1. Section 4: Sea-Floor Spreading
What is the process of sea-floor spreading?
What is the evidence for sea-floor spreading?
What happens at deep-ocean trenches?

2. Mid-Ocean Ridges
Mid-ocean ridge is an undersea mountain chain that is part of a long system of mountains that wind beneath all of Earth’s oceans.

3. Mid-Ocean Ridges Extends into all of Earth’s oceans
Most of the mountains lie under the water
There are a few places where the ridge pokes above the
water
Where does
this occur?
Iceland

4. What Is Sea-Floor Spreading?
In sea-floor spreading, the sea floor spreads apart along both
sides of a mid-ocean ridge as new crust is added. As a result,
the ocean floors move like conveyor belts, carrying the
continents along with them.

5. Sea-Floor Spreading Begins at the mid-ocean ridge
Molten material that forms beneath the surface rises
and erupts
Older rock moves outward on both sides of the ridge
As the molten material cools, it forms a strip of solid
rock in the center of ridge

6. Evidence for Sea-Floor Spreading
Several types of evidence supported sea-floor
spreading:
Eruptions of molten material

7. Evidence for Sea-Floor Spreading
Magnetic stripes in the rock of the ocean floor

8. Evidence for Sea-Floor Spreading
The ages of the rocks themselves from drilling

9. NO! Deep-Ocean Trenches
If the ocean floor is spreading, is our Earth getting
larger?
NO!
The ocean floor sinks beneath deep under water
canyons called deep-ocean trenches

10. Subduction at Trenches
In a process taking tens of millions of years, part of the
ocean floor sinks back into the mantle at
deep-ocean trenches.

11. The Process of Subduction
Subduction is the process by which the ocean floor
sinks beneath a deep-ocean trench and back into
the mantle.
Ocean floor closer to a mid-ocean ridge moves away
from the ridge and toward a trench.
Sea-floor spreading and subduction work together.
They move the ocean floor as if it were on a giant
conveyor belt.

12. What is happening to the Atlantic Ocean?

13. Growing an Ocean Because of sea-floor spreading, the distance between
Europe and North America is increasing by a few
centimeters per year.