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

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


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.


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.


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.


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


Links on Fossils

Click the SciLinks button for links on fossils.

- Fossils


End of Section:Fossils


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


Determining Relative Age

To determine relative age, geologists also study extrusions and intrusions of igneous rock, faults, and gaps in the geologic record.



Determining Relative Age


An unconformity occurs where erosion wears away layers of sedimentary rock. Other rock layers then form on top.


Using Fossils to Date Rocks

Index fossils are useful because they tell the relative ages of the rock layers in which they occur.



Index Fossil Activity

Click the Active Art button to open a browser window and access Active Art about index fossils.



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.



Rock Layers

Click the Video button to watch a movie about rock layers.



Index Fossils

Click the Video button to watch a movie about index fossils.



End of Section:The Relative Age

of Rocks


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.


Determining Absolute Ages

Geologists use radioactive dating to determine the absolute ages of rocks.


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%



Percentages

Practice Problem

What percent of a radioactive element will remain after five half-lives?

3.125%



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.


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.



More on Radioactive Dating

Click the PHSchool.com button for an activity about radioactive dating.



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.


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


More on the Geologic Time Scale

Click the PHSchool.com button for an activity about the geologic time scale.

- The Geologic Time Scale


End of Section:The Geologic Time Scale


Earth’s Surface Forms

During the first several hundred million years of Precambrian Time, an atmosphere, oceans, and continents began to form.

- 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.

- Early Earth


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


Links on Precambrian Earth

Click the SciLinks button for links on Precambrian Earth.

- Early Earth


End of Section:Early Earth


Mass Extinctions

The graph shows how the number of families of animals in Earth’s oceans has changed.

- Eras of Earth’s History


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?



Mass Extinctions

Slightly more than 50 million years ago

Interpreting Data:

How long ago did the most recent mass extinction occur?



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?



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?


A Trip Through Geologic Time - Eras of Earth’s History

Geologic History


Continental Drift Activity

Click the Active Art button to open a browser window and access Active Art about continental drift.



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.



End of Section:Eras of Earth’s

History


Graphic Organizer

Molds and casts

Fossils

include

Rock fossils

Carbon films

include

Petrified fossils

Trace fossils

Amber

Preserved fossils

IceTar

include


End of Section:Graphic Organizer

Documents

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