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Suggested levels for Guided Reading, DRA,™ Lexile,® and Reading Recovery™ are providedin the Pearson Scott Foresman Leveling Guide.

ISBN 0-328-13571-2

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GenreComprehension

Skills and StrategyText Features

Expository

nonfiction

• Cause and Effect

• Fact and Opinion

• Summarize

• Captions

• Diagrams

• Time Line

• Glossary

Scott Foresman Reading Street 5.5.4

Earth Science

Earth:The Inside Story

by Camilla Calamandrei



Editorial Offices: Glenview, Illinois • Parsippany, New Jersey • New York, New YorkSales Offices: Needham, Massachusetts • Duluth, Georgia • Glenview, Illinois

Coppell, Texas • Ontario, California • Mesa, Arizona

Earth:

The Inside Story

by Camilla Calamandrei



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correct errors called to its attention in subsequent editions.

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Illustrations by Jonathan Massie

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ISBN: 0-328-13571-2

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2 3 4 5 6 7 8 9 10 V0G1 14 13 12 11 10 09 08 07 06 05

Today, we understand a great deal about volcanoes,earthquakes, and tidal waves, and we understand how theyare related.

3

The Earth in Motion

Earth may look like a calm planet when viewed

from outer space. But it is anything but calm. In fact,

we live on a planet that is constantly moving and

changing.

Earth moves in a few different ways. First of all,

we now know that Earth revolves around the sun.

We also know that it spins on an axis. But the planet

itself is not one solid piece of rock. Different layers of

Earth itself are moving and changing all the time.



4

Earthquakes, volcanic eruptions, and tidal waves

are all dramatic side effects of layers of the planet

moving into and away from each other. They remind

us that things are always happening within the

planet.

Read on to find out how scientists learned that

Earth is made of layers.

Pieces of a Puzzle

For thousands of years, humans thought

Earth was a solid mass. Even the famous Greek

philosopher Aristotle, who lived more than 2,300

years ago correctly thought Earth was round, long

before others realized it. But he thought, incorrectly,

that Earth was solid.

It was not until 2,000 years later that some

evidence came to light that made a few peoplebegin to question whether the Earth really was solid.

Before the invention of the airplane, maps were

challenging to make. Early mapmakers had to

reconstruct entire coastlines on paper, even though

they could see and measure only the small area

that was in front of them at any given moment.

Today, our maps are very precise because planes and

satellites allow us to see Earth’s landmasses—and

many miles of coastline—from above.

An early map of the world 5

During the 16th century, explorers sailing the

seas in search of new routes from Europe to the East

came upon continents they had not known about.

Each of these expeditions included mapmakers.

These mid-16th century maps revealed something

amazing. It appeared that the western edge of the

African continent and the eastern edge of the South

American continent could fit together like two pieces

of a jigsaw puzzle. Could that really have happened

by chance?



African continent

South American

continent

6 7

The English philosopher Francis Bacon (1561–

1626) believed that the interlocking shapes of the

coastlines of Africa and South America did not

happen by chance. Other scholars and scientists

felt the same way. Some philosophers believed that

Africa and South America had once been a single,

large landmass.

They were less certain, however, about what had

caused the landmass to split. They did not know how

or why the continents floated so far from each other.

How could such a thing happen if Earth were solid

all the way through?

Some people decided that the waters of a great

flood—described in the Bible—had divided the

original continent and floated its parts to different

hemispheres. The idea that the flood had changed

Earth’s surface became popular. Francis Bacon even

went along with it.

If Earth were solid, though, as people believed,

then where did the water for such a huge flood

come from? Some scientists and scholars decided

that Earth’s entire surface must float on water. They

speculated that chemicals in the water had somehow

dissolved the solid core of Earth and caused it to

break apart.



Neptune,

Roman God of the Sea

Pluto,

Roman God of the Underworld

The idea that the continents floated on water

became known as the Neptunist idea. It was named

for the Roman god of the sea and earthquakes,

Neptune.

A competing idea that emerged in the late 1700swas called the Plutonist idea. Pluto was the god of

the underworld. The Plutonists believed that Earth’s

surface rested on a mass of molten rock, and they

pointed to Earth’s volcanoes as proof. They said that

if Earth’s crust floated on water, then volcanoes

would spew water instead of molten lava.

8

Solid core

Liquid rock

Earth’s crust

9

Three LayersBy the 1870s, most geologists (scientists who study

Earth’s origin and structure) believed that Earth was

made up of three layers—a rigid crust that encases

Earth, a solid core, and liquid rock that surrounds

Earth’s core.

