The Rock and Fossil Record

1
The Rock and Fossil Record
Chapter 6
Preview
Section 1 Earths Story and Those Who First
Listened Section 2 Relative Dating Which Came
First? Section 3 Absolute Dating A Measure of
Time Section 4 Looking at Fossils Section 5
Time Marches On
Concept Mapping
2
Chapter 6
Section 1 Earths Story and Those Who First
Listened
Bellringer
The Present Is the Key to the Past. This phrase
was the cornerstone of the uniformitarianist
theory developed by geologist James Hutton in the
late 1700s.   Write a few sentences in your
science journal about how studying the present
could reveal the story of Earths history. Use
sketches to illustrate processes that occurred
millions of years ago that you can still see
today.
3
Chapter 6
Section 1 Earths Story and Those Who First
Listened
Objectives

• Compare uniformitarianism and catastrophism.
• Describe how the science of geology has changed
  over the past 200 years.
• Explain the role of paleontology in the study of
  Earths history.

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Chapter 6
Section 1 Earths Story and Those Who First
Listened
The Principle of Uniformitarianism

• Scientist James Hutton, the author of Theory of
  the Earth, proposed that geologic processes such
  as erosion and deposition do not change over
  time.
• Uniformitarianism is the idea that the same
  geologic processes shaping the Earth today have
  been at work throughout Earths history.
• The next slide shows how Hutton developed the
  idea of uniformitarianism.

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Chapter 6
Section 1 Earths Story and Those Who First
Listened
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Chapter 6
Section 1 Earths Story and Those Who First
Listened
The Principle of Uniformitarianism, continued

• Uniformitarianism Versus Catastrophism Huttons
  theories sparked a scientific debate by
  suggesting the Earth was much older than a few
  thousand years, as previously thought.
• A few thousand years was not enough time for the
  gradual geologic processes that Hutton described
  to have shaped the planet.

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Chapter 6
Section 1 Earths Story and Those Who First
Listened
The Principle of Uniformitarianism, continued

• A Victory for Uniformitarianism Catastrophism
  was geologys guiding principle until the work of
  geologist Charles Lyell caused people to
  reconsider uniformitarianism.
• Lyell published Principles of Geology in the
  early 1830s. Armed with Huttons notes and new
  evidence of his own, Lyell successfully
  challenged the principle of catastrophism.

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Chapter 6
Section 1 Earths Story and Those Who First
Listened
Modern Geology -- A Happy Medium

• During the late 20th century, scientists such as
  Stephen J. Gould challenged Lyells
  uniformitarianism. They believed that
  catastrophes occasionally play an important role
  in shaping Earths history.
• Today, scientists realize that most geologic
  change is gradual and uniform, but catastrophes
  that cause geologic change have occurred during
  Earths long history.

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Chapter 6
Section 1 Earths Story and Those Who First
Listened
Uniformitarianism and Catastrophism
Click below to watch the Visual Concept.
Visual Concept
10
Chapter 6
Section 1 Earths Story and Those Who First
Listened
Paleontology -- The Study of Past Life

• The history of the Earth would be incomplete
  without knowledge of the organisms that have
  inhabited our planet and the conditions under
  which they lived.
• The science involved with the study of past life
  is called paleontology.
• Paleontologist study fossils, which are the
  remains of organisms preserved by geologic
  processes.

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Chapter 6
Section 2 Relative Dating Which Came First?
Bellringer
Arrange the following sentences in a logical
order to make a short story I stood in the
checkout line. I selected two apples. I
walked home from the store. I gave the
cashier money. I went to the store. The
cashier gave me change. I was
hungry.   Write your story in your science
journal.
12
Section 2 Relative Dating Which Came First?
Chapter 6
Objectives

• Explain how relative dating is used in geology.
• Explain the principle of superposition.
• Describe how the geologic column is used in
  relative dating.
• Identify two events and two features that
  disrupt rock layers.
• Explain how physical features are used to
  determine relative ages.

