Chapter 17
Plate Tectonics
Section 17.2
Sea-Floor Spreading
Vocabulary

Subduction:


Sea-flooring spreading:


The process by which oceanic crust sinks beneath a deepocean trench and back into the mantle at a convergent plate
boundary.
The process by which molten material adds new oceanic
crust to the ocean floor.
Deep-ocean trench:

A deep valley along the ocean floor through which oceanic
crust slowly sinks towards the mantle.
Vocabulary

Mid-ocean ridge:


The undersea mountain chain where new ocean
floor is produced; a divergent plate boundary.
Sonar:

A device that determines the distance of an object
under water by recording echoes of sound waves.
Mapping the Mid-Ocean Ridge

Circle the letter of each sentence that is true
about the mid-ocean ridge.




A. The mid-ocean ridge is the longest chain of
mountains in the world.
B. The mid-ocean ridge is found only below the
Pacific Ocean.
C. The mid-ocean ridge lies completely under
water.
D. The top of the mid-ocean ridge is split by a
steep-sided valley.
Mapping the Mid-Ocean Ridge

A device that bounces sound waves off
sonar
underwater objects is called __________.

What is sonar used for?

Sonar is used to determine the distance to an
object. It has been used to map the mid-ocean
ridge.
Evidence for Sea-Floor Spreading

The process that continually adds new material
to the ocean floor is called
sea-floor spreading
________________.
Molten rock erupts
through cracks.
Molten rock cools to
form strip of rock.
Older rock is
pushed aside.
Evidence for Sea-Floor Spreading

Three types of evidence for sea-floor
spreading:



Molten material
Magnetic stripes
Drilling samples
Evidence for Sea-Floor Spreading

Circle the letter of each sentence that is true
about Earth’s magnetism.




A. At times in the past, a compass needle on Earth
would have pointed south.
B. Rock that makes up the ocean floor lies in a
pattern of magnetized stripes.
C. The pattern of stripes is different on both sides
of the mid-ocean ridge.
D. Rocks that harden at the same time have the
same “magnetic memory.”
Evidence for Sea-Floor Spreading

How did drilling samples show that sea-floor
spreading really has taken place?

The farther away from the ridge that the
samples were taken, the older they were, and
the youngest samples were always in the center
of the ridge.
Subduction at Deep-Ocean Trenches

Deep underwater canyons are called
deep-ocean
trenches
________________.

True or False: At deep-ocean trenches,
conduction allows oceanic crust to sink back
into the mantle.

False
Subduction and Earth’s Oceans

True or False: The Pacific Ocean is shrinking.


True
Why is the Atlantic Ocean expanding?

The Atlantic Ocean is expanding because it has
only a few short deep-ocean trenches, so the
spreading ocean floor has virtually nowhere to
go.