Transcript A Trip Through Geologic Time
A Trip Through Geologic Time
Table of Contents
Fossils The Relative Age of Rocks Radioactive Dating The Geologic Time Scale Early Earth Eras of Earth’s History
A Trip Through Geologic Time
- Fossils
How a Fossil Forms
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
- Fossils
Changes Over Time
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
- Fossils
Changes Over Time
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
- Fossils
Using Prior Knowledge
Before 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.
2.
3.
What You Know
Fossils come from ancient organisms.
Fossils are found in hardened rock.
Fossils show us how some present-day organisms looked different in the past.
1.
2.
3.
What You Learned
Molds and casts are types of fossils.
Organisms are also preserved in amber, tar, and ice.
Fossils tell us about past climates, changes in Earth’s surface, and how organisms have changed over time.
A Trip Through Geologic Time
- Fossils
Links on Fossils
Click the SciLinks button for links on fossils.
A Trip Through Geologic Time
End of Section: Fossils
A Trip Through Geologic Time
- The Relative Age of Rocks
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.
A Trip Through Geologic Time
- The Relative Age of Rocks
Determining Relative Age
To determine relative age, geologists also study extrusions and intrusions of igneous rock, faults, and gaps in the geologic record.
A Trip Through Geologic Time
- The Relative Age of Rocks
Determining Relative Age
An unconformity occurs where erosion wears away layers of sedimentary rock. Other rock layers then form on top.
A Trip Through Geologic Time
- The Relative Age of Rocks
Using Fossils to Date Rocks
Index fossils are useful because they tell the relative ages of the rock layers in which they occur.
A Trip Through Geologic Time
- The Relative Age of Rocks
Index Fossil Activity
Click the Active Art button to open a browser window and access Active Art about index fossils.
A Trip Through Geologic Time
- The Relative Age of Rocks
Asking Questions
Before 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.
Question Answer
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?
How are fossils used to date rocks?
They examine the position of rock layer, extrusions and intrusions of igneous rock, faults, and gaps in the geologic record.
The age of an index fossil tells the age of the rock layer in which it occurs.
A Trip Through Geologic Time
- The Relative Age of Rocks
Rock Layers
Click the Video button to watch a movie about rock layers.
A Trip Through Geologic Time
- The Relative Age of Rocks
Index Fossils
Click the Video button to watch a movie about index fossils.
A Trip Through Geologic Time
End of Section: The Relative Age of Rocks
A Trip Through Geologic Time
- Radioactive Dating
Radioactive Decay
During radioactive decay, the atoms of one element break down to form atoms of another element.
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
- Radioactive Dating
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%
A Trip Through Geologic Time
- Radioactive Dating
Percentages
Practice Problem
What percent of a radioactive element will remain after five half-lives?
3.125%
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
- Radioactive Dating
Identifying Main Ideas
As 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.
Main Idea
Using radioactive dating, scientists can determine… Detail the absolute ages of the most ancient rocks using potassium-40.
Detail the absolute ages of fossils up to about 50,000 years ago using carbon-14.
Detail the ages of sedimentary rocks by dating the igneous intrusions and extrusions near the sedimentary rock.
A Trip Through Geologic Time
- Radioactive Dating
More on Radioactive Dating
Click the PHSchool.com button for an activity about radioactive dating.
A Trip Through Geologic Time
End of Section: Radioactive Dating
A Trip Through Geologic Time
- The Geologic Time Scale
The Geologic Time Scale
Because 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
- The Geologic Time Scale
Sequencing
As 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 Scale
Precambrian Time Paleozoic Era: Permian Paleozoic Era: Cambrian Period Mesozoic Era: Triassic Paleozoic Era: Ordovician Period Mesozoic Era: Jurassic Paleozoic Era: Silurian Paleozoic Era: Devonian Paleozoic Era: Carboniferous Mesozoic Era: Cretaceous Cenozoic Era: Tertiary Cenozoic Era: Quaternary
A Trip Through Geologic Time
- The Geologic Time Scale
More on the Geologic Time Scale
Click the PHSchool.com button for an activity about the geologic time scale.
A Trip Through Geologic Time
End of Section: The Geologic Time Scale
A Trip Through Geologic Time
- Early Earth
Earth’s Surface Forms
During the first several hundred million years of Precambrian Time, an atmosphere, oceans, and continents began to form.
A Trip Through Geologic Time
- Early Earth
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.
A Trip Through Geologic Time
- Early Earth
Comparing and Contrasting
As you read, compare and contrast the different types of mass movement by completing a table like the one below.
Feature Precambrian Earth Early Earth Later Precambrian Earth
Atmosphere Oceans Continents Hydrogen and helium Carbon dioxide, nitrogen, and water vapor Earth’s surface is too hot. All water evaporates into water vapor.
Less dense rock at surface forms continents.
Earth cools, water vapor condenses, and rain falls. Rain forms oceans.
Old continents break apart, and new continents form as a result of continental drift.
A Trip Through Geologic Time
- Early Earth
Links on Precambrian Earth
Click the SciLinks button for links on Precambrian Earth.
A Trip Through Geologic Time
End of Section: Early Earth
A Trip Through Geologic Time
Eras of Earth’s History
Mass Extinctions
The graph shows how the number of families of animals in Earth’s oceans has changed.
A Trip Through Geologic Time
Eras of Earth’s History
Mass Extinctions
Reading Graphs:
What variable is shown on the
x
-axis of the graph? On the
y
axis?
The
x
-axis shows time in millions of years before the present; the
y
-axis shows the number of families of ocean animals.
A Trip Through Geologic Time
Eras of Earth’s History
Mass Extinctions
Interpreting Data:
How long ago did the most recent mass extinction occur?
Slightly more than 50 million years ago
A Trip Through Geologic Time
Eras of Earth’s History
Mass Extinctions
Interpreting Data:
Which mass extinction produced the greatest drop in the number of families of ocean animals?
The one that occurred about 230 million years ago
A Trip Through Geologic Time
Eras of Earth’s History
Mass Extinctions
Relating Cause and Effect:
In general, how did the number of families change between mass extinctions? The number of families of ocean animals immediately dropped but then increased.
A Trip Through Geologic Time
Eras of Earth’s History
Geologic History
A Trip Through Geologic Time
Eras of Earth’s History
Continental Drift Activity
Click the Active Art button to open a browser window and access Active Art about continental drift.
A Trip Through Geologic Time
Eras of Earth’s History
Previewing Visuals
Before 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.
Earth’s History 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
Q
. What caused the mass extinction at the end of the Cretaceous Period?
A
. An object from space struck Earth and blocked the sunlight.
A Trip Through Geologic Time
End of Section: Eras of Earth’s History
A Trip Through Geologic Time
Graphic Organizer
Fossils include Rock fossils include Preserved fossils include Carbon films Petrified fossils Molds and casts Trace fossils Amber Tar Ice
A Trip Through Geologic Time