Transcript Continental Drift and Seafloor Spreading

Earth’s Structure and Pangaea
Review Inside the Earth
The Earth has 4
main layers.
1. Crust (rock)
2. Mantle (rock)
3. Outer Core
(liquid metal)
4. Inner Core
(solid metal )
Continental Drift Theory
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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 different continents lined up)-Landforms
2. Mesosaurus – Reptile fossils found on South America
and South Africa – It was a freshwater animal! –Animal
Fossils
3. Glossopteris- plant fossils found on different
continents- Plant fossils
4. Tropical plant fossils that were found on an island in
Artic Ocean! (Scratches in rocks made by glaciers in South Africa)
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 (Hint: think lava lamp) at the mid-ocean
ridge.
Convection currents in the Earth’s
asthenosphere drives the movement of the
tectonic plates
• Convection Currents-A circulation pattern
in which material is heated, becomes less
dense and rises in one area, then cools,
becomes more dense and sinks in another
area, flowing in a continuous loop
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Why WERE the continents
moving??
…because of sea-floor spreading!
When two oceanic plates pull apart, magma
rises through the gap in the middle, the
magma cools and forms new sea
floor(Oceanic crust). Sea-floor spreading occurs
at/along mid-ocean ridges
Continents are connected to
the sea-floor. When the seafloor moves, so do 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 or near ocean trenches.
Subduction Zones
3. Magnetic Reversals – Iron in the rocks
switched directions every time the
Earth’s polarity switched.
Sea-Floor Spreading
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Sea-Floor Spreading in detail…
Sea-Floor Spreading
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Subduction
• Subduction is the process of the oceanic
lithosphere colliding with and descending
(Sinking) beneath the continental
lithosphere. It is destroyed (melts) and
becomes magma.
• One edge of one crustal plate is forced
below the edge of another plate.
Subduction
Subduction Zone
Sea-Floor Spreading and Plate
Tectonics
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Mid-Ocean Ridge
• A series of volcanic mountain ranges on the
ocean floor
• New oceanic crust is formed at the mid-ocean
ridge spreads apart and magma is released.
Mid-Ocean Ridge
Deep ocean trenches
• Ocean trenches (underwater canyon) can be
formed by subduction between continental crust
and oceanic crust.
• Ocean trenches can also be formed when two
plates carrying oceanic crust meet.
• More dense oceanic crust sinks back into the
asthenosphere.(subduction)
Seafloor Spreading
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