Download Ch 5 S 4 Sea-Floor Spreading

Chapter 5: Plate
Tectonics
Section 4: Sea-Floor Spreading
1. Mid-Ocean Ridges: an undersea mountain chain where new ocean floor is
produced
a. Scientists mapped the mid-ocean ridges using sonar in the 1900’s
i. Sonar: a device that bounces sound waves off underwater objects
and then records the echoes of these sound waves
1. The time it takes for the echo to arrive indicates the distance to
the object
ii. Most of the mountains in the mid-ocean ridge system lie hidden
under hundreds of meters of water
iii. In a few places, the ridge pokes above the surface
1. Ex: The island of Iceland is a part of the mid-ocean ridge
2. What is Sea-Floor Spreading?
a. Harry Hess, an American geologist, carefully examined maps of the midocean ridge system
i. He began to think Wegener was right about Pangaea
ii. In 1960, he proposed sea-floor spreading
1. Sea-floor spreading: a process that continually adds new material to the ocean
floor
a. The sea floor spreads apart along both sides of a mid-ocean ridge as new
crust is added.
b. The ocean floors move like conveyor belts, carrying the continents along
with them
c. Begins at a mid-ocean ridge, which forms along a crack in the oceanic crust
d. Molten material beneath the surface rises and erupts along the ridge
e. Older rock moves outward on both sides of the ridge
f. As the molten material cools, it forms a strip of solid rock in the center of
the ridge
g. It forms a new strip of rock
3. Evidence for Sea-Floor Spreading
a. Supporting Evidence: eruptions of molten material, magnetic stripes in
the rock of the ocean floor, and the ages of the rocks themselves
i. Evidence from Molten Material
1. Scientists proved that new material is erupting along mid-ocean
ridges
2. They took a submarine, Alvin, to a ridge valley where they
found pillow shaped rocks, or like toothpaste squeezed from a
tube
a. Rocks like this only form when molten material hardens
quickly after erupting under water
ii. Evidence from Magnetic Stripes
1. While studying the ocean floors, scientists found that the Earth’s
magnetic poles have reversed themselves many times.
a. The last reversal happened 780,000 years ago
2. The rock that makes up the ocean floor lies in a pattern of
magnetized “stripes”
3. The rock contains iron
a. As the rock cooled, the iron bits lined up in the direction of
Earth’s magnetic poles
iii. Evidence from Drilling Samples
1. The Glomar Challenger, a drilling ship built in 1968, gathered
samples from the ocean floor
2. This showed scientists that the newest rocks were always in the
center of the ridges
a. Made scientists assured that sea-floor spreading really has
taken place
4. Subduction at Trenches
a. Deep-ocean trenches: a deep valley along the ocean floor beneath
which oceanic crust slowly sinks toward the mantle
i. The oceanic crust bends downward
ii. Part of the ocean floor sinks back into the mantle at deep-ocean
trenches
b. The Process of Subduction
i. Subduction: the process by which ocean floor sinks beneath a deepocean trench and back into the mantle
ii. Crust closer to a mid-ocean ridge moves away from the ridge and
toward a deep-ocean trench
iii.Sea-floor spreading and subduction work together
1. They move as if the ocean floor were a giant conveyor belt
c. Subduction and Earth’s Oceans
i. Subduction and sea-floor spreading can change the size and shape of
the oceans
ii. The ocean floor is renewed about every 200 million years
iii.The Pacific Ocean covers almost 1/3 of Earth
1. It is shrinking
2. Sometimes a deep ocean trench swallows more oceanic crust
than a mid-ocean ridge can produce
3. If the ridge does not add new crust fast enough, the width of the
ocean will shrink
iv. The Atlantic Ocean is expanding
1. It has only a few short trenches
2. The ocean floor has nowhere to go
a. As the ocean floor spreads, the continents along its edges
also move
b. The whole ocean gets wider
Review
• What device is used to map the ocean
floor?
Review
• How does new oceanic crust form?
Review
• Why does the rock of the ocean floor have
a pattern of magnetic stripes?
Review
• Why is the Pacific Ocean shrinking?