1. Sea Floor Spreading and Convection

2. Review

3. Alfred Wegener… No one believed him because he did not explain…
What force could be strong enough to move such large masses of rock over such great distances?
How are the continents moving?

4. Background First we’ll answer why the continents are moving.
We began to take a good look at the ocean floor because of submarine warfare in World War Two.
Until the mid 1900s, most scientists thought that the ocean floor, unlike the continents, was flat.
WHY?

6. Submarine Warfare
The demands of the war promoted periods of rapid technological advances.
For the first time, we began to use submarine warfare.
Cartographers needed to make detailed maps of the ocean floor for the military so that they may navigate through the water.

7. Sonar
Sonar (SOund Navigation And Ranging)is an echo-sounding technique that has been used to map ocean-floor topography.
The time it takes for the sound waves to travel is used to calculate the distance to the ocean floor.

8. Ocean Floor Topography
Underwater mountain ridges were discovered using sonar.
Ocean ridges = underwater mountain ranges.
Later we discovered that volcanism and earthquakes are common along the ridges.

9. Deep Sea Trenches
Deep sea trenches: narrow, elongated depressions in the seafloor with very steep sides.

10. Magnetometers
During WWII, the magnetometers were used to detect the magnetic fields generated by the steel hulls of submarines.
A magnetometer is a device that can detect small changes in magnetic fields.
Scientists began to use the magnetometers to measure the magnetic strength of rocks on the ocean floor.
The data helped scientists discover the theory of sea floor spreading.

11. Paleomagnetism Paleomagnetism is the study of the magnetic record.
The magnetic fields of basalt changes with time. The magnetic pattern matched on either side of ocean ridges.

12. Dating Ocean Rocks
Scientists also began to take samples of ocean rocks.
The ages of rocks appeared in a predictable pattern.
The age of rock increased the farther you moved away from the ocean ridge.

13. Mid-Atlantic Ridge-New RockB B A

14. Sea Floor Spreading
With the topographic, sedimentary and age data from the seafloor, an American Scientist named Harry Hess proposed the theory called sea floor spreading.
Sea floor spreading: states that new ocean crust is formed at ocean ridges and destroyed in deep-sea trenches.

15. Magma Role
Magma is very viscous, so it is less rigid than surrounding material. This forces the magma into the crack in the crust, away from the ocean ridge.