Chapter Introduction
Lesson 1
The Continental
Drift Hypothesis
Lesson 2
Development
of a Theory
Lesson 3
The Theory of
Plate Tectonics
Chapter Wrap-Up
Arctic_Images/Getty Images
What is the theory
of plate tectonics?
The Continental Drift Hypothesis
• What evidence supports continental
drift?
• Why did scientists question the
continental drift hypothesis?
The Continental Drift Hypothesis
• Pangaea
• continental drift
Pangaea
• Nearly 100 years ago, Alfred Wegener
proposed that all the continents were
once part of a supercontinent called
Pangaea.
• Over time, Pangaea began breaking
apart and the continents slowly moved
to their present position.
Pangaea (cont.)
• Wegener proposed the hypothesis of
continental drift, which suggested
that continents are in constant motion
on the surface of Earth.
• Wegener observed the similarities of
continental coastlines now separated
by oceans and how they could fit
together like pieces of a puzzle.
The eastern coast of
South America
mirrors the shape of
the west coast of
Africa.
Evidence That Continents Move
• Evidence to support Wegener’s
hypothesis is found in
• climate clues;
• fossil clues;
• rock clues.
• When Wegener pieced Pangaea
together, he proposed that the
continents were located closer to the
South Pole 250 million years ago.
Evidence That Continents Move (cont.)
• Wegener suggested that a large sheet
of ice covered the continents.
• Wegener studied the sediments left
behind and the glacial grooves that
formed when the ice sheets melted and
Pangaea spread apart.
• This provided climate evidence for
continental drift.
The presence of an ice sheet covering
Pangea could explain glacial features
found on some continents today.
Evidence That Continents Move (cont.)
• Animals and plants that live on
separate continents can be unique to
that continent alone.
• Fossils of similar organisms have been
found on several continents separated
by oceans.
• Fossils of a plant called Glossopteris
have been found on continents that are
now separated by oceans.
• The orange
area shows
where fossils of
Glossopteris
have been
found.
• Fossils provide
evidence for
continental drift.
Evidence That Continents Move (cont.)
• Wegener observed that mountain ranges
and rock formations on different
continents had common origins,
providing rock evidence for continental
drift.
• Volcanic rock that is identical in
chemistry and age has been found on
both the western coast of Africa and the
eastern coast of South America.
The Caledonia mountain range in northern
Europe and the Appalachian Mountains in
eastern North America are similar in age,
structure, and rock type.
Evidence That Continents Move (cont.)
How were similar rock types
used to support the
continental drift hypothesis?
What was missing?
• Wegener’s ideas were not widely
accepted until nearly four decades later.
• Scientists questioned continental drift
because it was a slow process and
Wegener could not measure how fast
continents moved or how they moved.
• Scientists could not understand how
continents could push their way through
the solid rock of the mantle and the
seafloor.
What was missing? (cont.)
mantle
Science Use the middle layer of
Earth, situated between the crust
above and the core below
Common Use a loose,
sleeveless garment worn over
other clothes
What was missing? (cont.)
Why did scientists argue
against Wegener’s
continental drift hypothesis?
• All continents were once part of a
supercontinent called Pangaea.
• Alfred Wegener proposed that
continents move around on Earth’s
surface.
What term did Wegener use to
describe the constant motion of
continents on the surface of Earth?
A. Pangaea
B. continental drag
C. continental movement
D. continental drift
In which of these did Wegener
observe similarities that suggested
continents might fit together like the
pieces of a puzzle?
A. fossils
B. ice sheets
C. plates
D. continental coastlines
Wegener proposed that 250 million
years ago, South America, Africa,
India, and Australia were located
closer to what?
A. equator
B. South Pole
C. North Pole
D. Pacific ocean
Do you agree or disagree?
1. India has always been north of the
equator.
2. All the continents once formed one
supercontinent.