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Sea-Floor Spreading
Chapter 4 Section 4
Key Terms
Mid-Ocean Ridge
Sonar
Sea-Floor Spreading
Deep-Ocean Trench
Subduction
Mid-Ocean Ridges
 Mid-Ocean Ridge: an undersea mountain chain that is part
of a long system of mountains that winds beneath Earth’s
oceans.
 Ex. East Pacific Rise
 Sonar: a device that bounces sound waves off underwater
objects and then records the echoes of these sound waves.
 Scientists used this to map the ocean floor.
 The time it takes for the echo to arrive indicates the
distance from you to an object.
 Mid-ocean ridges curve along the sea floor and extend into
every ocean.
 Most of the mountains in these ridges are hidden under
water, but some, like the island of Iceland, poke above the
surface.
What is Sea-Floor Spreading?
In 1960, one geologist proposed the idea of
sea-floor spreading
Sea-floor spreading: the sea floor spreads
apart along both sides of a mid-ocean ridge
as new crust is added.
As a result, the ocean floor moves like
conveyer belts, carrying the continents
along with them.
The Steps in Sea-Floor Spreading
 1. Begins at a mid-ocean ridge, which forms along
a crack in the oceanic crust.
 2. Molten material rises and erupts.
 3. Older rock moves outward on both sides of the
ridge.
 4. As the molten material cools,
it forms a strip of solid rock in the
center of the ridge.
 5. More molten material flows into
the crack and forms a new
strip of rock.
Evidence of Sea-Floor Spreading
Several types of evidence supported
the theory of sea-floor spreading:
eruptions of molten material, magnetic
stripes in the rock of the ocean floor,
and the ages of the rocks themselves.
http://www.youtube.com/watch?v=sgD
M6m0lUGY
http://www.youtube.com/watch?v=wtQ
p4jdzA9U
Evidence from Molten Material
Scientists used a submarine by the name
of Alvin to take a look at mid-ocean ridges.
They discovered rocks that looked like
pillows or toothpaste squeezed from a tube.
Rocks shaped like this only form when
molten material hardens quickly under the
water, so this proved that molten material
had erupted again and again.
http://www.youtube.com/watch?v=fwU0VzZ
iUgI
Evidence from Magnetic Stripes
 Remember that the earth is like a giant magnet,
and our compasses always point north.
 The earth’s magnetic poles have reversed
themselves throughout history. The last time this
happened was 780,00 years ago.
 Scientists discovered that the rock that makes up
the ocean floors lies in a pattern of magnetized
stripes.
 As the molten material
cooled, the iron bits lined
up in the direction of the
magnetic poles.
http://www.youtube.com/watch?v=YIAXiE8RedA
Evidence from Drilling Samples
 A ship built in 1968 called the Glomar Challenger
collected rock samples from drilling into the ocean
floor.
 The Glomar Challenger used drilling pipes to bring
up these samples.
 Scientists found out that the ages of the rock
samples got older as you moved farther away from
the ridge.
 http://www.youtube.com/watch?v=pzCz3gyen_k
Subduction at Trenches
 Deep-ocean trenches: deep underwater canyons.
 Subduction: a process taking tens of millions of years where part
of the ocean floor sinks back into the mantle at deep-ocean
trenches.
 Sea-floor spreading and subduction work together.
 As the sea floor spreads apart, ocean floor closer to a mid-ocean
ridge moves away from the ridge and toward a trench.
 Sea-floor spreading and subduction change the size and shape
of the ocean about every 200 million years.
 http://www.youtube.com/watch?v=NbDqJy28hBw