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A Trip Through Geologic Time
Fossils
The Relative Age of Rocks
Radioactive Dating
The Geologic Time Scale
Early Earth
Eras of Earth’s History
Table of Contents
A Trip Through Geologic Time
How a Fossil Forms
- Fossils
Most fossils form when living things die and are buried by sediment. The sediment slowly hardens into rock and preserves the shapes of the organisms.
A Trip Through Geologic Time
Changes Over Time
- Fossils
The fossils record provides evidence about the history of life and past environments on Earth. The fossil record also shows that different groups of organisms have changed over time. Fossils of many different kinds of organisms were formed in this ancient lakeshore environment that existed in Wyoming about 50 million years ago.
A Trip Through Geologic Time
Changes Over Time
- Fossils
From fossils, scientists have reconstructed the paleomastodon. This animal had a short trunk and short tusks on both the upper and lower jaws. The paleomastodon is an ancestor of the modern elephant.
A Trip Through Geologic Time
What You Know
What You Learned
Using Prior KnowledgeBefore you read, look at the section headings and visuals to see what this section is about. Then write what you know about fossils in a graphic organizer like the one below. As you read, write what you learn.
1. Fossils come from ancient organisms.2. Fossils are found in hardened rock.3. Fossils show us how some present-day organisms looked
different in the past.
1. Molds and casts are types of fossils.2. Organisms are also preserved in amber, tar, and ice.3. Fossils tell us about past climates, changes in Earth’s surface,
and how organisms have changed over time.
- Fossils
A Trip Through Geologic Time
Links on Fossils
Click the SciLinks button for links on fossils.
- Fossils
A Trip Through Geologic Time
End of Section:Fossils
A Trip Through Geologic Time
The Position of Rock Layers
According to the law of superposition, in horizontal sedimentary rock layers, the oldest layer is at the bottom. Each higher layer is younger than the layers below it.
- The Relative Age of Rocks
A Trip Through Geologic Time
Determining Relative Age
To determine relative age, geologists also study extrusions and intrusions of igneous rock, faults, and gaps in the geologic record.
- The Relative Age of Rocks
A Trip Through Geologic Time
Determining Relative Age
- The Relative Age of Rocks
An unconformity occurs where erosion wears away layers of sedimentary rock. Other rock layers then form on top.
A Trip Through Geologic Time
Using Fossils to Date Rocks
Index fossils are useful because they tell the relative ages of the rock layers in which they occur.
- The Relative Age of Rocks
A Trip Through Geologic Time
Index Fossil Activity
Click the Active Art button to open a browser window and access Active Art about index fossils.
- The Relative Age of Rocks
A Trip Through Geologic Time
Question Answer
Asking QuestionsBefore you read, preview the red headings. In a graphic organizer like the one below, ask a what or how question for each heading. As you read, write answers to your questions.
What does the position of rock layer reveal?
The oldest layers—and the oldest fossils—are at the bottom.
How do geologists determine the relative age of a rock?
They examine the position of rock layer, extrusions and intrusions of igneous rock, faults, and gaps in the geologic record.
How are fossils used to date rocks?
The age of an index fossil tells the age of the rock layer in which it occurs.
- The Relative Age of Rocks
A Trip Through Geologic Time
Rock Layers
Click the Video button to watch a movie about rock layers.
- The Relative Age of Rocks
A Trip Through Geologic Time
Index Fossils
Click the Video button to watch a movie about index fossils.
- The Relative Age of Rocks
A Trip Through Geologic Time
End of Section:The Relative Age
of Rocks
A Trip Through Geologic Time
Radioactive Decay
During radioactive decay, the atoms of one element break down to form atoms of another element.
- Radioactive Dating
A Trip Through Geologic Time - Radioactive Dating
Radioactive Decay
The half-life of a radioactive element is the amount of time it takes for half of the radioactive atoms to decay.
A Trip Through Geologic Time - Radioactive Dating
Determining Absolute Ages
Geologists use radioactive dating to determine the absolute ages of rocks.
A Trip Through Geologic Time
Percentages What percentage of a radioactive element will be left after three half-lives? First multiply 1/2 three times to determine what fraction of the element will remain.
You can convert this fraction to a percentage by setting up a proportion:
To find the value of d, begin by cross-multiplying, as for any proportion:
1 X 100 = 8 X d
d =
d = 12.5%
- Radioactive Dating
A Trip Through Geologic Time
Percentages
Practice Problem
What percent of a radioactive element will remain after five half-lives?
3.125%
- Radioactive Dating
A Trip Through Geologic Time - Radioactive Dating
Determining Absolute Ages
The age of a sedimentary rock layer can be determined relative to the absolute age of an igneous intrusion or extrusion near the sedimentary rock.
A Trip Through Geologic Time
Main Idea
Detail Detail Detail
Identifying Main IdeasAs you read the section “Determining Absolute Ages,” write the main idea in a graphic organizer like the one below. Then write three supporting details that further explain the main idea.
