LEQ: What is subduction , where does it occur, and why does it happen ?

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 continentalcrust.

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.