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• Chapter 8, Section 1 Objectives:
1. Explain Wegener’s hypothesis of
continental drift.
2. List evidence for Wegener’s
hypothesis of continental drift.
3. Describe seafloor spreading.
• Continental Drift (1912): the
hypothesis proposed by Alfred
Wegener, which stated that the
continents had moved.
• Pangaea (all lands): a single land
mass formed by all of the continents.
– Began to break up into smaller continents
about 200 million years ago.
• Panthalassa (all seas): the huge
ocean that surrounded Pangaea.
• Pangaea (all lands): a single land
mass formed by all of the continents.
– Began to break up into smaller continents
about 200 million years ago.
• Panthalassa (all seas): the huge
ocean that surrounded Pangaea.
• Pangaea (all lands): a single land
mass formed by all of the continents.
– Began to break up into smaller continents
about 200 million years ago.
• Panthalassa (all seas): the huge
ocean that surrounded Pangaea.
• Evidence of Continental Drift:
1. Fossil – links between Africa and
South America
• Evidence of Continental Drift:
2. Geological
- Age and type of rocks match
- Mountain ranges fit together
• Evidence of Continental Drift:
3. Climatic
-Matching layers of debris from glaciers
in areas that are warmer today
- Fossil evidence (coal deposits) indicate a
matching tropical or subtropical swamps
in the northern hemisphere
4. Seafloor spreading
5. Paleomagnetism
• Despite evidence, most scientists
disbelieved the hypothesis!
• Seafloor Spreading (1947): Magma
or molten rock welling up through rift
in center of mid-ocean ridge, as ocean
floor moved away from both sides of
ridges.
• Mid-Atlantic Ridge:
an undersea mountain
range with a steep,
narrow valley running
down it’s center.
• Mid-Ocean Ridges:
65,000 km long system
of undersea mountain
ranges that wind
around the Earth.
• Evidence of seafloor spreading:
– Age of rocks
• Oldest oceanic rocks no more than 150
million years old
• Oldest continental rocks are about 4
billion years old
• Paleomagnetism
of the ocean
floor (1960’s)
– Magma cools and
iron-bearing
minerals become
magnetized
– Orientation
becomes
permanent
pointing north
Normal magnetic polarity
Points north
• Paleomagnetism
of the ocean
floor (1960’s)
– Scientists have
found bands
pointing south
– Earth’s magnetic
field must reverse
itself
– Verified by dating
of rocks
Reverse magnetic polarity
Points south