This basic description of the Earth’s interior was

scientifically confirmed in the late 1800s and early

1900s. Scientific developments in the mid-1900s

allowed even greater, more complex, understanding

of the layers that make up the planet.



10

During the mid-1800s, scientists seemed to have

put aside the question inspired by the maps of the

1500s: How could the continents have moved or

floated away from one another if Earth’s crust is a

rigid shell? They focused on the fact that there was

molten, or liquid, rock under the crust. They called

this layer of liquid rock between Earth’s surface and

its core, the mantle, but they did not explain how

pieces of the crust could have moved.

Geologists didn’t know whether the mantle was

all molten rock or if part of it might be solid. They

also were eager to find proof that Earth had a solid

core, as they believed.

A Solid Core

In 1880, English geologist John Milne and his

team invented the first accurate seismometer. Thismachine is used to locate earthquake shocks and

measure the strength of an earthquake. A machine

called a seismograph is used to record earthquakes.

Geologists using seismometers and seismographs

noticed that the shocks, or waves, of earthquakes

varied.

During an earthquake, the frame of the seismograph shakes,but the weight with the pen attached stays still. When theframe shakes, it moves the paper below the pen, whichrecords the size of the shock waves. Scientists use this record tocalculate the earthquake’s strength.

Framemovement

How a Seismograph Works

Weight

Pen

Paper

Drum

Ground movement

11



12

Experts knew that the waves would have

measured the same if they were traveling through

the same kind of material. The fact that the shock

waves varied told them that the waves were

traveling through different materials. Geologists

began to study the measurements of these

waves for clues about what made up Earth’s

interior.

The English seismologist Richard Dixon

Oldham performed studies in 1906 indicating

that Earth’s core was dense.

In 1936, the Danish seismologist Inge

Lehmann documented that some seismic waves

during an earthquake did not travel through Earth’s

core but bounced back. She concluded that Earth

has a central inner core that is solid iron and a

surrounding outer core that is made of liquid iron.

It is now known that the inner core is a solid iron

ball approximately the size of the moon. It can reach

temperatures from 6,700º to 12,600º Fahrenheit

(possibly more). The intense heat of the inner core

keeps the outer core in a permanently molten state.

At the same time, the pressure from the outer core

keeps the inner core solid.

Inner core

Outer core

13



The pressure and temperature of the earth increases as onemoves closer to the center.

Inner core

about 6,700°F–12,600°F

Outer core

about 8,000°F–11,000°F

Crust

Mantle

about 1,600°F

14

This is how the continents mayhave looked millions of years ago.

15

Continental Drift

In 1912, the German scientist Alfred Wegener

suggested that Africa and South America had once

been one continent. As you know, he was not the

first to have this idea (remember the maps of the

1500s), but he did offer new thinking on the subject.

Wegener noticed two things in his research. First,

fossils of similar extinct plants had been discovered

in both Africa and South America. This suggested

that at the time the fossils were living plants, the

two continents had been one. Second, modern, more

detailed maps showed that other continents besides

South America and Africa had coastlines that seemed

to interlock. This suggested that other continents

may once have been connected as well.



16

After gathering more clues, Wegener decided

that all of the continents had once been one

super-continent. He suggested that approximately

200 million years ago, this super-continent (or

Pangaea, as he called it) had split, and its fragments

had gradually drifted apart. He also claimed the

continents were still floating away from one another.

Because most geologists in 1912 still thought

that Earth’s surface was an unbroken shell, they

were unsure of Wegener’s idea. Wegener died while

conducting research in Greenland in 1930. Though

his work still had not been widely accepted, soon it

would be.

In the 1950s, important information about Earth’s

crust came to light. A group of scientists exploring

the ocean floor discovered a long chain of ridges

that formed an underwater mountain range. As

they examined these ridges, they realized that these

undersea mountains had developed along a deep

crack in Earth’s crust. This proved that Earth’s crust is

not an unbroken shell.

Just as the philosopher

Aristotle had been right that

Earth was round but wrong

that it was solid, Francis Bacon

was right about one thing

but wrong about another.Alfred Wegener’s research in

the 1900s showed that Bacon

was right in the 1500s when

he proposed that Africa and

South America had once been

one continent. The discovery

that Earth’s crust floats on

molten rock proved that Bacon

was wrong in thinking thatthe continents float on water.

Francis Bacon

Alfred Wegener

17

By 1960, it was understood that the crust of Earth

is rigid, but it is not one solid piece like a layer of

armor. Instead, Earth’s crust is broken into separate

pieces that scientists call tectonic plates. These plates

float on the layer of molten rock below the crust’s

surface.

The molten rock moves, and it carries the tectonic

plates to different locations. This explains how the

continents moved in the past and introduces the idea

that they are still moving.