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Chapter 6
Section 2 Relative Dating Which Came First?
The Principle of Superposition

• Geologists try to determine the order in which
  events have happened during Earths history. They
  rely on rocks and fossils to help them in their
  investigation.
• The process of determining whether an event or
  object is older or younger than other events or
  objects is called relative dating.

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Chapter 6
Section 2 Relative Dating Which Came First?
The Principle of Superposition, continued

• Layers of sedimentary rock, such as the ones
  shown below, are stacked like pancakes.

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Chapter 6
Section 2 Relative Dating Which Came First?
The Principle of Superposition, continued

• As you move from the top to the bottom in layers
  of sedimentary rock, the lower layers are older.
• Superposition is a principle that states that
  younger rocks lie above older rocks, if the
  layers have not been disturbed.

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Chapter 6
Section 2 Relative Dating Which Came First?
The Principle of Superposition, continued

• Disturbing Forces Not all rock sequences are
  arranged with the oldest layers on the bottom and
  the youngest layers on top.
• Some rock sequences have been disturbed by
  forces within the Earth.
• These forces can push other rocks into a
  sequence, tilt or fold rock layers, and break
  sequences into moveable parts.

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Chapter 6
Section 2 Relative Dating Which Came First?
The Geologic Column

• The geologic column is an ideal sequence of rock
  layers that contains all the known fossils and
  rock formations on Earth, arranged from oldest to
  youngest.
• Geologists use the geologic column to interpret
  rock sequences and to identify the layers in
  puzzling rock sequences.

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Chapter 6
Section 2 Relative Dating Which Came First?
Geologic Column
Click below to watch the Visual Concept.
Visual Concept
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Chapter 6
Section 2 Relative Dating Which Came First?
Disturbed Rock Layers

• Geologists often find features that cut across
  existing layers of rock.
• Geologists use the relationships between rock
  layers and the features that cross them to assign
  relative ages to the features and the layers.
• The features must be younger than the rock
  layers because the rock layers had to be present
  before the features could cut across them.

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Chapter 6
Section 2 Relative Dating Which Came First?
Disturbed Rock Layers, continued

• Events That Disturb Rock Layers Geologists
  assume that the way sediment is deposited to form
  rock layers in horizontal layers has not
  changed over time.
• If rock layers are not horizontal, something
  must have disturbed them after they formed.
• The next slide describes four ways that rock
  layers may become disturbed.

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Chapter 6
Section 2 Relative Dating Which Came First?
Disturbed Rock Layers, continued

• A fault is a break in the Earths crust along
  which blocks of the crust slide relative to one
  another.
• An intrusion is molten rock from the Earths
  interior that squeezes into existing rock and
  cools.
• Folding occurs when rock layers bend and buckle
  from Earths internal forces.
• Tilting occurs when internal forces in the Earth
  slant rock layers.

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Chapter 6
Section 2 Relative Dating Which Came First?
Gaps in the Record -- Unconformities

• Missing Evidence Sometimes, layers of rock are
  missing, creating a gap in the geologic record.
  Missing rock layers create breaks in rock-layer
  sequences called unconformities.
• An unconformity is a break in the geologic
  record created when rock layers are eroded or
  when sediment is not deposited for a long period
  of time.

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Chapter 6
Section 2 Relative Dating Which Came First?
Unconformities
Click below to watch the Visual Concept.
Visual Concept
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Chapter 6
Section 2 Relative Dating Which Came First?
Types of Unconformities

• Most unconformities form by both erosion and
  nondeposition, but other factors may be involved.
• To simplify the study of unconformities,
  geologists place them into three major
  categories disconformities, nonconformities, and
  angular unconformities.

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Chapter 6
Section 2 Relative Dating Which Came First?
Types of Unconformities, continued

• Disconformities exist where part of a sequence
  of parallel rock layers is missing.

