Download sea-floor spreading

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
named
Harry 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 midocean ridge,
there is a crack
in the oceanic
crust.
As the molten material
cools, it forms a strip
of solid rock in the
center of the ridge.
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.
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.
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.
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 happens at
deep-ocean trenches.
This process is called
subduction.
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 Ocean
is
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 Ocean
is
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 Wegener
may have been right!