Download 2How Is Continental Movement Explained by Plate Tectonics?

2
How Is Continental Movement
Explained by Plate Tectonics?
LESSON GOALS
You will learn
• how scientists gathered
more data that enabled
them to build on and
change the theory of
continental drift.
• how the theory of plate
tectonics explains the
movement of continents.
• the three different ways
that plates can move.
sonar (so/nar), device that
uses sound waves to
measure distance.
Section of ocean floor
Scientists kept looking for data to support or to reject
the theory of continental drift. During the 1950s,
scientists began to study the crust that makes up the ocean
floor . You might wonder how studying the ocean floor
could help support a theory about the movement of the
continents. By studying the ocean floor, scientists learned
that the continents were moving-not
over the ocean
floor as Wegener had thought, but along with the ocean
floor.
Gathering Data from the Ocean Floor
Geologists used sonar to make a map of the ocean
floor. Sonar is a device that uses sound waves to measure
distance. Scientists can calculate how far an object is by
measuring the time it takes for sound waves to reach the
object and then bounce back. The farther away an object
is, the longer it takes for sound waves to travel to the
object and back.
Using sonar, geologists began to measure how deep the
oceans all over the world are. They learned that the ocean
floor is not all flat. The ocean floor has huge mountain
ranges called ridges. They also found deep valleys or
trenches. The diagram shows what a section of the ocean
floor looks like.
Ridge~-----._£~
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Legend
Millions of Years
Age of crust under the Atlantic Ocean.
-
Geologists also studied the age of the crust under the
oceans. They found that the crust is not the same age on
different parts of the ocean floor. The map shows the age
of the crust under part of the Atlantic Ocean. Each color
stands for a different age. Notice that the youngest crust is
found along the ridge in the middle of the ocean. Where is
the oldest crust?
Geologists noticed that the crust along ridges is always
younger than the crust away from ridges. Thus, new crust
must be forming at ridges. How does new crust form?
Geologists found cracks along ridges. Hot melted
rock-or
magma-from
the mantle rises up through
these cracks, as shown. When the magma comes out onto
the ocean floor, it hardens and forms new crust. As new
crust forms, the older crust is pushed away from the
ridge, causing the ocean floor to spread. The spreading of
the ocean floor near ridges is called seafloor spreading.
seafloor spreading,
spreading of ocean floor
caused by new crust
forming along ridges.
Seafloor spreading
occurs when magma pushes
upward through the ocean
floor.
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plate tectonics
(tek tori/iks), theory stating
that the earth's surface is
covered with moving
plates.
plate a large section of the
earth's surface that is made
up of the crust and upper
mantle.
Plate
Plate
Crust
Mantle
Core
Plate
Earth's structure
Plate Tectonics
Geologists used the information they learned
seafloor spreading to change parts of Wegener's the
continental drift. Remember that Wegener could
explain how continents moved. He thought that
continents moved over the ocean floor. Today, scie
have formed a new theory-called
plate tectonic
explain how continents move. According to this th
both the continents and the ocean floors are locat
plates that move over the earth.
Each plate is a huge block of rock. Notice in the pi
that a plate is made up of crust and the upper part of
mantle. The plates float on the partly melted rock in
mantle. Currents in the melted rock move the plates.
currents are caused by heat from within the earth.
The surface of the earth is made up of nine large pI
and several smaller ones. The map shows where some
the plates are located. Notice that a continent and 0
floor can be found on the same plate.
The arrows on the map show the directions in w the plates are moving. You can see that plates m
toward each other, away from each other, or past e
other. This theory of the earth's structure explains m
features of the earth such as mountain chains
volcanoes as well as the movement of continen
Tectonics comes from the Greek word for builder.
theory of plate tectonics explains how features of
earth are built up and torn down.
The surface of the earth is divided into moving plates.
324
Plate Boundaries
The arrows on the map on page 324 show the three
kinds of plate boundaries-places
where two plates meet.
Notice how the plates move in three general directions.
A spreading boundary is an area where two plates are
moving away from one another. New crust is formed at
spreading boundaries. Find spreading boundaries on the
map. Where are most of these boundaries located?
