Download Plate Tectonics and Sea Floor Spreading

Plate Tectonics and Sea Floor Spreading1 Name:________________________________ Period:_________
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The earth, just like a middle schooler sitting at his desk all afternoon, is restless! Even though we think
of the earth as solid and steady under our feet, it is actually moving and shifting all the time. The scientific
study of this earth movement is called plate tectonics. A plate is just a huge slab of rock, and tectonics
comes from an old Greek word meaning "to build." So plate tectonics is the theory that explains how the
earth's surface is built up from rocky plates which are moving.
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This theory is fairly new - it began in the 1960s. But even before that, scientists had the idea that the
earth was always shifting.. Way back in 1596, a Dutch mapmaker named Abraham Ontelius noticed that it
looked like America had been "torn away from Europe and Africa." Scientists thought that, at one time, all of
the continents might have been just one huge continent. They called it the supercontinent, and named it
Pangaea. It may have existed around 225 million years ago. If you take a world map and cut out the
continents, you can try to piece them together like a puzzle to make Pangaea.
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In 1912, Alfred Wegener published his theory called "continental drift" which said that maybe the seven
continents sort of floated around on the earth In 1910, Alfred Wegener proposed the theory of continental
drift. He believed that the continents were drifting apart. He found evidence to prove his theory. But he could
not answer one question. How were the continents moving? He thought they were plowing through the
ocean crust. Other scientists knew this could not happen.
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In the 1960's scientists finally found what was missing in Wegener's theory. Geologist Harry Hess finally
proved that Wegener was right. The continents are moving apart. Hess believed that sea-floor spreading is
the answer to the mystery of continental drift.
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The mid-ocean ridge is found throughout the oceans of the earth. This is a 50,000 mile chain of volcanic
mountains in the middle of the ocean. These mountains have a deep crack that runs through them. It is
called a rift valley. Sea-floor spreading happens here. It is a slow, regular process. There are no explosive
bursts like volcanic eruptions on land.
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During sea-floor spreading, magma rises from the mantle. It oozes out of the rift valley. It cools and
forms new crust. A small amount of new ocean floor is added to the surface of the earth. It fills the gap at
the ridge. As this cycle continues, the newly created crust slowly moves away from the ridge.
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You might think that because of sea-floor spreading, the earth should be growing larger. But sea-floor
spreading is a recycling process. New crust is created at the mid-ocean ridge. Old crust is pushed back into
deep-sea trenches. This process is called subduction. Much of it melts into the mantle. This keeps the earth
from growing larger. There is evidence to prove this theory. Scientists studied rocks from the ocean floor.
Rocks closest to the ridge were younger than rocks found farther away. Magnetic mapping also helped to
prove this theory. Scientists found identical magnetic stripes on both sides of the ridge.
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Scientists have learned much more about these moving plates since the 1960s by studying the
boundaries where the plates bump together. Since many of these boundaries are under the ocean, they use
modern technology to search for clues. Sonar has been used to find out how deep the ocean is in various
areas. Satellites have mapped out the boundaries of the plates, and shown how they changed over time.
Magnetic strips on the ocean floor have been discovered by scientists using magnetometers.
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One really big discovery was the mid-ocean ridge. The mid-ocean ridge is a huge underwater mountain
chain that zig-zags around some of the continents and circles much of the earth. The mid-ocean ridge has
been built up from magma that pushes up from below. At the same time that magma is pushing up and
causing the sea floor to spread in some places, in other places the sea floor is gradually disappearing down
inside ocean floor trenches.
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A part of the mid-ocean ridge, called the Mid-Atlantic Ridge, goes right through the middle of Iceland. It
causes big splits called fissures, lots of volcanoes, and lava fountains.
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By Sharon Fabian and Patti Hutchison
Copyright © 2007 edHelper
Scientists studied Hess's and Wegener's work. The theory of continental drift was combined with the theory
of sea-floor spreading. This led to the theory of plate tectonics
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Another example of spreading, or plates pulling apart, happened in
northern Africa many years ago. There Saudi Arabia became separated
from the rest of the continent of Africa as the plates spread apart. That is
how the Red Sea was formed. When plates pull apart, forming new crust
for the earth, they are called divergent plates.
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Plates can also move toward each other. Then they are called
convergent plates. Sometimes they crash together, sometimes one plate
slowly slides under the other, and sometimes the plates scrape together
side by side. The San Andreas Fault, which runs down the middle of
California, is an example of convergent plates that are scraping along side
by side. If you have heard people joking about California falling into the
ocean, this is what they are talking about! Of course these things take
time, often millions of years.
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Altogether there are about 12 plates that make up the surface of the earth. Many of the continents have
their own plate. For example there is a North American plate, which includes all of North America and
extends out into the ocean on both sides. Europe and Asia share a plate, the Eurasian Plate. There are also
plates that are mostly under the oceans. Plate tectonics show us powerful forces at work within the earth.
These forces cause earthquakes to happen, volcanoes to erupt, and mountains to form. People cannot
control these powerful forces, but we can learn a lot from them.
1. The theory that explains how the earth's
surface is made up of moving rocky plates is
called
Sea floor spreading
Plate tectonics
Supercontinent
Continental drift
2. At one time, all of the continents may have been combined into
one giant continent called
Europe
Eurasia
Pangaea
North America
3. The mid-ocean ridge is
The Rocky Mountains
Another name for Pangaea
A huge mountain range under the
ocean
A volcano
4. The word that means plates pulling apart is
Boundary
Convergent
Continental
Divergent
5. The word that means plates pushing together 6. This article is mainly about
is
Volcanoes caused by the Mid-Atlantic Ridge in Iceland
Boundary
The supercontinent
Continental
The theory of plate tectonics
Divergent
How Saudi Arabia separated from the continent of Africa
Convergent
7. List and describe two powerful forces of
nature caused by plate tectonics.
8. What do you think will happen as the plates continue to
move?