in the mid-1900’s, scientists realized that the ocean floor had many mountain ranges similar to...
TRANSCRIPT
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!