Name: ____________________________
Science / Block: _______
Date: _________________
Teacher: ______________
Chapter 3 Lesson 2 – Seafloor Spreading - (5 words)
mid-ocean ridge- A mid-ocean ridge is where seafloor spreading occurs or new ocean floor is
produced. These undersea mountains form the longest mountain ranges on Earth.
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Sonar- Sonar uses sound waves (send out a ping and listen for the echo to return) to measure
the distance to an object. Sonar is used to map the ocean floor. Sonar stands for sound
navigation and ranging.
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seafloor spreading- In seafloor spreading new crust is continually added to the ocean floor at
a mid-ocean ridge. This happens as:
1. A crack forms in the mid-ocean ridge.
2. From this crack, magma erupts, cools, and hardens into new ocean crust.
3. The seafloor spreads as new crust pushes older strips of rock outward from both sides
of the ridge.
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deep-ocean trenches- Deep-ocean trenches are deep underwater canyons.
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subduction – Subduction is the process by which the ocean floor sinks into the mantle.
Subduction occurs at deep ocean trenches because:
1. Oceanic crust eventually cools & becomes denser as it moves away from the midocean ridge.
2. Gravity eventually pulls denser oceanic crust down beneath the trench into the mantle.
3. As subduction happens, crust closer to the mid ocean ridge is pulled toward the trench
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Evidence that supports the theory of seafloor spreading:
Evidence from ocean material:
 Pillow lava formed along ridges when lava hardens quickly after erupting under water.
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Evidence from magnetic stripes:
 Earth's magnetic field or polarity reverses itself from time to time; North becomes South
and South becomes North (last time was about 730 MYA).
 Lava that cools and becomes rock at a given time in Earth's history stores a record of
our planet's magnetic polarity at the time of the rock's formation.
 Rocks on the seafloor on either side of a mid-ocean spreading ridge preserve a record
of the Earth's magnetic field over time.
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Evidence from drilling samples:
 Older rock further away from mid-ocean ridge because the ocean rock is gradually
pushed away from the ridge by the younger, new rock – like a conveyer belt.
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Explain seafloor spreading helps support Wegener’s hypothesis
of Continental Drift:
Seafloor spreading supports Wegener’s hypothesis of Continental Drift because it provides a
partial explain of how ocean crust and continental crust (continents) move.
Why didn’t Wegener use seafloor spreading to support
Continental Drift?
Wegener couldn’t use seafloor spreading to prove Continental Drift because sonar and deepdiving vessels didn’t exist in Wegner’s lifetime to identify and explore mid-ocean ridges.
Name: ____________________________
Science / Block: _______
Date: _________________
Teacher: ______________
Chapter 3 Lesson 2 – Seafloor Spreading - (5 words)
mid-ocean ridge- ____________________________________________________________
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Sonar- ____________________________________________________________________
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seafloor spreading- __________________________________________________________
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deep-ocean trenches- ________________________________________________________
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subduction – _______________________________________________________________
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Evidence that supports the theory of seafloor spreading:
Evidence from ocean material:
___________________________________________________________________________
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Evidence from magnetic stripes:
___________________________________________________________________________
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___________________________________________________________________________
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Evidence from drilling samples:
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Explain seafloor spreading helps support Wegener’s hypothesis
of Continental Drift:
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___________________________________________________________________________
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Why didn’t Wegener use seafloor spreading to support
Continental Drift?
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