1. The Theory of Sea Floor Spreading
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2. Claimed the ocean floor was spreading apart, and as it spreads apart, new crust material rises from the mantle to fill the gap
Supports Wegener’s idea that Continents were moving
Harry Hess (1960)

4. Hess’s Evidence 1. The discovery of the Mid-ocean Ridge
During World War II, sonar was used to locate mines/subs- discovered mid-ocean ridge.
Mid-ocean Ridge- underwater volcanic mountain chains; serve as location where sea-floor spreading could occur.
Hess’s Evidence 1. The discovery of the Mid-ocean Ridge

5. Map of Mid-ocean Ridges

6. Hess’s Evidence 2. Age dating of Ocean floor
older crust is further away from ridges; younger crust is located near ridges.
If ocean floor wasn’t being pulled apart, new crust should just form on top of old crust as lava spews out and down the side of a volcano.

7. Standard volcano NOT being spread apart over time: younger layers form on top of volcano

8. Sea Floor Spreading: Land is pulled apart, so new crust forms NEXT to old crust, rather than on top of it.

9. Hess’s Evidence 3. Ocean floor shows Magnetic Reversals
Earth’s magnetic poles “flip” from north to south every so often, called a reversal.
Evidence of Magnetic Reversals form in horizontal bands on ocean floor, rather than stacked layers.
Land was pulled apart so new crust forms NEXT to old crust