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Chapter 6
Section 2 Relative Dating Which Came First?
Types of Unconformities, continued

• Nonconformities exist where sedimentary rock
  layers lie on top of an eroded surface of
  nonlayered igneous or metamorphic rock.

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Chapter 6
Section 2 Relative Dating Which Came First?
Types of Unconformities, continued

• Angular Unconformities exist between horizontal
  rock layers and rock layers that are tilted or
  folded.

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Chapter 6
Section 2 Relative Dating Which Came First?
Rock-Layer Puzzles

• Rock-layer sequences often have been affected by
  more than one geological event or feature.

• For example, intrusions may squeeze into rock
  layers that contain an unconformity, as shown at
  right.

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Chapter 6
Section 2 Relative Dating Which Came First?
Rock-Layer Puzzles, continued

• Determining the order events that led to a
  sequence that has been disturbed by more than one
  rock-disturbing feature is like solving a jigsaw
  puzzle.
• Geologists must use their knowledge of the
  events that disturb rock-layer sequences to piece
  together the history of the Earth.

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Chapter 6
Section 3 Absolute Dating A Measure of Time
Bellringer
Do the following statements describe relative or
absolute age? 1. She is my younger sister.
2. He is 12 years old.   Why do geologists use
both absolute and relative dating to interpret
the past? Why are both absolute and relative
dates valid dates for geologists, and other earth
scientists to use? Write a paragraph in your
science journal.
31
Chapter 6
Section 3 Absolute Dating A Measure of Time
Objectives

• Describe how radioactive decay occurs.
• Explain how radioactive decay relates to
  radiometric dating.
• Identify four types of radiometric dating.
• Determine the best type of radiometric dating to
  use to date an object.

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Chapter 6
Section 3 Absolute Dating A Measure of Time
Radioactive Decay

• Absolute dating is any method of measuring the
  age of an event or object in years.
• To determine the absolute ages of fossils and
  rocks, scientists analyze isotopes of radioactive
  elements.
• Atoms of the same element that have the same
  number of protons but different numbers of
  neutrons are called isotopes.

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Chapter 6
Section 3 Absolute Dating A Measure of Time
Radioactive Decay, continued

• Most isotopes are stable, meaning that they
  stay in their original form.
• Other isotopes are unstable. Scientists call
  unstable isotopes radioactive.

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Chapter 6
Section 3 Absolute Dating A Measure of Time
Radioactive Decay, continued

• Radioactive isotopes tend to break down into
  stable isotopes of the same or other elements in
  a process called radioactive decay.

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Chapter 6
Section 3 Absolute Dating A Measure of Time
Radioactive Decay, continued

• Because radioactive decay occurs at a steady
  rate, scientists can use the relative amounts of
  stable and unstable isotopes present in an object
  to determine the objects age.

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Chapter 6
Section 3 Absolute Dating A Measure of Time
Radioactive Decay, continued

• Dating Rocks How Does It Work? In radioactive
  decay, an unstable radioactive isotope of one
  element breaks down into a stable isotope. The
  stable isotope may be of the same element or of a
  different element.
• The unstable radioactive isotope is called the
  parent isotope.
• The stable isotope produced by the radioactive
  decay of the parent isotope is called the
  daughter isotope.

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Chapter 6
Section 3 Absolute Dating A Measure of Time
Radioactive Decay, continued

• The rate of radioactive decay is constant, so
  scientists can compare the amount of parent
  material with the amount of daughter material to
  date rock.
• The more daughter material there is, the older
  the rock is.

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Chapter 6
Section 3 Absolute Dating A Measure of Time
Radiometric Dating

• Determining the absolute age of a sample, based
  on the ratio of parent material to daughter
  material is called radiometric dating.
• If you know the rate of decay for a radioactive
  element in a rock, you can figure out the
  absolute age of the rock.

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Chapter 6
Section 3 Absolute Dating A Measure of Time
Radiometric Dating, continued

• A half-life is the time needed for half of a
  sample of a radioactive substance to undergo
  radioactive decay.
• After every half-life, the amount of parent
  material decrease by one-half.

