Open internet for Slide 6, Slide 7
Go to favorites and open animations and look for
sea floor spreading and convection in the mantle
and open…right click refresh to keep animation
playing
DVD clip on sea floor spreading… 4 minutes
Sea-Floor Spreading
Tubeworms have no
mouth, eyes, or
stomach ("gut").
Their survival
depends on a
symbiotic
relationship with the
billions of bacteria
that live inside of
them. These
bacteria convert the
chemicals that shoot
out of the deep sea
vents into food for
the worm.
Sea-Floor Spreading
Sonar - a device
that bounces
sound waves off
under-water
objects and then
records the
echoes of these
sound waves to
make a 3
dimensional map.
Sea-Floor Spreading
1. Mid-Ocean Ridge – the longest chain
of mountains in the world---these are
divergent plate boundaries.
Evidence for Continental Drift
• When Wegener first
noticed the similarities in
the shoreline of continents
on both sides of the
Atlantic Ocean, he then
began fitting them
together. He particularly
liked the way in which
South America and Africa
fit.
• Wegener used documented
cases of fossil organisms
that had been found on
different landmasses that
could not have crossed the
current oceans.
More Evidence…
• Matching mountain ranges
on each side of the
Atlantic Ocean: The
Appalachian Mountains that
disappear off the coast of
Newfoundland match
mountains in the British
Isles and Scandinavia which
are comparable in age and
structure.
• Ancient climates: When the
continents are put together
to form Pangaea the
remains of glacial materials
fit together to form a
pattern like the large ice
sheets that cover our poles
today.
THE PROOF-----SEAFLOOR SPREADING
•An American scientist named Harry Hess
proposed the seafloor spreading theory in
1962.
•The Sea floor spreading theory states that
new ocean crust is being created at midocean ridges (which are large mountain
chains underwater) and destroyed at deepsea trenches.
•This is proof that the plates are moving
along on a “conveyor belt” so Wegner’s
idea on continent drift was correct.
The force responsible for driving or moving the plates is
_____________________.
convection
currents
Convection Currents occur within the mantle of the earth
when hot magma rises and cool magma sinks
•Magma, because it is hotter and less
dense, is forced up towards the ocean
floor cooling off when it touches
water.
•When the magma hardens, a small
amount of new ocean floor is added
to the Earth’s crust. As more magma
rises and cools it pushes the new sea
floor away from the ridge. This process
is called sea floor spreading.
PROOF THAT THE SEA FLOOR IS SPREADING
1. The oldest ocean floor rocks ever found are 180
million years old. Remember the Earth is 4.6 billion
years old. This proves that ocean floor is being
destroyed therefore all ocean floor rocks are young
compared to the age of Earth.
2. The rocks closest to the ocean ridge were younger
than the rocks found further from the ridge. This means
that new rocks are formed at the ridges and push the
older rocks away from the ridge.
Animation of Sea Floor Spreading and Ages…2.5
Sea-Floor Spreading
1. Evidence from
Molten Material –
Rocks shaped like
pillows(rock pillows)
show that molten
material has erupted
again and again from
cracks along the
mid-ocean ridge and
cooled quickly
3. Geologists also discovered that the rocks found at
oceanic ridges showed the magnetic reversals that the
Earth has undergone.
Animation of Magnetic Reversals…2.3 on Previous Animation
Magnetic Reversals occur
• When magma cools, the iron cools into the mineral
magnetite. It lines up parallel to the Earth’s present
magnetic field. This iron is like compass needles,
pointing north. So when the rock hardens, a record of
the Earth’s magnetic field at that time is locked in stone---so to speak!!
2. Studies of these magma flows have showed a
magnetic reversal over geologic time.
3. A magnetic reversal (called a FLIP) is a change in
Earth’s magnetic field. North pole becomes south pole
and south becomes north.
4. A magnetic field that has the same orientation as Earth’s
present field is said to have a normal polarity
5. A magnetic field that is opposite to the present field
has reversed polarity
Sea-Floor Spreading
3. Evidence
from Drilling
Samples – Core
samples from the
ocean floor show
that older rocks
are found farther
from the ridge;
youngest rocks are
in the center of
the ridge
YOU NEED TO LOOK AT THE ARROWS
AND LEARN HOW
TO READ THEM FOR PLATE
MOVEMENT
→← CONVERGING
←→ DIVERGING
↑↓SLIDING PAST
AMAZING EARTH SCIENCE FACTS
New sea floor
1. ____________________is
created at mid oceanic
ridges.
increases as distance from ridge
2. Age of rock ____________
increases (____________
rock is at the ridge).
youngest
Convection
3. __________________
currents
__________________
drive plate
movement.
Sea-Floor Spreading
8. Subduction –
Process by
which the ocean
floor sinks
beneath a deepocean trench
and back into
the mantle;
allows part of
the ocean floor
to sink back
into the mantle
Sea-Floor Spreading:
Subduction zone
Deep-Ocean
Trench –
Occurs at
subduction
zones. Deep
underwater
canyons form
where oceanic
crust bends
downward
Sea-Floor Spreading