Transcript Sea-Floor Spreading

Sea-Floor Spreading
Deep Sea Environments
The very deepest parts of the sea are very
cold and completely dark.
Water sinks through cracks, or vents in the
crust and it heated by hot mantle rock.
This heated water then shoots back into the
ocean – we call these hydrothermal vents!
There are many different types of creatures
that live at these vents, but many of them
are very bizarre and can only live in these
environments.
Fauna of Hydrothermal Vents
Mussels, worms and
spider crabs
Tubeworms
Mapping the Mid-Ocean Ridge
This is the longest chain of mountains
in the world – it extends into all of the
world’s oceans.
Most of the Ridge’s mountains are
underwater, but a few aren’t (Iceland).
Mid Ocean Ridge
Mid-Ocean Ridge cont.
Scientists map this area using Sonar – a
device that bounces sound waves off
underwater objects then records the
echoes of the sound waves. This time
then determines the distance to the
object.
Mid-ocean ridges
Magnetometer – a device that can
detect small changes in magnetic fields
These are used to construct magnetic
maps of the sea floor.
The study of the magnetic record is
called paleomagnetism.
A magnetic reversal is a change in the
earth’s magnetic field.
Sea-Floor Spreading
At the Mid-Ocean Ridge, molten
material rises from the mantle and
erupts. The material then spreads out
and pushes older rock to both sides of
the ridge. This process continues, over
and over (think of a conveyor belt).
New material is added to the sea floor
all the time – and older material is lost.
Sea Floor Spreading @ Mid
Ocean Ridge
Evidence supporting Sea Floor
Spreading: Molten Material
Scientists dove to the ocean floor in a
submersible called Alvin that can resist the
pressure of 4 km of water.
The crew found strange rocks in the central
valley of the Ridge – they looked like pillows!
These rocks hardened quickly after erupting,
and only underwater and concluded that
material must be erupting continually
underwater at this location!
Evidence: Magnetic Reversal
The rocks from all over the ocean floor that
scientists have studied show periodic reversal
of magnetic North and South.
This rock lies in magnetized stripes that hold
the record of Earth’s previous magnetism
What changes would we see if North and
South switched today?
Evidence: Magnetic Reversal
How is this recorded?
Oceanic rock is very
Iron rich, and when
the material cooled
the iron bits lined up
in the direction of the
poles. This magnetic
memory is
permanent once the
rock is cool.
Evidence: Magnetic Stripes
Scientists can tell when the Iron bits are
pointing toward the North or the South- they
can detect when the magnetic reversal
occurred.
The magnetism recorded was identical on
both sides of the Mid-Ocean Ridge – proving
again that the material forms in the center,
then breaks apart and moves outward.
Evidence: Cores
A drilling ship sent pipes down through the
water 6 km deep to take samples of the
ocean floor.
These samples were brought up and
scientists figures out their age.
They found that the oldest rocks are farthest
away from the Ridge (to both sides), while
the rocks close to the Ridge are younger.
Evidence: Cores
Subduction
So the ocean floor keeps getting wider and
wider. The cause?
Subduction is the process by which the ocean
floor sinks beneath a deep-ocean trench and
back into the mantle.
Convection currents push new crust that
forms at the Ridge away and toward a deepocean trench.
Subduction cont.
As the rock erupted out of the Mid-Oceanic
Ridge cools down, it became denser and
gravity pulls this beneath the trench.
The subducted rock is melted and recycled
back into the mantle.
What does this mean to us?

The ocean floor is renewed every 200 million
years (this is a sloooow process!)
Our Oceans today
The Pacific Ocean is
actually shrinking!
More oceanic crust
is being subducted
in its trenches than
is being produced at
the Mid-Ocean Ridge
The Atlantic Ocean
is expanding!
The Atlantic has
very few trenches,
but it
accommodates the
spreading by moving
with the surrounding
continents (including
North America).