives are constant
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Absolute Age

• What can be done to change the
• half-life of a radioactive isotope?
• Why is this important?

NOTHING!
it is reliable to calculate age
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Absolute Dating Techniques

• Absolute dating allows scientists to determine
  the actual age of a rock, fossil or other object
• Scientists use radioactive decay to determine the
  ages of rocks

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Radioactive Decay

• Radioactive substances emit particles at a set
  rate
• As they emit particles the number of protons and
  neutrons change and the element is converted into
  a different element
• Radioactive dating is the emission of radioactive
  particles and the resulting change into other
  elements

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Radiometric Dating

• Radiometric dating is the process in which
  scientists determine the ratio of parent nuclei
  to daughter nuclei
• After they determine the ratio of parent to
  daughter nuclei they can figure out the actual
  age of the object

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Radiometric dating

• As this process takes place the Parent decays
  into the daughter
• The parent isotope is what the element originally
  was
• The daughter isotope is what the parent isotope
  is turning into

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Radiometric Dating

• Example
• Uranium-238 will decay into Lead-206 during a
  specific span of time
• The rate at which these particles decay remains
  constant making them good indicators of the
  actual age of the object

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Half Life

• Half life-The period of time it takes for one
  half of the isotope to decay

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Useful Isotopes
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Useful Isotopes

• Carbon 14-Useful for finding the age of materials
  that are of organic origin
• Amber
• Humanoid Bones
• Papyrus
• Charcoal Fragments
• The half life of Carbon 14 is 5730 therefore it
  would be used to date rocks that are thousands of
  years old

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Half Lives
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Half Lives

• The half life of U-238 is 4.5 Billion Years old
  so therefore it would not be useful to date
  something that is thousands of years old
• The isotope used is based on the age range of the
  rock

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Absolute Age

• What radioactive isotope could be
• used to determine the absolute
• age of material that was recently living?
• carbon-14

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If there is a 100g sample of C14, how many grams
of C14 would remain after three half-lives?
How long would this take? Show all work.

?
?
?
100g
50g
25g
12.5g
3 half-lives x (5.7 x 103) 1.71 x 104
17,100 years
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Practice

• How old are the following rocks
• Contains 50 U-235, 50 Pb-207
• Contains 25 K-40, 75 Ar-40
• Contains 12.5 C-14, 87.5 N-14
• Contains 50 U-238, 50 Pb-206

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Other Absolute Dating Techniques

• Dendrochronology-The science of comparing annual
  growth rings in trees to date event and changes
  in the past environments
• During the spring a tree experiences its
  greatest growth
• During the winter its growth is less

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Dendrochronology

• The widths of a trees rings are related to the
  climate conditions during growth periods

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Varves

• Vares are bands of alternating light and dark
  colored sediments of sand, clay and silt
• During the Pleistocene
• (11,000 years ago) there was mass glaciation
• When the glaciers melted the glacial sediments
  were deposited in lakes

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Varves

• The glacier sediment is dark in color
• When they mix with sediments that accumulated
  during warm temperatures varves are formed

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Key Beds

• Key Beds-Sediment layers that serves as a time
  marker in the rock record.
• Key beds result from
• Volcanic Ash
• Meteorite Impact debris
• Found in rocks deposited during the Cretaceous
  Period

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Key Beds

• Key Beds are released by volcanic eruptions as
  well
• Will be used to date rocks back to the eruption
  of Mount St. Helens(1980)

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Evolution

• Explain the theory of evolution.

organisms adapt to their environment in order to
survive
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Evolution

• Explain how the fossil record supports this theory

can see that different organisms have changed
over time
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