Transcript Continental Drift and Sea-Floor Spreading 7.2

Earth’s Structure and Pangaea
Review Inside the Earth
The Earth has 4
layers.
1. Crust (rock)
2. Mantle (rock)
3. Outer Core
(metal)
4. Inner Core
(metal)
Continental Drift Theory
• Continental Drift states
that the continents were
once a single landmass
(Pangaea), broke apart,
and “drifted” to their
current locations.
• Proposed by Alfred
Wegener (1915).
He had evidence that
Pangaea existed…
Lived in the early 1900’s
Pangaea
•
•
Large single landmass that existed 245 m.y.a.
Evidence that Pangaea existed
1. Continents fit together like puzzle pieces (mountain
ranges lined up)
2. Mesosaurus – Reptile fossil found on South America
and Africa – It couldn’t swim!
3. Glossopteris – Tropical plant fossil that was found in
Antarctica!
The continental drift theory
was NOT accepted
because Wegener could
not explain HOW the
continents were
moving/drifted apart.
When magma is
heated it becomes
less dense and
rises.
Why WERE the continents
moving??
…because of sea-floor spreading!
Two oceanic plates pull apart, magma rises
through the gap in the middle, the magma
cools and forms new sea floor. (Oceanic crust)
Continents are connected to
the sea-floor. When the seafloor moves, so does the
continents!
Sea-Floor Spreading Evidence
1. Brand new rock was found at the midocean ridge.
2. Rock gets older and older the farther you
get away from the ridge. Oldest rocks
were found at the trenches.
3. Magnetic Reversals – Iron in the rocks
switched directions every time the
Earth’s polarity switched.
Sea-Floor Spreading in detail…
Check these sites out!
• http://www.wwnorton.com/college/geo/ege
o/flash/2_5.swf
• http://education.sdsc.edu/optiputer/flash/s
eafloorspread.htm