1500s Maps show that Africanand South American coastlinesappear to interlock. PhilosopherFrancis Bacon (1561–1626) proposes

that Africa and South America musthave once been one continent.

1500 190

1890

1880

1880 Geologist JohnMilne invents themodern seismograph.

18

Once they accepted Wegener’s theory that Earth’s

crust is not an unbroken shell, geologists began to

piece together the history of Earth’s continents.

It is now believed that 225 million years ago,

Earth had one great landmass (Pangaea), as Wegener

had proposed. Then, about 200 million years ago,

Pangaea began to split apart, and there were two

great landmasses. The southern landmass included

the land that would become South America, Africa,

India, Australia, and Antarctica. The northern

landmass included the land that would become all of

Asia (except India), Europe, and North America.

Scientists believe that about 135 million years

ago the two landmasses started to break into the

continents that we know today.

P

A

N

G

A E A

Northern landmass (Laurasia) and Southern

landmass (Gondwanaland) 200 million years ago

1960

L A U R A S I A

G O N

D W

A N

A L A N D

Pangaea 225 million years ago

0 1920 1940

19301910 1950

1906 SeismologistRichard Dixon Oldhamshows that Earth’s core is

dense.

1936 SeismologistInge Lehmanndocuments that

Earth’s core is solid.

1950s Scientists exploringthe ocean floor provethat Earth’s crust is not anunbroken shell.

1960s PlateTectonic Theory

is introduced.

1912–1930 ScientistAlfred Wegenerintroduces the conceptof continental drift.

19



20

Plate Tectonics

The more scientists learn about Earth’s inner

structure, the more they understand how events

occurring in the interior of the planet actually shape

the surface of the planet.

You may have heard people talk about old

mountains versus new mountains. Or perhaps you

know that some volcanoes spew lava and other

volcanoes produce a huge amount of ash during an

eruption. You may have heard that earthquakes tend

to happen along something called a fault line. All

of this is related to plate tectonics—how the plates

of the Earth move into one another, against one

another, away from one another, and so on.

Plates form a fault or fault line where they meet.

Fault lines can run over many miles in a rather

straight line, or they can curve a little like a serpent.

The plates on either side of the fault move past each

other in different directions. Usually, they move

slowly. When they move quickly, an earthquake

occurs.

21

When two plates collide, the plate edges can fold

and produce mountains. Also, when Earth’s crust

is squeezed together by tectonic plate movement,

blocks of land may lift up between two faults and

form mountains.

Finally, a heavier plate and a lighter plate can

meet and produce volcanic mountains. The heavier

plate sinks under the lighter plate, and the edge of

the heavier plate is plunged into the mantle of Earth

where the edge melts. This molten rock then forces

its way to the surface as a volcano. In some cases,

lava will flow out of a volcanic mountain and run

down the sides for miles and miles.

In other cases, huge amounts of ash will explode

from the volcano and fill the sky. This kind of

explosion can do hideous damage to animal and

plant life.



22

Summary

Although scientists have learned a great deal

about the interior of Earth, they continue to do

experiments and conduct research in an effort to

learn more.

As recently as 1996, scientists from Columbia

University’s Lamont-Doherty Earth Observatory in

Palisades, New York, announced that they have

proof that the inner core is spinning independently

of the rest of Earth, and that the core is spinning

faster than Earth itself. This is the first time that the

motion of the inner core has ever been measured.

This discovery provides new information that

may help scientists understand changes in Earth’s

magnetic field.

Earth is a living, growing, changing thing. Earth’s

systems seem to maintain a kind of balance just as

human bodies do. Earth’s interior and exterior parts

interact to keep Earth growing and to maintain

conditions necessary for life on the surface of the

planet.

The more we learn about Earth, the more we appreciatehow much we still have to learn about our amazing planet.

23



24

Glossaryarmor n. a protectiveouter layer.

encases v. encloses,surrounds.

eruption n. the violentejection of material,sudden outburst.

extinct adj. no longerexisting.

hideous adj. shocking,frightening.

plunged v. to havebeen thrown suddenly

downward or forward.

serpent n. snake.

Reader Response

1. What caused people to question whether Earth wassolid? What effect did the discovery of underseamountain ridges have? Use a graphic organizer likethis one to record two more cause-effect relationships

discussed in the book.

2. In your own opinion, which scientist or scholarfeatured in the book is most interesting and why?

3. The term “Continental Drift” is not defined in thebook. What do you think it means? How did youfigure that out?

4. Think about the volcanoes and earthquakes describedin this book. How do you think scientists should usewhat they learn about Earth to help people?

Cause Effect

Cause Effect

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