1. LEQ: What is subduction, where does it occur, and why does it happen?
Key Terms: deep-ocean trench,
density,
subduction

2. Sea Floor Spreading Review
Turn to page 82 in your textbook.
At the mid ocean ridges:
Molten material rises from the mantle and erupts.
The molten material then spreads out on both sides of the mid ocean ridge.
As the molten material spreads out, it pushes older rock to both sides of the ridge.
This process is known as sea floor spreading.
This process adds new oceanic crust to Earth’s surface.
If new crust is being added at the mid ocean ridges, why isn’t the Earth getting larger? (

3. Deep Ocean Trench A deep valley along the ocean floor.
Located far away from the mid ocean ridge.
The deepest parts of the ocean.
Form where the oceanic crust bends downward and sinks toward the mantle.
Why does the oceanic crust sink?

4. Density
Density is the amount of mass in a given space (mass per unit volume).
All matter (solids, liquids, and gases) have a density.
The density of a specific substance is always the same.
Different substances have different densities.
When two substances come in contact with each other, the less dense substance will float while the more dense substance will sink.
The density of oceanic crust is more than the density of continental crust.
Therefore oceanic crust sinks below continental crust.

5. Subduction Turn to page 84 in your textbook.
Subduction is the process by which oceanic crust sinks beneath a deep-ocean trench and back into the mantle.
Where there are deep ocean trenches, subduction occurs.
At deep ocean trenches, subduction allows part of the ocean floor to sink back into the mantle.
Subduction Animation

6. Addition and Subtraction
New oceanic crust is being added to Earth’s surface at the mid-ocean ridges by the process of sea floor spreading.
Old oceanic crust is being subtracted from Earth’s surface at the deep-ocean trenches by the process of subduction .
These process work together to cause a balance so the Earth is not getting larger or smaller!

7. Geologic Time These processes happen all the time, but VERY slowly.
When we factor in Geologic Time (millions and millions of years), these processes have a substantial affect on Earth’s surface.

8. Subduction in the Pacific Ocean
More oceanic crust is being subducted at the deep ocean trenches than is being produced at the mid ocean ridges.
The Pacific Ocean is getting smaller!

9. Subduction in the Atlantic Ocean
More oceanic crust is being produced at the mid ocean ridge than is being subducted at the deep ocean trenches.
The Atlantic Ocean is getting larger!
Lets see how this is predicted to affect future Continental Drift.

10. Labeling Sea Floor Spreading and Subduction
Use the image on the next slide to help you complete your labeling worksheet.

12. Turn in your worksheet when complete.