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Chapter 6
Section 3 Absolute Dating A Measure of Time
Types of Radiometric Dating

• Scientists use different radiometric-dating
  methods based on the estimated age of an object.
  There are four radiometric-dating techniques.
• Potassium-Argon Method Potassium-40 has a
  half-life of 1.3 billion years, and it decays
  leaving a daughter material of argon.
• This method is used mainly to date rocks older
  than 100,000 years.

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Chapter 6
Section 3 Absolute Dating A Measure of Time
Types of Radiometric Dating, continued

• Uranium-Lead Method Uranium-238 is a
  radioactive isotope with a half-life of 4.5
  billion years. Uranium-238 decays in a series of
  steps to lead-206.
• The uranium-lead method can be used to date
  rocks more than 10 million years old.

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Chapter 6
Section 3 Absolute Dating A Measure of Time
Types of Radiometric Dating, continued

• Rubidium-Strontium Method The unstable parent
  isotope rubidium-87 forms a stable daughter
  isotope strontium-87.
• The half-life of rubidium-87 is 49 billion
  years. This method is used for rocks older than
  10 million years.

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Chapter 6
Section 3 Absolute Dating A Measure of Time
Types of Radiometric Dating, continued

• Carbon-14 Method Carbon is normally found in
  three forms, the stable isotopes carbon-12 and
  carbon-13, and the radioactive isotope carbon-14.
• Living plants and animals contain a constant
  ratio of carbon-14 to carbon-12. Once a plant or
  animal dies, no new carbon is taken in. The
  amount of carbon-14 begins to decrease as the
  plant or animal decays.

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Chapter 6
Section 3 Absolute Dating A Measure of Time
Types of Radiometric Dating, continued

• The half-life of carbon-14 is 5,730 years.
• The carbon-14 method of radiometric dating is
  used mainly for dating things that lived within
  the last 50,000 years.

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Chapter 6
Section 3 Absolute Dating A Measure of Time
Radiometric Dating
Click below to watch the Visual Concept.
Visual Concept
46
Chapter 6
Section 4 Looking at Fossils
Bellringer
Describe the fossil record of your own life that
might be found 65 million years from now. What
items, or artifacts, might be likely to survive?
What kinds of things would decay and disappear?
Do you think your fossil record would produce an
accurate picture of your life? What might be
missing?   Write your description in your science
journal.
47
Chapter 6
Section 4 Looking at Fossils
Objectives

• Describe five ways that different types of
  fossils form.
• List three types of fossils that are not part of
  organisms.
• Explain how fossils can be used to determine the
  history of changes in environments and organisms.
• Explain how index fossils can be used to date
  rock layers.

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Chapter 6
Section 4 Looking at Fossils
Fossilized Organisms

• The trace or remains of an organism that lived
  long ago, most commonly preserved in sedimentary
  rock is called a fossil.
• Fossils are most often preserved in sedimentary
  rock, but other materials can also preserve
  evidence of past life.

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Chapter 6
Section 4 Looking at Fossils
Fossilized Organisms, continued

• Fossils in Rocks When an organism dies, it
  either begins to decay or is consumed by other
  organisms. Sometimes dead organisms are quickly
  buried by sediment, which slows down decay.
• Shells and bones are more resistant to decay
  than soft tissues, so when sediments become rock,
  the harder structures are more commonly preserved.

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Chapter 6
Section 4 Looking at Fossils
Fossilized Organisms, continued

• Fossils in Amber Organisms occasionally become
  trapped in soft, sticky tree sap, which hardens
  and becomes amber.
• Insect fossils have often been preserved in this
  way, but frogs and lizards have also been found
  in amber.

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Chapter 6
Section 4 Looking at Fossils
Fossilized Organisms, continued

• Petrifaction is a process in which minerals
  replace and organisms tissues.
• One form of petrifaction is called
  permineralization, a process in which the pore
  space in an organisms hard tissue is filled up
  with mineral.
• Replacement is a process in which an organisms
  tissues are completely replaced by minerals.

