1. Sea-Floor Spreading

2. Proposed By
Harry Hess, an American geologist who studied the mid-ocean ridge.

3. Hess Proposed
In 1960, he suggested that the ocean floor moved like conveyor belts carrying the continents.

4. Mid-Ocean Ridge Longest mountain range in the world. Mapped by sonar.
Runs like the seams of a baseball along the sea floor.
Most under water. Some above water like Iceland.
Mapped by sonar.

5. How it Works
At mid-ocean ridge, molten material rises from mantle and spreads out, pushing older rock to both sides of the ridge.

6. Evidence From Molten Material
In 1960, scientist found new material that is erupting along Mid-ocean ridge. Rocks shaped like squeezed toothpaste called pillows.

7. Evidence from Drilling Samples
Drilled down into the rock and brought up samples. Scientists then found the age of the rocks.
Older rocks were farther away from the MOR.

8. Evidence From Magnetic Strips
Evidence shows that Earth’s magnetic north and south poles reverse.
The rock that makes up the ocean floor has a pattern of magnetic strips.
The strips on both sides match and show that they were made at the same time.

10. Growing Ocean?!
The ocean does not keep growing wider and wider because of subduction.
This is where the ocean floor sinks beneath a deep-ocean trench and back into the mantle.