Chapter 9
Plate Tectonics
Section 9.1
Continental Drift
Continental drift
• As people have studied maps , they were impressed by the
similarity of the continental shorelines on either side of the
Atlantic Ocean.
• In 1915, Alfred Wegener, a German scientist, proposed his
hypothesis of continental drift.
• His hypothesis stated that the continents had once been
joined to form a single continent.
• He called this super continent Pangaea, meaning all land.
• Surrounding Pangaea was a huge ocean, Panthalassa,
meaning “all seas.”
• Wegener also hypothesized that about 200 million years ago
Pangaea began breaking up into smaller continents.
• These continents then drifted to their present positions.
Continental drift
Wegener’s continental drift
hypothesis stated that all the
continents once joined together to
form
A. Two major supercontinents.
B. Two major supercontinents and three
smaller continents.
C. One major supercontinent.
D. Three major supercontinents.
The supercontinent in the
continental drift hypothesis was
called
A.
B.
C.
D.
Panthalassa.
Pangaea.
Mesosaurus.
Africa.
What hypothesis states that the
continents were once joined to
form a single supercontinent?
A.
B.
C.
D.
Plate tectonics
Continental drift
Seafloor spreading
Paleomagnetism
Continental drift
Evidence to support continental drift
1. The Continental Puzzle:
• When looking at maps of the time Wegener
thought that the continents might have been
joined when he noticed the similarities
between coastlines on opposite sides of the
South Atlantic Ocean.
• His opponents correctly argued that erosion
continually changes shorelines over time.
CONTINENTAL DRIFT
Continental drift
2. Matching Fossils:
• If the continents had once been joined, Wegener reasoned, research
should uncover fossils of the same plants and animals in areas that had
been adjoining parts of Pangaea.
• Wegener knew that identical fossil remains of Mesosauras, a small,
extinct land reptile that lived 270 million years ago, had already been
found in both eastern South America and western Africa.
• Wegener knew that it was impossible for these reptiles to have swum
across the Atlantic.
• His opponents thought that there were land bridges that might have
connected the continents at some earlier time.
– Ex: Bering Straight between Asia and North America
• There was no evidence that suggested that South America and Africa
were connected by a land bridge.
• Wegener thus concluded that South America and Africa must have been
joined at one time.
Continental drift
Continental drift
3. Rock Types and Structures:
• The clear picture in the continental drift puzzle is
one of matching rock types and mountain belts.
• Rock evidence for continental drift exists in the form
of several mountain belts that end at one coastline,
only to appear on a landmass across the ocean.
– Ex: Appalachian Mountain Belt runs northeastward
through the eastern U.S., ending off the coast of
Newfoundland. Mountains of the same age with similar
rocks and structures are found in the British Isles and
Scandinavia.
• When these landmasses are fit together the
mountain chains form a nearly continuous belt.
Continental drift
Continental drift
4. Ancient Climates:
• Wegener was a meteorologist, so he was interested in
obtaining data about ancient climates to support continental
drift.
• He found glacial deposits showing that between 220 million
and 300 million years ago, ice sheets covered large areas of
the Southern Hemisphere.
• Layers of glacial till were found in southern Africa and South
America, as well as in India and Australia.
• The land area that shows evidence of this glaciation now lies
near the equator in a subtropical or tropical climate.
• Large tropical swamps existed during the same time in the
Northern Hemisphere with lush vegetation which eventually
became the coal fields of the eastern U.S., Europe, and
Siberia.
Continental drift
One kind of evidence that supports
Wegener’s hypothesis is that
A. The same magnetic directions exist on
different continents.
B. Major rivers on different continents match.
C. Land bridges still exist that connect major
continents.
D. Fossils of the same organism have been
found on different continents.
Evidence about ancient climates
indicates that
A. Glacial ice once covered much of what is now
India and Australia.
B. Continents in the Northern hemisphere today
were once centered over the South Pole.
C. Continents in the Southern Hemisphere today
were once centered onver the North Pole.
D. No continents occupied the Southern
Hemisphere.
The geographic distribution of the
swimming reptile Mesosaurus
provides evidence that
A. Europe was covered by a shallow sea
when Mesosaurus lived.
B. A land bridge existed between Australia
and India.
C. South America and Africa were once
joined.
D. The Atlantic Ocean was wider when
Mesosaurus lived than it is now.
Which of the following was not
used in support of the continental
drift hypothesis?
A.
B.
C.
D.
Fossil evidence
Paleomagnetism
Ancient climate
Fit of South America and Africa
How did opponents of continental drift
account for the existence of similar
fossils on widely separated
continents?
A.
B.
C.
D.
Parallel evolution
Oceanic currents
Large ocean rafts
Migration across land bridges
Continental drift
• Wegener’s drift hypothesis faced a great deal of
criticism from other scientists.
• One objection was that Wegener could not describe
a mechanism that was capable of moving the
continents across the globe.
• Wegener proposed that the tidal influence of the
Moon was strong enough to give the continents a
westward motion.
• Physicists responded that tidal friction of the size
needed to move the continent would stop Earth’s
rotation.
Continental drift
• Wegener also proposed that the larger and
smaller continents broke through the oceanic
crust, much like ice breakers cut through ice.
• There was no evidence to suggest that the
ocean floor was weak enough to permit
passage of the continents without the ocean
floor being broken and deformed in the
process.
• Most scientists rejected Wegener’s
hypothesis, but a few geologists continued to
search for additional evidence.
Continental drift
• With major strides in technology, scientists
were able to map the ocean floor.
• Extensive data on earthquake activity and
Earth’s magnetic field also became available.
• By 1968, these findings led to a new theory,
known as plate tectonics.
• This theory provides the framework for
understanding most geologic processes.
What was the main reason
Wegener’s continental drift
hypothesis was rejected?
A. He was not well liked by other scientists.
B. He could not provide a mechanism for the
movement of the continents.
C. He could provide only illogical
explanations for the movement of the
continents.
D. His evidence was incorrect.
Wegener’s idea that tidal forces
might cause continental drift was
shown to be impossible when it
was
A. Determined that Earth’s magnetic field was too
strong.
B. Shown that the tidal forces needed to move
continents would stop Earth’s rotation.
C. Determined that Earth’s density was too low.
D. Shown that no tides occurred 200 million years
ago.
Science Journal
Label the top of the page with Section 9.1 and
the date. Answer in complete sentences.
1. Explain the theory of Continental Drift
including the four pieces of evidence.