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Sea-Floor Spreading
Oceanic crust forms at ocean ridges and
becomes part of the seafloor.
Review Vocabulary
basalt: a dark-gray to black fine-grained
igneous rock
I. Mapping the Ocean Floor
Until the mid-1900’s, many scientists
thought that the ocean floors were
essentially flat and that oceanic
crust was unchanging and was
much older than continental crust.
Advances in technology during the 1940’s
and 1950’s showed that all of these
widely accepted ideas were
incorrect.
A. Magnetometer - device that can
detect small changes in
magnetic fields
B. Sonar
1. Bounces sound
waves off
underwater
objects
2. Echoes determine
distance to
object
3. Maps ocean floor
topography
II. Ocean Floor Topography
A. Mid-Ocean Ridges
What name was given to the large
underwater mountain ranges that were
discovered during the 1940s and 1950s?
A. ocean ridges
B. ocean
trenches
C. sea mounts
D. sea stacks
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B. Deep-sea trenches
Pg. 474 Fig. 17.8
The deepest trench,
the Mariana Trench,
is more than 11 km
deep. Mount Everest,
the world’s tallest
mountain, stands at
9 km above sea level,
and could fit inside
the Mariana Trench
with six Empire State
buildings stacked on
top.
III. Ocean Rocks & Sediments
A. Glomar Challenger (drilling ship)
1. Youngest rock sample near ridge
2. Oldest rocks near continents
B. Evidence
1. Molten material
a. Alvin (submarine)
b. Rock samples
C. Sediments
1. Thickness of ocean-floor sediments
increases with distance from an ocean
ridge
How did scientists discover that rocks farther away from
the mid-ocean ridge were older than those near it?
A. by observing eruptions of
molten material on the sea
floor
B. by mapping rocks on the sea
floor using sonar
C. by determining the age of rock
samples obtained by drilling
on the sea floor
D. by measuring how fast seafloor spreading occurs
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What technology did scientists use in the
mid-1900s to map the mid-ocean ridge?
A. satellites
B. deep-sea
diving
C. submarines
D. sonar
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IV. Magnetism
A. Earth’s magnetic field
1. Flow of molten iron in the outer core
2. Magnetic reversal
a. Flow in the outer core changes
b. Earth’s magnetic field changes direction
B. Magnetic polarity time scale
1. Paleomagnetism - the study of the history
of Earth’s magnetic field
2. Lava solidifies
a. iron-bearing minerals crystallize
b. align with Earth’s magnetic field
Pg. 476 Fig. 17.11
Periods of normal polarity
alternate with periods of reversed
polarity. Long-term changes in
Earth’s magnetic field, called
epochs, are named as shown
here. Short-term changes are
called events.
FIG 17. 12 Pg. 477 Regions of
normal and reverse polarity
form a series of stripes across
the ocean floor parallel to the
ocean ridges. The ages and
widths of the stripes match
from one side of the ridges
to the other.
3. Isochron - imaginary line on a map
a. points that have the same age
Which layer of Earth generates
Earth’s magnetic field?
a. the crust
b.the mantle
c. the outer core
d.the inner core
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A rock cools from lava and acquires a
normal magnetic orientation. What
happens to this rock’s magnetism when
Earth’s magnetic field next reverses?
A. It changes direction.
B. It stays the same.
C. It gets erased.
D. It has a different
strength.
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V. Sea-floor spreading
A. Harry Hess studied mid-ocean
ridges
1. Maybe Wegener was right
B. New material added to ocean floor
1. Spreads apart at ridges
2. Moves ocean floor & continents
C. Molten material rises
1. Pushes older rock outward
2. New material cools
In sea-floor spreading, molten material
rises from the mantle and erupts
A. along the edges of
all the continents.
B. along mid-ocean
ridges.
C. in deep-ocean
trenches.
D. at the north and
south poles.
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What happens to an oceanic plate
after it moves to a trench?
A. It is welded to a
continent.
B. It sinks into the mantle.
C. It is uplifted to form
mountains.
D. It breaks into small
pieces.
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VI. Subduction at Trenches
A. Ocean floor sinks back into mantle
1. Gravity pulls old, dense rock
down
2. Occurs at deep-ocean trenches
The process by which the ocean floor
sinks beneath a deep-ocean trench
and back into the mantle is known as
A. convection.
B. continental
drift.
C. subduction.
D. conduction.
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Where in an ocean basin is the
oldest oceanic crust usually
located?
A. near an ocean ridge
B. near an ocean trench
C. along the middle of
the basin
D. along any edge of the
basin
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Visualizing Seafloor Spreading
Data from topographic,
sedimentary, and paleomagnetic
research led scientists to propose
seafloor spreading.
B. Changes size & shape of oceans
1. Pacific Ocean is shrinking
a. More subduction than new
crust forming
2. Atlantic is expanding
seafloor401spreading
Oceanic crust forms at
ocean ridges and becomes part of
the seafloor.
 Studies of the seafloor provided
evidence that the ocean floor is
not flat and unchanging.
 Oceanic crust is geologically young.
 New oceanic crust forms as
magma rises at ridges and
solidifies.
 As new oceanic crust forms,
the older crust moves away
from the ridges.