4-4: Sea Floor Spreading

6th Grade Earth Science

4-4: Sea Floor Spreading

• Very large mountain ranges found under the oceans.

• Found using Sonar – sound waves bounced off underwater objects.

Mid Ocean Ridges

4-4: Sea Floor Spreading

• The sea floor spreads apart along both sides of the ridge as new crust is added.

Sea floor spreading

4-4: Sea Floor Spreading

4-4: Sea Floor Spreading

1. Molten lava found along the ridge.

2. Magnetic stripes in the sea floor.

3. Drilling samples dated.

Evidence of Sea floor spreading

4-4: Sea Floor Spreading

• Molten lava found along the ocean ridges.

4-4: Sea Floor Spreading

• Magnetic stripes on both sides of the ridge match each other.

• Click Here • Actual measured magnetic stripes in the Atlantic ocean. Try and match stripes on either side of ridge on the next slide.

4-4: Sea Floor Spreading

4-4: Sea Floor Spreading

4-4: Sea Floor Spreading

• While drilling and dating rocks on the ocean floor, they found that rocks are older further away from the ridge.

4-4: Sea Floor Spreading Deep Ocean Trenches

4-4: Sea Floor Spreading

• Since the Earth isn’t getting bigger, crust must be getting destroyed somewhere.

• Ocean floor sinks into deep trenches.

Deep Ocean Trenches

4-4: Sea Floor Spreading

• Subduction is the sinking of cold crust at the trench back into the mantle.

• The trench is called a subduction zone.

4-4: Sea Floor Spreading

• The spreading and subducting of crust changes the size and shape of the ocean.

• The Pacific Ocean is getting smaller – there are more trenches.

• The Atlantic Ocean is getting bigger – there are less trenches.

The changing oceans

Click-Here

4-4: Sea Floor Spreading Summary

1. Large mountain ranges exist under the ocean, where new crust is being made.

2. Evidence comes from undersea lava, magnetic stripes, and dated rocks.

3. Crust subducts back into Earth at ocean trenches.

4. Oceans continually change size and shape.