1. Sea Floor Spreading

2. Mid-Ocean Ridges
An undersea mountain chain where new ocean floor is produced
Scientists use SONAR to discover the ridges and other under sea objects.

3. Mid-Ocean Ridges

4. Sonar
A device that bounces sound waves off underwater objects and then records the echoes of these sound waves.
The time it takes the waves to travel to the object and return is measured. This indicates the distance to the object.

5. Sea Floor Spreading- The Mantles Conveyor
The sea-floor opens on both sides and new crust is added.
Molten material created underneath moves upwards while the older crust is pushed aside.
??Where does this molten material come from?

6. Evidence comes from --------

7. Molten Material
In 1960’s scientists dove to the ocean floor and discovered dark pillow like rocks.
Such rocks only form from quick hardening molten material.

8. Magnetic stripes Geomagnetic Reversal
Throughout earth’s history the pole have reversed themselves many times.
These events last a few hundred to a few thousand years
Magnetic field is weakened and the quickly recover after a new field is established
The last reversal 780,000 y.a.

9. Drilling Samples
The Glomar Challenger was used to drill holes into the ocean floor.
The age of the rocks were then determined.

10. Subduction At Trenches
Subduction is the process in which the ocean floor sinks back under the crust.
Deep-open trenches are the canyons that the ocean floor move into.
The time it takes for new rock to form and sink is approx. 200 million years.
What is often the result of a subduction zone?

11. Review What are their numbers? Trench Volcanic Islands Mid-ocean ridge
Magma
Oceanic Crust
Subduction Zone