1. The mechanism for continental drift
Sea-Floor Spreading
The mechanism for continental drift

2. Theory of Continental Drift
Remember Wegner proposed that over time, continents drift apart?
Well how did this happen?

3. Mid-ocean Ridges SEA-FLOOR SPREADING
Scientist (Harry Hess) noticed that there are mountain ranges that exist under water in the Oceans
Ex: Mid-Atlantic Ridge
These mid-ocean ridges exist due to
SEA-FLOOR SPREADING

4. Sea Floor Spreading Video
The process by which new oceanic lithosphere is created as older materials are pulled away.
As tectonic plates move away from each other the sea floor spreads apart and magma rises to fill in the gap.
Sea Floor Spreading Video

5. Sea-Floor Spreading Explained

6. Sea-Floor Spreading Sea-floor spreading creates new oceanic crust
Newer crust is found at the center or near the ridge
Older crust is found further away from the ridge (because it is being pushed away from the center of the ridge by new rising magma)

8. Magnetic Reversals Strongest evidence for sea-floor spreading:
Magnetic Reversals recorded in ocean floor
Magnetic reversals:
The switching of Earth’s north and south pole over time

9. Magnetic Reversals and the Sea Floor
Magnetic minerals found in the magma that bubbles up at the ridge align with the Earth’s magnetic field.
Once the molten rock cools, it’s like tiny compasses set in stone
This record is carried slowly away from the center of the ridge, forming bands of alternating magnetic reversals
Every time there was a magnetic reversal, a new band was formed

11. Magnetic Reversals
Why would Magnetic Reversal bands be evidence of sea-floor spreading?
These bands show that different parts (bands) of the crust were formed over different time periods
Because with each band, the rock patterns that form that part of the crust are different from the other bands nearby

12. Magnetic Reversals

13. Magnetic Reversals