Mid-Ocean Ridges
 Mid-ocean ridges are an undersea mountain chain
where new ocean floor is produced.
 In the mid-1900’s, scientists begin mapping the midocean ridges using SONAR.
 Mapping mid-ocean ridges, made scientists more
curious.
 What are these ridges?
 How did they form?
Sea-Floor Spreading
 In sea-floor spreading, the sea floor spreads apart
along both sides of a mid-ocean ridge as a new crust is
added.
 The ocean floor moves like a conveyor belt, carrying the
continents along with them.
 Along these ridges, molten material that has formed
several km beneath the surface rises and erupts.
 As the molten material cools on the ridge, new solid
rock is formed, more molten material flows into the
cracks and new strips of rock is formed.
Evidence of Sea-Floor
Spreading
• Evidence from molten
material
• In the 1960’s scientists
sent Alvin, a
submarine that could
withstand the
crushing pressures
deep in the ocean.
• Strange rocks were
found. Rocks that
looked like toothpaste
squeezed from the
tube. These types of
rock only form when
molten rock cools and
hardens quickly after
erupting.
More Evidence of Sea-Floor Spreading
Evidence from Magnetic Stripes
 History shows that the earth’s
magnetic field has reversed many
times.
 Rocks that make up the ocean floor
reveal this pattern in magnetized
stripes.
 These rocks contain iron. As they
cool and harden, the iron lined up in
the direction of the Earth’s magnetic
field.
Evidence from Drill Samples
 Rock samples from drilling into
the ocean floor are the final
proof of sea-floor spreading.
 The Glomar Challenger sent
pipes 6 km deep in the ocean to
drill hole sin the ocean floor.
 Scientists found that the farther
away from the ridge, the older
the rock sample were.
Subduction at Trenches
 Deep-ocean trenches are deep underwater canyons,
where the oceanic crust bends downward.
 SUBDUCTION is a process by which the ocean floor
sinks beneath a deep-ocean trench and back into the
mantle.
 It takes about 200 million years for new rock to form,
move across the mid-ocean ridge, move across the
ocean, and sink into a trench.
Did you know the Pacific Ocean is Shrinking
and the Atlantic ocean is expanding?
 Sometimes a deep ocean trench swallows more oceanic crust
than a mid-ocean ridge can produce.
 This is why the Pacific Ocean is shrinking
 The opposite is happening to the Atlantic Ocean.
 More oceanic crust is being produced at mid-ocean ridges that
the deep ocean-trenches and swallow up.