Unit 2: Earth’s History
Big Idea: Rock, fossils, and other types of natural evidence are used to study Earth’s history and measure geologic time.
Lesson 1: Geologic Change Over Time:
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Essential Question: How do we learn about Earth’s history?
o Be able to explain how Earth materials, such as rock, fossils, and ice, show that Earth has changed over
time.
Key Topics:
o State the principle of uniformitarianism.
 Theory that the same geologic processes shaping Earth today have been at work throughout
Earth’s history.
 This theory was proposed in 1788 by James Hutton.
o Define fossils.
 Fossils are the traes or remains of organisms that lived in the past, most commonly preserved in
sedimentary rock.
o Describe the ways organisms can be preserved as fossils.
 Remains of organisms preserved in:
 Asphalt
 Amber
 Ice
 Some fossils form by petrification, a process in which an organism’s tissues are completely
replaced by minerals.
o List examples of trace fossils.
 Tracks
 Burrow
 Coprolites
o The Rock Record
 Explain how fossils supply evidence of geologic change.
 The fossils preserved in sedimentary rock serve as a record of evolution and provide
data about past environments and climate.
 Relate the composition and texture of sedimentary rocks to the environment in which the rocks
formed.
 Its composition indicates erosive sources of the sediment.
 Its texture indicates the environment in which the sediment was transported and
deposited.
 Describe and give evidence for the movement of continents over time.
 The continent have been moving throughout Earth’s History
 At one time, they formed a single landmass, which is evident:
o By the shapes of continents.
o By the distribution of rock types, mountains, and fossils.
o Earth’s Changing Climate
 List types of evidence that support that Earth’s climate has changed over time.
 Ice cores, sea-floor sediments, and tree rings provide evidence that Earth’s climate has
changed over time.
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Vocabulary (Study tip: make flash cards and practice writing the vocabulary words in a sentence in the proper
context):
o Uniformitarianism
o Fossil
o Trace fossil
o Climate
o Ice core
Lesson 2: Relative Dating:
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Essential Questions: How are the relative ages of rocks measured?
Key Topics:
o Dating Undisturbed Rock Layers.
 Define relative dating.
 Relative dating is used to find if one object or event is older or younger than another.
 Describe deposition of sedimentary rock layers.
 Sedimentary rock forms in horizontal layers and will stay that way if not disturbed.
 Summarize and apply the law of superposition.
 The law of superposition states that an undisturbed sedimentary rock layer is older than
the layers above it and younger than the layers below it.
o Dating Disturbed Rock Layers.
 Define unconformity.
 An unconformity is a gap in the geologic record caused by erosion or a pause in
deposition.
 Explain and apply the law of crosscutting relationships.
 The law of crosscutting relationships states that a fault or body of rock is younger than
any rock it cuts through
o Fossils and Relative Dating
 Summarize how scientists can use fossils to determine the relative age of rock layers.
 A rock layer that contains a fossil of an earlier life form is relatively older than a rock
layer that contains a fossil of a more recent life form
o Geologic Columns
 Describe geologic columns and their use.
 An ordered arrangement of rock layers based on their relative ages that is used to
identify the relative ages of rock layers that are located in different regions.
Vocabulary:
o Relative dating
o Fossil
o Superposition
o Geologic column
o unconformity
Lesson 3: Absolute Dating:
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Essential Question: How is the absolute age of rock measured?
Key Topics:
o Absolute Dating
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 Define absolute dating.
 Define radioactive decay.
 Apply the concept of half-life to problems of determining the age of a sample.
o Radiometric Dating
 Explain radiometric dating.
 Identify radiometric dating methods.
o The Age of the Earth
 Identify approximately how old Earth is.
 Explain how scientists have determined the age of Earth.
o Index Fossils
 Define index fossil.
 Explain how index fossils can be used to determine the age of rock.
Vocabulary
o Absolute dating
o Radiometric dating
o Radioactive decay
o Half-life
Lesson 4: The Geologic Time Scale
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Essential Question: What is the geologic time scale?
Key Topics
o A Historical Perspective of Geologic Change
 Describe the principle of catastrophism in terms of the rate of geologic change.
 Describe the principle of uniformitarianism in terms of the rate of geologic change.
 Explain what geologists mean today when they use the term uniformitarianism.
o The Geologic Time Scale
 Explain how the age of Earth has been determined.
 Describe the geologic time scale and explain how it is divided into different spans of time.
o Milestones in Earth History
 List the four major divisions of Earth’s history
 Describe major events in the geologic history of Earth during Precambrian time, the Paleozoic
era, the Mesozoic era, and the Cenozoic era.
Vocabulary
o Geology
o Geologic time scale