Using radioactive dating, scientists can determine…
the absolute ages of the most ancient rocks usingpotassium-40.
the absolute ages of fossils up to about 50,000 years ago using carbon-14.
the ages of sedimentary rocks by dating the igneous intrusions and extrusions near the sedimentary rock.
- Radioactive Dating
A Trip Through Geologic Time
More on Radioactive Dating
Click the PHSchool.com button for an activity about radioactive dating.
- Radioactive Dating
A Trip Through Geologic Time
End of Section:Radioactive
Dating
A Trip Through Geologic Time - The Geologic Time Scale
The Geologic Time ScaleBecause the time span of Earth’s past is so great, geologists use the geologic time scale to show Earth’s history.
A Trip Through Geologic Time
SequencingAs you read, make a flowchart like the one below that shows the eras and periods of geologic time. Write the name of each era and period in the flowchart in the order in which it occurs.
Geologic Time ScalePrecambrian Time
Paleozoic Era: Cambrian Period
Paleozoic Era: Ordovician Period
Paleozoic Era: Silurian
Paleozoic Era: Devonian
Paleozoic Era: Carboniferous
Paleozoic Era: Permian
Mesozoic Era: Triassic
Mesozoic Era: Jurassic
Mesozoic Era: Cretaceous
Cenozoic Era: Tertiary
Cenozoic Era: Quaternary
- The Geologic Time Scale
A Trip Through Geologic Time
More on the Geologic Time Scale
Click the PHSchool.com button for an activity about the geologic time scale.
- The Geologic Time Scale
A Trip Through Geologic Time
End of Section:The Geologic Time Scale
A Trip Through Geologic Time
Earth’s Surface Forms
During the first several hundred million years of Precambrian Time, an atmosphere, oceans, and continents began to form.
- Early Earth
A Trip Through Geologic Time
Life Develops
Scientists have found fossils of single-celled organisms in rocks that formed about 3.5 billion years ago. These earliest life forms were probably similar to present-day bacteria.
- Early Earth
A Trip Through Geologic Time
Feature Early Earth Later Precambrian Earth
Comparing and ContrastingAs you read, compare and contrast the different types of mass movement by completing a table like the one below.
Atmosphere Hydrogen and helium Carbon dioxide, nitrogen, and water vapor
Oceans Earth’s surface is too hot. All water evaporates into water vapor.
Earth cools, water vapor condenses, and rain falls. Rain forms oceans.
Continents Less dense rock at surface forms continents.
Old continents break apart, and new continents form as a result of continental drift.
Precambrian Earth
- Early Earth
A Trip Through Geologic Time
Links on Precambrian Earth
Click the SciLinks button for links on Precambrian Earth.
- Early Earth
A Trip Through Geologic Time
End of Section:Early Earth
A Trip Through Geologic Time
Mass Extinctions
The graph shows how the number of families of animals in Earth’s oceans has changed.
- Eras of Earth’s History
A Trip Through Geologic Time
Mass Extinctions
The x-axis shows time in millions of years before the present; the y-axis shows the number of families of ocean animals.
Reading Graphs:
What variable is shown on the x-axis of the graph? On the y-axis?
- Eras of Earth’s History
A Trip Through Geologic Time
Mass Extinctions
Slightly more than 50 million years ago
Interpreting Data:
How long ago did the most recent mass extinction occur?
- Eras of Earth’s History
A Trip Through Geologic Time
Mass Extinctions
The one that occurred about 230 million years ago
Interpreting Data:
Which mass extinction produced the greatest drop in the number of families of ocean animals?
- Eras of Earth’s History
A Trip Through Geologic Time
Mass Extinctions
The number of families of ocean animals immediately dropped but then increased.
Relating Cause and Effect:
In general, how did the number of families change between mass extinctions?
- Eras of Earth’s History
A Trip Through Geologic Time - Eras of Earth’s History
Geologic History
A Trip Through Geologic Time
Continental Drift Activity
Click the Active Art button to open a browser window and access Active Art about continental drift.
- Eras of Earth’s History
A Trip Through Geologic Time
Previewing VisualsBefore you read, preview Figure 22. Then write three questions you have about Earth’s history in a graphic organizer like the one below. As you read, answer your questions.
Q. What geologic events happened during Precambrian Time?
A. Earth, the oceans, and the first sedimentary rocks formed.
Q. When did the dinosaurs appear on Earth?
A. About 225 million years ago
Earth’s History
Q. What caused the mass extinction at the end of the Cretaceous Period?
A. An object from space struck Earth and blocked the sunlight.
- Eras of Earth’s History
A Trip Through Geologic Time
End of Section:Eras of Earth’s
History
A Trip Through Geologic Time
Graphic Organizer
Molds and casts
Fossils
include
Rock fossils
Carbon films
include
Petrified fossils
Trace fossils
Amber
Preserved fossils
IceTar
include
A Trip Through Geologic Time
End of Section:Graphic Organizer
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