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Chapter 6
Section 4 Looking at Fossils
Fossilized Organisms, continued

• Fossils in Asphalt There are places where
  asphalt wells up at the Earths surface. These
  thick, sticky pools can trap and preserve
  organisms.
• Frozen Fossils Since cold temperatures slow
  down decay, many types of fossils have been found
  preserved in ice.

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Chapter 6
Section 4 Looking at Fossils
Other Types of Fossils

• Trace Fossils are naturally preserved evidence
  of animal activity. Preserved animal tracks are
  an example of a trace fossil.
• Other types of trace fossils include preserved
  burrows or shelters that were made by animals,
  and coprolite, which is preserved animal dung.

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Chapter 6
Section 4 Looking at Fossils
Other Types of Fossils, continued

• Molds and Casts are two more examples of
  fossils.
• A mold is a mark or cavity made in a sedimentary
  surface by a shell or other body.
• A cast is a type of fossil that forms when
  sediments fill the cavity left by a decomposed
  organism.

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Chapter 6
Section 4 Looking at Fossils
Using Fossils to Interpret the Past

• The Information in the Fossil Record The fossil
  record offers only a rough sketch of the history
  of life on Earth. The fossil record is incomplete
  because most organisms never became fossils.
• Scientists know more information about organisms
  that had hard body parts and that lived in
  environments that favored fossilization.

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Chapter 6
Section 4 Looking at Fossils
Using Fossils to Interpret the Past, continued

• History of Environmental Changes The fossil
  record reveals changes in an areas climate over
  time. By using the fossils of plants and land
  animals, scientists can reconstruct past
  climates.
• History of Changing Organisms By studying the
  relationships between fossils, scientists can
  interpret how life has changed over time.

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Chapter 6
Section 4 Looking at Fossils
Using Fossils to Date Rocks

• Scientists have learned that particular types of
  fossils appear only in certain layers of rock.
• By dating the rock layers above and below these
  fossils, scientists can determine the time span
  in which the organisms that formed the fossils
  lived.

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Chapter 6
Section 4 Looking at Fossils
Using Fossils to Date Rocks, continued

• If a type of organism existed for only a short
  period of time, its fossils would show up in a
  limited range of rock layers. These fossils are
  called index fossils.
• Index fossils are fossils that are found in the
  rock layers of only one geologic age, and can be
  used to establish the age of the rock layers.

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Chapter 6
Section 4 Looking at Fossils
Using Fossils to Date Rocks, continued

• Ammonites An example of an index fossil is the
  fossil of a genus of ammonites called Tropites.
• Tropites, a marine mollusk similar to a modern
  squid, lived between 230 million and 208 million
  years ago.

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Chapter 6
Section 4 Looking at Fossils
Using Fossils to Date Rocks, continued

• Trilobites Fossils of a genus of trilobites
  called Phacops are another example of an index
  fossil.
• Trilobites are extinct and lived approximately
  400 million years ago. When scientists find
  Phacops in a rock, they assume that the rock is
  approximately 400 million years old.

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Chapter 6
Section 5 Time Marches On
Bellringer
Archaeologists and paleontologists believe that
modern humans have lived on Earth for 150,000 to
200,000 years. If we imagine the history of Earth
to be the length of one calendar year, on which
date do you think modern humans arrived?
  Record your answer in your science journal.
62
Chapter 6
Section 5 Time Marches On
Objectives

• Explain how geologic time is recorded in rock
  layers.
• Identify important dates on the geologic time
  scale.
• Explain how changes in climate resulted in the
  extinction of some species.

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Chapter 6
Section 5 Time Marches On
Geologic Time

• The Rock Record and Geologic Time Grand Canyon
  National Park is one of the best places in North
  America to see Earths history recorded in rock
  layers.
• These rock layers represent almost half, or
  nearly 2 billion years, of Earths history.

