23.6 Earth's History
Reading Focus
Key Concepts
How do geologists determine the relative and absolute ages of rock layers?- What forms the basis for the geologic time scale?
- What are the major divisions of Earth's history?
Vocabulary
fossils
relative age
law of superposition
extinct
index fossils
absolute age
era
periods
mass extinction
Reading Strategy
Previewing Copy the table below. Before you read, examine Figures 34 and 36 to help you understand about geologic time. Write at least two questions about them in the table. As you read, write answers to your questions.
Questions on Geologic Time
Figure 32 Layers of rock are deposited horizontally, like the layers of the Grand Canyon shown here.
The Grand Canyon slices down nearly two kilometers through many horizontal layers of rock. Each layer formed millions of years ago, as a shallow sea repeatedly flooded this part of North America. The sea slowly filled up with a flat layer of sediment that had eroded from the nearby land. The next time the sea flooded the land, a new, flat sedimentary layer formed on top of the older layer beneath it. As the layers of sediment increased, they slowly changed to rock. At the same time, the remains of living things trapped in the sediment became fossils. Fossils are the preserved remains or traces of once living things.
Determining the Age of Rocks
Suppose that a geologist finds a fossil in a sedimentary rock near the rim of the Grand Canyon. Is this fossil older or younger than a fossil found near the canyon bottom? The geologist is trying to determine the relative age of the fossil as well as that of the rock containing it. The relative age of a rock is its age compared to the ages of other rocks above or below it in a sequence of rock layers. Figure 33 shows the sequence of rock layers in the Grand Canyon.
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