In the mid-1900’s, scientists realized that the ocean floor had many mountain ranges similar to

those on the continents.

These underwater mountain ranges are called

mid-ocean ridges.

Scientists used sonar to map these mid-ocean ridges.

Sonar is a device that determines the

distance of an object under water

by recording echoes of sound waves.

Mid-ocean ridges were discovered under all of Earth’s

oceans!

Most are underwater, but a few can be seen above the surface.

Look at the map on the next slide. Can you find a place where a ridge rises

above the surface of the ocean?

In 1960, an American geologist namedHarry Hess suggested that new material is constantly being added to the ocean floor.

He called this process

sea-floor spreading.

This is what happens in sea-floor spreading:

At each mid-ocean ridge,

there is a crack in the oceanic

crust.

Along the ridge, molten material that

forms several kilometers beneath

the surface rises and erupts.

This pushes the older rock

outward away from the ridge.

As the molten material cools, it

forms a strip of solid rock in the center of

the ridge.

1.Evidence of the presence of molten material was found.

2.The rock that makes up the ocean floor lies in a pattern of magnetized stripes.

3.Rock samples found farther away from ridges are older than rock samples found close to the ridges.

1.Evidence of the presence of molten material was found.

2.The rock that makes up the ocean floor lies in a pattern of magnetized stripes.

This evidence led scientists to take another look at Wegener’s hypothesis of continental drift!

Hmm…

But if new ocean floor is constantly being

produced, why don’t the oceans keep getting wider and

wider?

As new ocean floor is being produced at mid-ocean ridges, other parts of the ocean floor

are sinking back into the mantle.

This process is called subduction.

This happens atdeep-ocean trenches.

New oceanic crust is very hot.

As it cools, it becomes more dense.

Eventually, gravity pulls the

older, denser crust down beneath the

trench.

Sea-floor spreading and subduction work together to move the ocean

floor as if it were on a huge conveyor belt.

The Pacific Oceanis

shrinking!

The Pacific Ocean is full of trenches. It is almost

completely surrounded by them! Because of this, oceanic crust is being

subducted faster than it is being produced.

The Atlantic Oceanis

expanding!

The Atlantic Ocean has very few trenches. Because of this, the

ocean floor has almost nowhere to go. As the ocean floor spreads, the

continents along its edges also move making the ocean wider.

The sea floor and the continents generally move at rates of only a few centimeters each year.

It takes about 200 million years for new rock to form at a mid-ocean

ridge, move across the ocean, and sink into a deep-ocean trench.

It seems that Wegenermay have been right!