1
How Have the Continents Changed
Positions?
LESSON GOALS
You will learn about
• the theory of continental
drift.
• evidence that supports
the theory of continental
drift.
continental
(kon/to neri/tl) drift,
theory that continents have
moved over time.
The continents have
changed positions over
time.
Eurasia
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Sometimes you make a decision and then find out more
information. As a result, you might change your decision.
Sometimes, new information causes scientists to change
their hypotheses or make new ones. Scientists are alway
gathering new information-or
data. They use data to
form theories about their observations. A theory is an
explanation that is based on the data available at the time.
As new data are discovered a theory might have to be
changed or even discarded. In this lesson you will learn
how geologists, scientists who study the earth, used data
to form a theory about the earth.
Moving Continents
In the past, almost everyone thought the earth'
landmasses-the
continents-were
always in the same
places. However, a few people questioned this idea. In
1620, geographers noticed that the coastlines of some
continents seemed to fit together like pieces of a jigsa
puzzle. Over the years, some people began to wonder
whether the continents were once joined together.
In 1912, Alfred Wegener (va/go nor) , a German
meteorologist, suggested the theory of continental drift.
This theory states that the continents have moved over
time and were not always located where they are now.
Wegener thought the continents were once joined
together in one large supercontinent about 200 million
years ago. He named this supercontinent Pangaea (pan
je/g), which means "all lands" in Greek. Look at the
maps to see how the continents might have fit together
200 million years ago and then moved apart.
Evidence for Continental Drift
Wegener needed more proof, or evidence, to support
his theory of continental drift. Wegener gathered data
about fossils and landforms to support his theory. Two
examples of fossil evidence are shown in the pictures.
Glossopteris (glos op/tor is) was an ancient fernlike
plant. It produced seeds instead of spores as modem ferns
do. Mesosaurus (rnes/o sor/os) was an ancient reptile
that lived in water and hunted fish. Fossils of both of
these organisms were found in South America and
southern Africa. Wegener did not think these organisms
could have crossed through the ocean that now separates
these two continents. The seeds of Glossopteris were too
large to be carried by winds. Mesosaurus could swim, but
lived only in freshwater.
Wegener found more evidence when he studied some
landforms on the earth. For example, the Appalachian
Mountain chain extends across the northeastern United
States and Canada, but it stops at the coast of
Newfoundland. Mountains formed in the same way and
of the same age as the Appalachians are found in the
United Kingdom and Norway.
Wegener thought the best explanation of why these
fossils and landforms are found on different continents
was that the continents were once joined together.
However, he could not explain how the continents moved
apart. Wegener thought that somehow the continents
moved over the ocean floor . Yet, he could find no
evidence to support this idea. At that time, scientists
knew of no way in which continents could move.
Even though Wegener could not explain how the
continents moved, some scientists thought his theory was
possible. Wegener's evidence was strong enough to get
scientists to look for more data. The scientists found data
that suggested the climate on the continents had changed.
They found evidence that glaciers once covered India,
which is now located near the Equator. Also, the
scientists found large amounts of coal in cool regions of
North America and Europe. Coal, however, is formed
from plants that grew in tropical regions millions of years
ago. All these clues suggest that the continents were once
located in different regions of the earth than they are now.
SCIENCE IN YOUR LIFE
One kind of salt, NaCl,
is found in large deposits
or beds in western Texas
and New Mexico. These
salt beds were formed
during the late Paleozoic
era. Scientists think that at
that time, the southern
portion of the United States
was located on the Equator.
Glossopteris
Mesosaurus
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Fossils
Glacier
1\
c Coal
Much of the evidence for continental drift is shown 0
the maps. The world map shows the evidence on th
continents in their present positions. The map to the le
shows the evidence on Pangaea. Compare the two map .
Notice how the same fossils and landforms that are far
apart today, could have been close together 200 rnillio
years ago.
/\/\/\
esson Review
1. What is the theory of continental drift?
2. What are two kinds of evidence that support the theory
of continental drift?
3. Challenge! The continents fit together better at
edges of the continental shelves than at the shoreline.
Why do you think this is so?
Study on your own, pages 482-483.
Use library books to find out the era and periods
when Pangaea existed. Describe the kinds of plants
and animals that lived on the earth during that time.
Write a brief report on your findings.
320
Chapter 12 Study Guide
On a separate sheet of paper, write the word or words that best
complete the sentence or answer the question.
LESSON 1
pages 318-320
1. Scientists use rim to form theories about their observations.
2. When did geographers first notice that the coastline of some
continents seemed to fit together like jigsaw puzzles?
3. Who proposed the theory of continental drift?
4. What was the name of the supercontinent proposed in the
continental drift theory?
5. From what was data gathered that supported the theory of
continental drift?
6. Why did Wegener think that Glossopteris could not have
crossed the oceans?
7. Where are chains of mountains located that are the same age
and the same type as the Appalachian Mountains in the
United States?
8. :1@, formed from tropical plants that grew millions of years
.. ago, was found in cool regions of North America and Europe.