Download Continental Drift

Continental Drift and Sea
Floor Spreading
Chapter 17
Continental Drift
• In 1912 Alfred
Wegener came up
with the hypothesis
– The continents
were all one
– Called Pangaea
Continental Drift
• Pangaea
– Super continent
– Existed during
the Permian
Period
– North half called
“Laurasia”
– South half called
“Gondwanaland”
Continental Drift
•
Wegener had 4 major pieces of evidence
for his theory
1. Similar rocks found in Africa and South
America
2. Glaciation and Ancient Climates
3. Fossils
4. Continents seem to fit like puzzle pieces
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
Continental Drift
2. Glaciation/Ancient
Climates
•
•
•
During
Carboniferous
period
Glaciers in India,
South Africa, South
America and
Australia
Northern
hemisphere was
warm and tropical
Continental Drift
3. Fossils
• Mesosaurus = aquatic (shallow) reptile
Continental Drift
3. Fossils
• Glossopteris = fern
that liked cold
climates that has
large seeds that do
not spread out well
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
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
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
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.
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!