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4.2 Reinforcing Key Concepts

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4.2 Reinforcing Key Concepts
SECTION
ROCKS PROVIDE A TIMELINE FOR EARTH.
4.2 Reinforcing Key Concepts
BIG IDEA Rocks,
KEY CONCEPT
fossils, and other types of natural evidence tell Earth’s story.
Rocks provide a timeline for Earth.
1. Layers of sedimentary rocks show
relative age. Index fossils are used to
determine when rock layers were formed.
Two rocks are found. Rock A contains
Inoceramus labiatus. Rock B contains
Nerinea trinodosa. Based on the Index
Fossil Chart to the right, about when
was each rock formed?
Index Fossils
Millions of
years ago
Rock layers
65
Inoceramus labiatus
144
Nerinea trinodosa
206
248
Tropites subbullatus
2. Radioactive dating can show absolute age. The
rate of change of a radioactive
element is measured in half-lives. Uranium 235 is an unstable element that breaks down
into lead 207. Fill in the chart below to show the percentage of each element remaining
after 0, 1, 2, and 3 half-lives.
Half-life
0
Percentage Uranium 235
100%
CHAPTER 4
Views of Earth’s Past
1
2
3
12.5%
Percentage Lead 207
0%
Copyright © by McDougal Littell, a division of Houghton Mifflin Company
Why are index fossils useful for figuring out
the age of a set of disturbed rock layers?
224 THE CHANGING EARTH, CHAPTER 4, REINFORCING KEY CONCEPTS
224-224-sean-urb-b0402-rkc 224
3/11/04, 10:39:53 AM
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