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Earthforce in the Crust
The Franklin Institute
The Franklin Institute. (1998). Earthforce in the crust.
Philadelphia, PA: The Franklin Institute Science Museum.
Used by permission.
Retrieved from http://www.fi.edu/earth/crust.html
Copyright laws may prohibit photocopying this document without express permission.
Earthforce in the Crust
by the Franklin Institute
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Go outside, find a flat piece of ground, and stand perfectly still. You won’t be able to notice
it, but the ground beneath your feet is moving. Imagine that you are balancing on a raft on a
pond, gently drifting. If you live on the North American continent, the ground beneath you is
one giant raft drifting westward very slowly, about 2.5 centimeters (one inch) per year. Get
a ruler and think about that. Imagine that you move just one inch during an entire year. That
seems pretty slow, but, considering the size of the continent, the force of that movement
can be huge. If a bicycle moved an inch and bumped into you, it wouldn’t really hurt. If a car
moved an inch and bumped into you, it might make a bruise. If a school bus moved an inch,
the bump might break a bone. If a tractor trailer bumped into you, you’d notice.
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Scientists think that the entire crust of the Earth is broken into big pieces called plates. All of
the plates are drifting on the liquid molten magma1 of the mantle2 beneath the crust. Even
though they’re drifting very slowly, when they bump into each other, the force is huge.
People who live near the edges of a plate notice the bumping. Most often, the earth
beneath them quakes and shakes as the plates bump and rub.
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Scientists call this action plate tectonics. Over millions of years, plate tectonics has
changed the appearance of the Earth’s crust. Besides the crustal plates drifting and moving
to different locations, the pushing and pulling between plates causes mountains and valleys
to form. Scientists think that, long ago, the crustal plate of India collided with the huge
crustal plate of Asia to form the Himalayas, the highest mountains in the world. While
crustal rock is solid, it does have some elastic properties. With enough force and heat, rock
can bend and change shape. When the two plates bumped into each other, rock along the
edges pushed upward. Even today, scientists think that the Indian plate is still colliding with
the Asian plate, moving about five centimeters (two inches) per year, and causing the
highest mountains to rise even higher.
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Most of the plates meet under the oceans where the force of their pushing and pulling may
sometimes not even be noticed. Some plates, like the Indian and Asian plates, meet on dry
land. The Himalayan mountains were formed when the two plates pushed together. The
Pacific and North American plates also meet on dry land. These two giant crustal plates are
not pushing together, however. They are scraping against each other. The Pacific plate is
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molten magma: rock that has been melted by great heat
mantle: a 1,800-mile-thick layer of very hot liquid rock under the Earth’s crust
The Franklin Institute. (1998). Earthforce in the crust. Philadelphia, PA: The Franklin Institute
Science Museum. Used by permission. Retrieved from http://www.fi.edu/earth/crust.html
ELA Grades 4–5
This material is copyrighted and therefore must be securely destroyed
immediately after use. DO NOT provide a copy of this material to anyone
(teacher, student, or otherwise) who is not directly involved with this test
administration.
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drifting Northward, scraping along the edge of the North American plate. The result of this
tremendous force is the San Andreas fault3, which cuts vertically through the state of
California for nearly 1,125 kilometers (700 miles).
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Scientists think that the San Andreas fault may be one hundred million years old. The crack
in the crust between the plates is at least thirty-two kilometers (twenty miles) deep. For
many years, scientists have used the fault to study the crustal plates, properties of rock,
and the core of the Earth.
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The San Andreas fault is a dangerous laboratory, however. In some places, the force of the
plates scraping has broken roads, relocated streams, and lowered mountains. The biggest
danger, of course, is from the quaking. The crustal plates are pushing and pulling, causing
motion. The mantle beneath the crust is pushing and pulling, causing motion. All of that
force builds up energy. When the energy gets released, the crust vibrates.
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Each month, scientists observe several small earthquakes along the San Andreas fault.
Most of them are barely noticeable. Once in a while, however, the energy really builds and
releases a tremendous vibration.
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Earthquakes are a natural part of the Earth’s geology and are not necessarily dangerous. In
the past 150 years, a tiny blip in the lifespan of the fault, Americans have moved to
California and built structures near the fault. During that time, many lives have been lost
due to quakes along the fault. In 1906, for example, the crust vibrated strongly for forty-five
seconds. In the history of the fault, the 1906 quake was minor. However, in nearby San
Francisco, a young city was destroyed. Five hundred people died—not from the quake, but
from the falling buildings and raging fires that resulted. Buildings tumbled to the ground,
breaking streets below. Woodstoves and oil lamps ignited the rubble. The city burned for
three days.
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In spite of the destruction and potential for future earthquakes, the citizens of San Francisco
stayed and rebuilt their city. Today, buildings are “earthquake-resistant.” Emergency
procedures are ready and waiting. Yet, in 1989, another quake rocked San Francisco,
killing sixty-two people, destroying buildings, and igniting fires across the city. Clearly, there
is no way to be prepared completely for violent vibrations in the crust.
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fault: a break in the Earth’s surface where two plates meet
The Franklin Institute. (1998). Earthforce in the crust. Philadelphia, PA: The Franklin Institute
Science Museum. Retrieved from http://www.fi.edu/earth/crust.html
ELA Grades 4–5
This material is copyrighted and therefore must be securely destroyed
immediately after use. DO NOT provide a copy of this material to anyone
(teacher, student, or otherwise) who is not directly involved with this test
administration.
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