Transcript Fossils Chapter 9, Sections 2 to 5 in brief Powerpoint

Chapter 9 Fossils:
Sections 2, 3, 4, 5 in Brief
Rusty Sturken
Duluth Middle School
Sixth Grade Earth Science
September, 2010
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Personal photo from the American Museum of Natural History July, 2007
9-2 Relative Age & Absolute Age
• Relative Age- the age of a rock
compared to the ages of other
rocks
• Absolute Age- the number of
years since a rock formed
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9-2 The law of superposition
• In (undisturbed) horizontal layers of
sedimentary rock, the oldest layer is at
the bottom. Each higher layer is
younger than the layers below it.
Youngest Layers
Oldest Layers
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9-2 Geologic Column
• Geologic column-The vertical layers of
horizontal rock of different
ages that have formed
over the history of Earth.
• In the geologic column,
the oldest rocks
are on the bottom
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9-2 Index Fossils
• Index fossils are useful to geologists
because they help determine the
relative ages of the layers of rock
where they are found, they can match
rock layers of similar age
• To be an index fossil a fossil must:
- be widely distributed (found in many
areas)
- have existed briefly (a short amount of
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time)
9-2 The Principle of Original
Horizontality
• The principle of original
horizontality says that most
sedimentary layers of rock are
deposited in a horizontal
position
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9-3 Radioactive Dating
• Some elements (radioactive elements)
break down over time to form atoms of
a different element, this is called
radioactive decay
• Half-life: the time it takes for half of the
radioactive atoms in a sample to decay
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9-3 Radioactive Dating
• Geologists use radioactive dating to
determine the absolute ages of rocks
• Potassium-Argon for very old rocks due
to long half-life
• Carbon-14 for dating plant and animal
materials (they contain carbon)
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9-3 Radioactive Dating/Age of
Earth
• Radioactive dating is usually only used
to date igneous rock
• Radioactive dating shows that the
oldest moon rocks are about 4.6 billion
years old. Scientists infer that Earth is
only a little older than those moon
rocks so Earth is about 4.6 billion years
old.
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9-4 The Geologic Time Scale
• The geologic time scale is a record of
the life forms and geologic events in
Earth’s history
• Because the time span of Earth’s
past is so great, geologists use the
geologic time scale to show
Earth’s history.
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9-4 The Geologic Time Scale
• The basic units of the geologic time
scale are eras, period, and epochs
– Cenozoic Era: 65 million years ago to
present, “Age of Mammals” we are now
living in the Cenozoic Era
– Mesozoic Era: 245 million years - 65
million years ago sometimes called “Age
of Dinosaurs”
– Paleozoic Era: 245-544 million years ago
Paleo- means “ancient” –zoic means “life”
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9-5 Earth’s History
• Earth formed from a mass of dust and
gas about 4.6 million years ago
• The mass extinction at the end of the
Paleozoic Era affected plants and
animals, on land and in the seas.
Scientists do not know what caused the
mass extinction but up to 95 percent of
life in the oceans disappeared
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Geologic History
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