1. Sea Floor Spreading

2. Seafloor Ridges and Trenches

3. Ridges
New seafloor has been forming for millions of years at the plate boundaries
Molten rock called magma from inside the Earth rises to the Sea Floor.
The new rock is pulled apart in opposite directions and is called a divergent boundary.

4. Trenches Trenches are formed where one plate slides beneath another.
The older seafloor rock descends, or subducts into trenches and is removed.
The older continental rocks which don’t descend into trenches to be recycled, can be as old as the earth about 4 billion years old.

5. Subduction
Point out the Direction of Seafloor spreading, Trench, Subduction Zone, Volcano/Mountain formation

6. Earth’s Magnetic Polarity Reversal
Magnetic Polarity Reversal and Seafloor Spreading
Is the proof that the seafloor(s) are actually spreading.

7. Oceanic Ridges and Trenches
A continuous process of pushing (seafloor pushing against continental crust where there are no trenches), pulling (Earth’s magma rising to Earth’s surface causing seafloor spreading), and recycling (older seafloor being recycled in trenches by the process of subduction).

8. An Issue With Density
Oceanic (or seafloor) crust has a greater density than continental crust.
Therefore, oceanic crust will subduct (go under) continental crust if there is a trench or ‘push’ the continent if there is no trench.

9. Seafloor Spreading Activity
You can work in groups of two or three.
You must demonstrate that your model works.
Answer all of the questions for full credit!