Download Theory of Plate Tectonics

8.9AB: Plate Tectonic Theory
Earth and Space
Theory of Plate Tectonics
(Lexile 970L)
1 As you may have discovered, Earth is not like the
other planets in the solar system. Much like
peeling an onion, Earth's layers answer questions
that have baffled scientists for years. Earth's crust,
the outermost layer, is not entirely one piece. It is
broken into sections which resemble an oversized
jigsaw puzzle. These giant sections, known as
plates, are always on the move, creeping along at
a snail's pace. The plates seem to float on top of
the mantle, the layer of Earth composed of partially
melted rock.
2 Have you ever put together a puzzle? The interlocking pieces must fit together perfectly in
order to form a picture. Over 100 years ago, a German scientist named Alfred Wegener
discovered something fascinating about a map of Earth. He realized that the continents seem
to fit together like the pieces of a jigsaw puzzle. This giant piece of land, called Pangaea,
existed over 250 million years ago. Over time, Pangaea's tectonic plates slowly drifted apart.
These plates in their current positions make up the seven continents as we know them.
3 Earth's plates are continually being created or recycled. By studying the ocean floor,
oceanographers have discovered mountainous ridges along the bottom of the ocean. These
ridges form where two plates have started to move apart. Molten rock from the underlying
mantle oozes out and fills the space that has been created by the plates' movement. Plate
tectonic theory has also helped scientists explain another amazing Earth landform: the deep
ocean trenches. These deep ocean trenches are areas where plates are being recycled. As
plates come together, one plate may go on top of the other. The crust of the lower plate is
slowly forced back down into Earth's mantle where it becomes molten rock again. This type of
area is known as a subduction zone. It forms when continental land masses collide with ocean
plates, or when two ocean plates collide with each other.
4 Movement of the plates does not just cause trenches and ridges on the ocean floor. It can also
form mountains on land. When two continental plates push together, there is so much force
that the plates buckle. Massive mountain ranges are formed. The friction during crustal
movement can sometimes cause earthquakes and tsunamis.
5 The plates on Earth's crust change Earth's surface features in many ways. Having previously
been pieced together in a giant landmass, they slowly moved apart to their current locations.
The plates will continue to move, forming new rocks and recycling the old. Imagine how a map
of Earth might look after another 250 million years of continental drift.
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8.9AB: Plate Tectonic Theory
Earth and Space
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2
3
What analogy does the author use to describe how Earth's continents fit
together?
A
The plates on Earth form a picture like a jigsaw puzzle does.
B
The mountains and volcanoes on Earth are pieced together.
C
The plates on Earth fit together like the pieces of a jigsaw puzzle.
D
The plates on Earth are slowly drifting apart.
Which of the following best describes Pangaea?
A
the seven continents that we know now
B
an area on Earth that contains a ring of volcanoes
C
a giant landmass that existed over 250 million years ago
D
the process by which plates move
How is Earth's crust recycled?
A
Earth's crust is not recycled, only created.
B
As molten rock seeps out of weak spots in the crust, volcanoes are
formed.
C
When plates slide past each other, creating friction, an earthquake
forms.
D
As new crust is created, old crust is forced down deep inside Earth's
mantle, where it becomes molten rock again.
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8.9AB: Plate Tectonic Theory
Earth and Space
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5
Scientists predict that in another 250 million years, the continents will be
located in different positions. How is this possible?
A
The tectonic plates on Earth are continually moving at a slow rate.
B
The gravity on Earth causes the continents to move around.
C
Scientist think that the continents will move back into the shape of
Pangaea again.
D
All of the above
The diagram above illustrates a type of plate boundary. Which crustal
feature is most likely to form at this type of boundary?
A
trench
B
ridges
C
tsunami
D
earthquake
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