LEQ:%20What%20is%20subduction,%20where%20%20does%20it%20occur,%20and%20why%20does%20it%20%20happen?

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 Earths
  surface.
• If new crust is being added at the mid ocean
  ridges, why isnt 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 Earths
  surface at the mid-ocean ridges by the process of
  sea floor spreading.
• Old oceanic crust is being subtracted from
  Earths 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 Earths 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.

11
(No Transcript)
12

• Turn in your worksheet when complete.