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CHAPTER 2
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Earth as a System
SECTION
1 Earth: A Unique Planet
KEY IDEAS
As you read this section, keep these questions in mind:
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What are the size and shape of Earth?
What are Earth’s compositional and structural layers?
What is a possible source for Earth’s magnetic field?
What does Newton’s law of gravitation state?
What Shape Is Earth?
Earth formed about 4.6 billion years ago. It is the only
planet in our solar system that scientists think has liquid
water on its surface. Earth is the only planet we know of
that supports life.
If you looked at Earth from space, it would look like
a perfect sphere, or ball. However, Earth is not perfectly
round. It is an oblate spheroid, or flattened sphere. The
diagram below shows what an oblate spheroid looks
like.
Perfect sphere
Oblate spheroid
Earth’s shape is exaggerated
in this picture. In real life,
you would not be able to
see the difference between
Earth and a perfect sphere.
READING TOOLBOX
Re-Read After you read the
section, write out answers to
the Key Ideas questions. If
you cannot answer the questions, re-read the text under
the appropriate heading.
Critical Thinking
1. Apply Concepts Which
would take longer, a trip
around Earth’s equator or a
trip around Earth from pole
to pole? Why? (Assume you
travel at the same speed for
both trips.)
What Is Inside Earth?
Scientists can study Earth’s interior directly by drilling.
However, scientists can drill only a few kilometers into
Earth’s surface. To learn about the rest of Earth’s interior,
scientists must use indirect methods.
One way scientists learn about Earth’s interior is by
studying seismic waves. Seismic waves are vibrations
that travel through Earth. Earthquakes cause most seismic waves. Seismic waves move differently in different
substances. Therefore, by studying these waves, scientists can learn what Earth’s interior is made of. Scientists
divide Earth’s interior into three compositional zones and
five structural zones.
READING CHECK
2. Explain Why must
scientists use indirect methods to study Earth’s interior?
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Earth as a System
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SECTION 1
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Earth: A Unique Planet continued
Talk About It
Lithosphere
Discuss In a small group,
talk about ways you have
learned about something
when you could not observe
it directly. For example, many
people try to learn what is in
a gift box by shaking it. Talk
about how these methods
are related to how scientists
learn about Earth’s interior.
Ocean
Continental
crust
Asthenosphere
Mesosphere
Outer core
(liquid)
Crust
Moho
Structural
Zones
Mantle
Inner core
(solid)
Core
Crust
Lithosphere
Mantle
Compositional
Zones
LOOKING CLOSER
3. Identify Relationships
Which three structural zones
overlap with the mantle?
Oceanic
crust
Asthenosphere
The black labels show the compositional
zones. The white labels show the structural
zones. The compositional and structural
zones of Earth’s interior overlap.
COMPOSITIONAL ZONES
Math Skills
4. Calculate Earth’s layers
have the following thicknesses: crust, 35 km; mantle,
2,900 km; outer core, 2,250
km; inner core, 1,228 km.
A seismic wave moves at the
following speeds through
each layer: crust, 8 km/s;
mantle, 12 km/s; outer core,
9.5 km/s; inner core,
10.5 km/s. How long would
a seismic wave take to travel
from Earth’s surface to its
center?
READING CHECK
5. Compare How are the
inner core and outer core
different?
Compositional zones are made up of different materials. The thin, solid, outermost compositional zone is
the crust. Crust beneath oceans is called oceanic crust.
Crust that makes up continents is called continental
crust. In general, continental crust is much thicker than
oceanic crust. The crust is between 5 km and 35 km
thick.
The mantle is the compositional zone that lies
beneath the crust. The mantle is made of denser rock
than the crust and is almost 2,900 km thick. The innermost compositional zone is the core. The core is a sphere
with a radius of about 3,500 km. Scientists think the core
is made up mainly of iron and nickel.
STRUCTURAL ZONES
Structural zones have different properties. The
lithosphere is made up of the crust and the top part of
the mantle. The lithosphere is relatively cool and brittle.
The asthenosphere is made of hot, solid mantle rock.
The rock of the asthenosphere is also under a great deal
of pressure. The heat and pressure allow the solid rock
to flow. The mesosphere is a layer of solid mantle rock
beneath the asthenosphere.
The core is divided into the outer core and inner
core. The outer core is made of liquid iron and nickel.
Scientists think the inner core is a made of solid iron and
nickel.
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Holt McDougal Earth Science
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Earth as a System
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Earth: A Unique Planet continued
What Is the Source of Earth’s Magnetic
Field?
Earth acts as a giant magnet. Like all magnets, it
has two magnetic poles. Earth’s magnetic field extends
beyond Earth’s atmosphere, and it affects a region of
space called the magnetosphere.
Magnetic
field lines
Solar wind
The magnetic field lines
around Earth show the
shape of Earth’s magnetosphere. The solar wind
affects the shape of the
magnetosphere.
Most scientists think that the liquid iron in Earth’s
outer core is the source of Earth’s magnetic field. They
think that motions within the core produce electric
currents that produce the magnetic field.
What Is Newton’s Law of Gravitation?
Gravity is a force that pulls matter together. In the
1600s, Isaac Newton explained how gravity affects
objects in his law of universal gravitation. This law
states that the force of gravity between two objects
depends on the masses of the objects and the distance
between them.
Weight = 500 N
Mass = 50 kg
READING CHECK
6. Identify What do most
scientists think is the source
of Earth’s magnetic field?
LOOKING CLOSER
Weight = 250 N
Mass = 50 kg
The force of gravity
decreases as an
object moves farther
from Earth’s center.
Weight = 50 N
Mass = 50 kg
7. Describe How does the
distance between two objects affect the gravitational
force between them?
Earth’s gravity pulls objects toward Earth’s center.
Weight is a measure of the strength of this pull. The
newton (N) is the SI unit of weight. The mass of an
object does not change with location, but the weight
of an object can change.
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Earth as a System
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Section 1 Review
SECTION VOCABULARY
asthenosphere the solid, plastic layer of the
mantle beneath the lithosphere; made of
mantle rock that flows very slowly, which
allows tectonic plates to move on top of it
crust the thin and solid outermost layer of the
Earth above the mantle
core the central part of the Earth below the
mantle
lithosphere the solid, outer layer of Earth that
consists of the crust and the rigid upper part of
the mantle
mantle in Earth science, the layer between
Earth’s crust and core
mesosphere literally the “middle sphere”; the
strong, lower part of the mantle between the
asthenosphere and the outer core
1. Organize Complete the concept map below to show the relationship between
Earth’s compositional zones and structural zones.
lithosphere
Earth’s interior has five
Earth’s interior has three
zones
zones
core
2. Apply Concepts A compass needle is a very small magnet that can move. Why can
you use a compass to determine direction on Earth?
3. Analyze Ideas Why would you weigh less on a high mountain peak than you
would at sea level?
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Holt McDougal Earth Science
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Earth as a System