2. Mapping the Mid-Ocean Ridge
• The mid-ocean ridge is the longest chain of
mountains in the world.
• In the 1950’s 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.
Mid-Atlantic Ridge

5. 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.
• 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.

6. • Hess called this process
Sea-Floor Spreading.
• Molten material, magnetic
stripes and drilling samples
supported Hess’s theory.

7. Evidence from Molten Material
•In the ’60’s, 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.

8. 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 it’s
magnetic field.

10. 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”.

11. 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.

12. 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.

14. 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.

15. • 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.

16. 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.

17. 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.

18. 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.