2. Mapping the Mid-Ocean Ridge
• The mid-ocean ridge is the
longest chain of mountains in the
world.
• In the 1950s scientist mapped
the mid-ocean ridge using sonar.
•Sonar is an instrument that uses
sound waves to measure
distance.
• It bounces sound waves off
underwater objects and records
the echoes of these sounds.
3. •The time that it takes the echo
indicates the distance to the
object.
• The scientists found out that
the ocean floor was not flat.
•This discovery peaked their
curiosity to discover what the
ridge was and how it got there.
4. There are huge mountain ranges
called ridges.
Ex: Mid-Atlantic
Ridge
7. Evidence for Sea-Floor Spreading
• In 1960, Harry Hess studied Wegener’s theory.
• Hess proposed the radical idea that the ocean floors
move like a conveyer belt, which in turn move the
continents.
8. Evidence for Sea-Floor Spreading
•This movement begins at the mid-ocean ridge,
which forms along in a crack in the oceanic crust.
• At the mid-ocean ridge, molten materials rise from
the mantle and erupts.
•The molten material spreads out, pushing older
rock to both sides of the ridge.
9. • Hess called this process
Sea-Floor Spreading.
• Molten material, magnetic
stripes, and drilling samples
supported Hess’s theory.
10. Evidence from Molten Material
• In the 1960s, scientist used a
small submarine called Alvin to
explore the ocean floor.
• Alvin’s crew found rocks
shaped like pillows or
toothpaste squeezed from a
tube.
• These rocks showed that molten material
had erupted many different times from cracks
along the mid-ocean ridge.
11. Evidence from Magnetic Stripes
• The Earth is like a giant magnet with
a north and south pole.
• The Earth’s magnetic poles reversed
themselves 780,000 years ago.
•Rocks on the ocean floor are in a
pattern of magnetized stripes.
•These stripes show when the Earth
reversed its magnetic field.
12.
13. Reversals
happen on
average only
about once every
250,000 years,
and they take
hundreds if not
thousands of
years to
complete. The
magnetic field
does not vanish
during this time.
14. More Evidence from Magnetic Stripes
• Molten material contains iron.
•As it cooled, the iron bits lined up
in the direction of Earth’s magnetic
poles.
•When the rock hardened, the iron
was locked in place, giving the
rocks a permanent “magnetic
memory”.
15. More Evidence from Magnetic Stripes
•Scientist recorded this “magnetic
memory” on both sides of the mid-
ocean ridge.
• They found a stripe of when the
magnetic field pointed north and a
parallel stripe that pointed south.
•Rock that hardens at the same time
would have the same magnetic
memory.
16. Evidence from Drilling Samples
The Glomar Challenger is a
drilling ship that recovered
drilling samples from the
ocean floor.
• They studied the age of the
rocks sampled.
• They found that the farther from
the ridge, the older the rock.
• The youngest rocks were at the
center of the ridge.
17. Subduction at Deep-Ocean Trenches
• The ocean floor plunges into deep
underwater canyons called deep-ocean
trenches.
• Subduction takes place where there are
deep-ocean trenches.
• New oceanic crust is hot.
• It moves away from the mid-ocean ridge
and cools, making it more dense.
18. • Gravity pulls the denser, older
crust down beneath the trench.
• Subduction allows
the ocean floor to
sink back into the
mantle that takes
tens of millions
of years to
recycle.
19. Subduction and Earth’s Ocean’s
•Subduction and sea-floor
spreading change the size
and shape of the oceans.
• The ocean floor is renewed
every 200 million years.
20. Subduction in the Pacific Ocean
• The Pacific Ocean covers 1/3 of the planet,
but it is shrinking.
• There is a ring of trenches that surrounds the
Pacific Ocean.
• This occurs because a deep ocean trench
swallows more oceanic crust than the mid-
ocean ridge can produce.
• If new crust is not added fast enough, the
width of the ocean shrinks.
21.
22. Subduction in the Atlantic Ocean
• The Atlantic Ocean is expanding.
• The Atlantic Ocean has only a few
trenches.
• The Atlantic Ocean floor is attached to the
continental crust of the continents.
• As the sea-floor spreads, the continents
along that edge also move.