The top picture on the right shows how plates move at a
spreading boundary and what the result can be. The Great
Rift Valley in Africa is one place where new crust is
being added to the earth's surface. As the crust builds up,
it forms a wider and deeper valley. Eventually, the valley
will widen and deepen to sea level. Water will flow in,
forming a sea. This process describes how the Atlantic
Ocean began over 200 million years ago.
A fracture boundary is an area where two plates are
moving past one another. New crust is not added at a
fracture boundary. Find a fracture boundary on the map.
The middle picture shows how plates move at a
fracture boundary and what the result can be. The San
Andreas Fault in California is a fracture boundary. The
two plates along the San Andreas Fault move past each
other at an average rate of about five centimeters per year.
Los Angeles is located on a plate on' one side of the San
Andreas Fault while San Francisco is located on the other
plate. At the rate these two plates move, Los Angeles and
San Francisco will be located next to each other in about
ten million years!
A colliding boundary is an area where two plates are
moving toward each other. Find colliding boundaries on
the map. Notice that colliding boundaries are located on
the ocean floor, on the surface of continents, and at the
edges of continents.
The picture on the bottom shows how plates move at a
oUiding boundary and what the result can be. Notice that
e crust of one plate moves under the crust of the other
late. Trenches are formed by colliding boundaries on the
ean floor. Mountains are formed when one or both of
e plates contain a continent. The crust on the upper
late buckles and rises, producing a mountain chain. The
des Mountains were formed in this way.
spreading boundary, an
area where two plates are
moving away from one
another.
fracture boundary, an area
where two plates are
sliding past one another.
colliding boundary, an
area where two plates are
moving toward one
another.
Colliding boundary
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INVESTIGATE!
Find out how the
thickness of the earth's
crust affects its ability to
float on the earth's mantle.
Write a hypothesis and
carry out an experiment to
test your hypothesis. Use
wooden blocks to represent
the earth's crust and water
to represent the earth's
mantle. Add masses to the
blocks to see which sinks
first.
Why do the continents not sink into the mantle?
rocks that make up the continents are less dense th
those in the mantle. Continental crust is also much thick
than ocean floor crust. These factors cause the continen
to stay afloat on the plates. Therefore, the oldest rocks
the earth are on the continents.
Although the earth's structure is better understood th
in the past, geologists still have questions. Data from ne
technology may help complete this picture.
Lesson Review
1. How did scientists find out that the ocean floor .
spreading in some places?
2. How does the theory of plate tectonics explain
movement of continents?
3. What three ways can plates move?
4. Challenge! How does the earth stay the same size .
new crust is formed at spreading boundaries?
Study on your own, pages 482-483.
The picture shows the Glomar Challenger, a
research ship that was built in the 1960s. The drilling
platform in the middle of the ship is nearly 43 meters
high. Scientists drill into the ocean floor to get
samples of the earth's crust. Use library books to
write a report about the Glomar Challenger. Use the
following questions as a guide. What problems did
scientists have as they drilled into the ocean floor?
What special instruments did scientists use to solve
these problems? What kinds of data can the Glomar
Challenger gather?
326
LESSON 2
pages 322-326
1. During the Wt , scientists began to study the crust of the
ocean floor.
2. Geologists used tm to rrieasure distances on the ocean floor.
3. What does sonar use to measure distances?
4. The farther an object, the E@ it takes for sound waves to
reach an object and bounce back.
5. What features are found on the ocean floor?
6. The mm of the crust under the oceans varies.
7. Where is the youngest crust found in the Atlantic Ocean?
8. New crust often forms along the t!@! .
9. What material forms new crust?
10. What happens to the ocean floor as older crust is pushed away
from the ridges?
11. The theory of Wl explains how continents move.
12. According to this theory, both continents andMl
are located
on moving plates.
13. The plate is made of crust and the upper part of theFY .
14. What moves the plates?
15. About illm;! large plates make up the surface of the earth.
16. Two plates meet at a plate my .
17. New crust is formed at fiB. .
18. Name one place where new crust is forming.
19. The San Andreas Fault in California is a }:~r
20. At what rate are the two plates along the San Andreas Fault
moving past each other?
21. A colliding boundary is an area where two plates are moving
@@ each other.
22. Wi!; are formed by colliding boundaries on the ocean floor.
23. What mountains were formed by colliding boundaries?
24. Identify the kind of boundary shown in each of the pictures
below.
A
B
c