1. During the 1940s and 1950s, using technology developed during World War I, scientists began using sound waves to map the ocean floor.

2. Researchers discovered an underwater system of ridges (mountains) and valleys like those found on the continents.

3. The theory of seafloor spreading explains the formation of the underwater mountain ranges.

4. Seafloor Spreading
Hot, less dense material below the Earth’s crust rises toward the surface at the mid-ocean ridges.
The seafloor spreads apart and magma is forced upward pushing the older seafloor away from the ridge in opposite directions.
The magma becomes solid as it cools and sinks forming new seafloor.

5. Evidence: Rock Age
The new oceanic lithosphere forms as magma rises toward the surface and solidifies.
As the new tectonic plates move away from each other, the seafloor spreads apart and magma fills in the gap.
As this new crust forms, the older crust gets pushed away from the mid-ocean ridge.

6. Evidence: Magnetic Strips
780,000 years ago Earth magnetic pole switched
Molten material contains iron, points to Earth’s magnetic pole
North Strip of rock
followed by South
Strip of rock

7. Seafloor Spreading

10. No, it does not keep expanding, but what happens to it?
If new crust is being added by seafloor spreading, does the Earth’s surface just keep expanding?
No, it does not keep expanding, but what happens to it?
Let’s continue to find out.

11. The ocean crust bends downward, causing deep-ocean trenches (DOT).
Subduction - ocean floor sinks beneath a DOT and back into mantle
So, is older crust more or less dense?
More dense