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Chapter 6
Section 5 Time Marches On
Geologic Time, continued

• The Fossil Record and Geologic Time Fossils of
  plants and animals are common in sedimentary
  rocks that belong to the Green River formation.
• These fossils are well preserved. Burial in the
  fine-grained lake-bed sediments preserved even
  the most delicate structures.

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Chapter 6
Section 5 Time Marches On
The Geologic Time Scale

• The geologic column represents the 4.6 billion
  years that have passed since the first rocks
  formed on the Earth. To aid in their study,
  geologists have created the geologic time scale.
• The geologic time scale is the standard method
  used to divide the Earths long natural history
  into manageable parts.

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Chapter 6
Section 5 Time Marches On
67
Chapter 6
Section 5 Time Marches On
The Geologic Time Scale, continued

• Divisions of Time Geologists have divided the
  Earths history into sections of time.
• An eon is the largest division of geologic time.
  
• The four eons are the Hadean eon, the Archean
  eon, the Proterozoic eon, and the Phanerozoic eon.

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Chapter 6
Section 5 Time Marches On
The Geologic Time Scale, continued

• Eons are divided into eras. For example, the
  Phanerozoic Eon is divided into three eras.
• Periods are the third-largest divisions of
  geologic time and are the units into which eras
  are divided.
• Periods are divided into epochs, the
  fourth-largest division of geologic time.

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Chapter 6
Section 5 Time Marches On
The Geologic Time Scale, continued

• The Appearance and Disappearance of Species At
  certain times during Earths history, the number
  of species has increased or decreased
  dramatically.
• An increase or decrease in the number of species
  often comes as a result of a relatively sudden
  increase or decrease in competition among species.

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Chapter 6
Section 5 Time Marches On
The Geologic Time Scale, continued

• The number of species decreases dramatically
  over a relatively short period of time during a
  mass extinction event.
• Extinction is the death of every member of a
  species.
• Events such as global climate change can cause
  mass extinctions.

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Chapter 6
Section 5 Time Marches On
The Geologic Time Scale, continued

• The Paleozoic Era Old Life This era lasted
  from about 542 million to 251 million years ago.
  It is the first era that is well represented by
  fossils.
• Marine life flourished at the beginning of the
  era and the oceans became home to a diversity of
  life. However, there were few land organisms.
• By the middle of the Paleozoic era, most modern
  groups of land plants had appeared.

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Chapter 6
Section 5 Time Marches On
The Geologic Time Scale, continued

• By the end of the Paleozoic era, amphibians and
  reptiles lived on the land, and insects were
  abundant.
• The era came to an end with the largest mass
  extinction in Earths history.
• Some scientists believe that changes in seawater
  circulation were a likely cause of this
  extinction, which killed nearly 90 of all marine
  species.

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Chapter 6
Section 5 Time Marches On
The Geologic Time Scale, continued

• The Mesozoic Era The Age of Reptiles This era
  began about 251 million years ago. During this
  era, reptiles, such as dinosaurs, dominated the
  land.
• Small mammals appeared about the same time as
  dinosaurs, and birds evolved late in the era.
• At the end of the Mesozoic era, about 15 to 20
  of all species on Earth, including the dinosaurs,
  became extinct. Global climate change may have
  been the cause.

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Chapter 6
Section 5 Time Marches On
The Geologic Time Scale, continued

• The Cenozoic Era The Age of Mammals The
  Cenozoic era began about 65.5 million years ago
  and continues to the present. This era is known
  as the Age of Mammals.
• After the mass extinction at the end of the
  Mesozoic era, mammals flourished. Mammals were
  able to survive the environmental changes that
  probably caused the extinction of the dinosaurs.

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Chapter 6
The Rock and Fossil Record
Concept Mapping
Use the terms below to complete the concept map
on the next slide.
sedimentary rocks fossils half-life radioactive isotope absolute dating faults

76
Chapter 6
The Rock and Fossil Record
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Chapter 6
The Rock and Fossil Record