1. Sea-Floor Spreading

2. Mapping the Mid-Ocean Ridge
The mid-ocean ridge is the longest chain of mountains in the world.
Scientists mapped the mid-ocean ridge using sonar.
Sonar is a device that bounces sound waves off underwater objects and records it.

3. Sea-Floor Spreading
At the mid-ocean ridge, molten material rises from the mantle and erupts. The molten material spreads out, pushing older rock to both sides of the ridge.
This process that continually adds to the ocean floor is sea floor spreading.
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4. Evidence for Sea Floor Spreading
In the 1960s scientists found evidence that new material is erupting along the mid-ocean ridge.
The presence of pillow shaped rocks has showed that molten material has erupted again and again.

5. Evidence for Sea-Floor Spreading
Evidence in the rocks shows that Earth’s magnetic poles have reversed.
The iron bits lined up in the opposite direction and shows this evidence.

6. Evidence of Sea-Floor Spreading
Evidence from drilling into the sea floor shows evidence of the ages of rocks
The closer you get to the mid-ocean ridge the younger the rock. The farther you get, the older the rock.

7. Subduction in Deep-Ocean Trenches
As the ocean spreads apart, it also plunges into deep water canyons called deep-ocean trenches.
Subduction is the process by which the ocean floor sinks beneath a deep-ocean trench and back into the mantle.

8. Subduction
The processes of subduction and sea-floor spreading can change the size and shape of the oceans.
Because of this processes, the ocean floor is renewed about every 200 million years.

9. Subduction in the Oceans
The Pacific Ocean is shrinking! This is due to the fact that a deep-ocean trench is swallowing more crust than the mid-ocean ridge can produce.
The Atlantic Ocean however, is expanding!