1. Earth’s Structure and Pangaea

2. Review Inside the Earth
The Earth has 4 main layers.
Crust (rock)
Mantle (rock)
Outer Core (metal)
Inner Core (metal)

3. 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

4. Pangaea Large single landmass that existed 245 m.y.a.
Evidence that Pangaea existed
Continents fit together like puzzle pieces (mountain ranges lined up)
Mesosaurus – Reptile fossil found on South America and Africa – It was a freshwater animal! It couldn’t swim thousands of miles in ocean water.
Glossopteris- plant fossils found on different continents
Tropical plant fossils that were found on an island in Artic Ocean!
The continental drift theory was NOT accepted because Wegener could not explain HOW the continents were moving/drifted apart.

5. When magma is heated it becomes less dense and rises at the mid-ocean ridge.

6. Convection currents in the Earth’s asthenosphere drives the movement of the tectonic plates
Convection Currents-A circulation pattern in which material is heated and rises in one area, then cools and sinks in another area, flowing in a continuous loop

7. 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 sea-floor moves, so do the continents!

8. Sea-Floor Spreading Evidence
Brand new rock was found at the mid-ocean ridge.
Rock gets older and older the farther you get away from the ridge. Oldest rocks were found at or near ocean trenches. Subduction Zones
Magnetic Reversals – Iron in the rocks switched directions every time the Earth’s polarity switched.

9. Sea-Floor Spreading in detail…

10. Mid-Ocean Ridge A series of mountain ranges on the ocean floor
New oceanic crust is formed at the mid-ocean ridge spreads apart and magma is released.

11. 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.

12. 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)

13. Seafloor Spreading

14. Check these sites out!