B.5.2_Our Changing Earth

Our Changing Earth
1

2

From tall mountains to flat plains, Earth’s surface has
extremes. The physical features of Earth that we observe
today did not always exist. Instead, they formed over millions
of years. Those long, slow changes still take place, even if we
can’t see them happening.
But there are other physical changes on Earth that we can
witness. These changes happen much more quickly. Some are
sudden changes, while others are more gradual. Earth is a
restless planet. As it changes, so do those who live on it.

Reading 1
Content
Science

Reading
Strategy
Synthesize What
do this paragraph
and the photograph
­suggest about some
of Earth’s changes?

✓

Reading Check
1. Recall details

What are two
extreme features
on Earth’s surface?

View of Earth from the moon’s surface

2. Predict What is
one change on
Earth that we can
see happen?
restless not able to rest





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Earthquakes
3

Earth’s crust is the layer of rock closest to Earth’s surface.
An earthquake is a sudden, violent shaking in Earth’s crust.
The crust is divided into a number of blocks of rock, called
plates, which cover the planet. An earthquake happens when
two of these plates, under great pressure, move past each
other along a fault. The violent shaking is caused by seismic

waves that travel through the planet. These waves can knock
down buildings and bridges. The place inside Earth where an
earthquake starts is the focus. The epicenter is the place on
Earth’s surface directly above the focus.

Tsunami

Plates

Plates

Fault
Seismic Waves

Reading
Strategy

Earthquake

Focus

Synthesize What
clear idea do the
pictures give you
about earthquakes?
violent very strong
pressure force

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Epicenter


The San Andreas Fault in California

How Do Earthquakes Happen?
4

Most earthquakes result when plates move over, under, or
past each other. Plate boundaries are places where two plates
meet. Earthquakes happen when the pressure on the rocks at
plate boundaries builds up.

5

This happens in different ways. Plates can move apart, or
diverge. As the rocks pull apart, magma rises from below.
Plates can come together, or converge. The rocks are pushed
from two different directions. This makes them bend and
break. Earthquakes can also happen when plates move past
one another without much motion up or down.

Surface Waves and Tsunamis
6

Most waves on a lake or an ocean can occur when wind
energy moves over the surface. The energy that causes a
tsunami comes from an earthquake on the sea floor. All of the
water above the epicenter is affected. A wave forms. It does
not seem large at first, but the energy is great. When the

water hits the shore, it crashes over the land.

Reading
Strategy
Synthesize How
is plate movement
related to faults, like
the San Andreas Fault
pictured above?

✓

Reading Check
1. Recall details
What builds up at
plate boundaries?
2. Infer Where are
you most likely to
notice a tsunami
wave?

magma hot liquid rock
motion movement



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Reading
Strategy
Synthesize What
effect does a volcano
have on Earth’s
surface? Connect the
text and pictures on
this page.

Volcanoes
7

A volcano forms at an opening, or vent, in the crust
near the edges of plates. A volcanic eruption occurs when
the plates move apart or when one plate is pushed under
the other. Water vapor, particles, gases, and magma pour
out onto the surface. Volcanoes form new land. Gases and
particles from the volcano may affect the weather.

Eruption
Volcano

Lava

Vent

Crater

particles very small pieces

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Magma


Lava from Kilauea Volcano in Hawaii
flows into the ocean. How does Earth
benefit from volcanic eruptions?

How Do Volcanoes Form?
8

Volcanoes often form at the boundaries between plates.
They occur when the plates pull apart and magma flows up
from below through cracks. Volcanoes can also form when
plates come together. One plate moves under the other. The
rock gets very hot and becomes magma. The magma rises,
forming a volcano.

9

The mantle is the layer of earth under the crust. Some
areas of mantle are hotter than others. Volcanoes can form
at these hot spots. As the plates move, the volcanoes form an
island chain. The Hawaiian Islands are on a hot spot.

Hot Spot Forming Islands

Reading

Strategy
Synthesize What
creates a hot spot?
Pull together
information from the
text and diagram.

✓

Crust

Reading Check
1. R
 ecall details
Where can you find
volcanoes?

Mantle

2. I nfer How might
gases and particles
affect the weather?




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Changes on Earth’s Surface
10

Earthquakes and volcanoes can change Earth’s surface.
These changes happen quickly.

11

Earth’s surface can change slowly, too. Weathering, such
as abrasion and ice wedging, breaks rock into tiny pieces.
Abrasion happens when tiny pieces of rock wear down
larger pieces of rock. Erosion happens when water, glaciers,
or wind carries sediment to new places. Sediment is tiny
pieces of rock and soil. Deposition happens when sediment
builds up in a place. Deposition builds new landforms such as
deltas, moraines, and sand dunes. Glaciers, water, and wind
can all cause deposition.

Reading
Strategy
Synthesize What
changes to the
earth’s surface are
represented by the
pictures? How are
they different from
each other?

Erosion

Weathering

Glacier
A glacier is a slowly moving mass
of ice. A glacier can cut into rock
and carry the pieces away.

Abrasion
Tiny bits of rocks moved by air,
water, or a glacier can cause
abrasion. Ice wedging is the
result of ice forming in small
spaces in rocks. The ice breaks
up the rock.

Deposition

Sand Dune

Wind can cause a sand dune to
build up.

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Weathering
12

Mechanical weathering happens when rocks

are broken into small pieces. Abrasion occurs when
pieces of rock are rubbed off by tiny bits of rock
moved by water, wind, or ice. Ice and plants also cause
weathering. A seed may fall into a crack in a rock. When
the seed sprouts and grows, the plant’s roots make the
crack bigger. Ice wedging also affects cracks in rock.
Water gets into a crack. When the water freezes, it
Plant roots push apart rock and cause
mechanical weathering.
expands, and the crack gets bigger. The cracked rock
finally breaks in two.

13

In chemical weathering, chemicals in water, such as
oxygen and carbon dioxide, react with chemicals in rock. This
breaks down the rock over time.

Glaciers
14

Over thousands of years, ice and snow can build up on
mountains. They form glaciers. As a glacier moves, sand and
rock inside the glacier wear down mountains and change
their shape. A mountain’s shape can tell us where glaciers
formed in the past.

Erosion and Deposition
15

16

Weathering breaks rock into smaller pieces. Wind, water,
or ice moves these small pieces, called sediment, to new
places. This process is erosion. The Grand Canyon was
formed by water erosion. Over millions of years, water cut
into the rock and carried pieces away. Sand is eroded from
beaches by waves. Glaciers carry rocks and soil as they
move. Wind carries dust and sand for miles.
When the wind or water
slows down, the sediment
drops out. This is deposition.
Deposition forms river
deltas and sand dunes.
When a glacier deposits
sediment, it can form a
moraine, a type of hill.

Reading
Strategy
Synthesize What kind
of weathering is the
Grand Canyon an
example of?

✓

Reading Check
1. R
 ecall details

What kinds of
changes are caused
by wind?
Water erosion formed this canyon.





2. F
 ind main idea
What is the main
idea of page 310?

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