1. Continental Drift and Sea Floor Spreading
Chapter 17

2. Continental Drift In 1912 Alfred Wegener came up with the hypothesis
The continents were all one
Called Pangaea

3. Continental Drift Pangaea Super continent
Existed during the Permian Period
North half called “Laurasia”
South half called “Gondwanaland”

4. Continental Drift
Wegener had 4 major pieces of evidence for his theory
Similar rocks found in Africa and South America
Glaciation and Ancient Climates
Fossils
Continents seem to fit like puzzle pieces

5. Continental Drift 1. Similar Rocks in Africa and South America
Rocks on both coasts are the same kind
2 continents must have been joined when rocks formed

6. Continental Drift 2. Glaciation/Ancient Climates
During Carboniferous period
Glaciers in India, South Africa, South America and Australia
Northern hemisphere was warm and tropical

7. Continental Drift
Fossils
Mesosaurus = aquatic (shallow) reptile

8. Continental Drift 3. Fossils
Glossopteris = fern that liked cold climates that has large seeds that do not spread out well

9. Continental Drift 4. Continents fit like puzzle pieces
First looked at South America - looked like it would fit like a puzzle piece into Africa
Most continents fit together

10. Continental Drift
Wegener tried to explain why and how the continents moved
He thought that the continents plowed through the ocean floor
This was physically impossible
So the other scientists of the time didn’t believe him

11. Sea Floor Spreading Enter Harry Hess (1962) Naval Officer
Used Fathometer (measures depth)
Took a map of the sea floor
Noticed mountains and valleys in the ocean

12. Sea Floor Spreading He proposed a theory to explain mid-ocean ridges
The magma wells up from deep inside the Earth and forms new rocks.
Some rocks contain minerals that are magnetic.
When the rock forms it records the direction of Earth’s magnetic field.
Like a compass, the rocks “point” towards magnetic north.
From the rocks we know that the earth’s magnetic field has reversed over time.

14. Sea Floor Spreading From this theory, he could estimate age of land
And the speed of movement of plates
Noted that if new crust is formed, there must be a place where old crust is destroyed
It is a continuous cycle
Even though new crust is forming, the Earth remains the same size!