17.2 Seafloor
Spreading
Objectives
• Summarize the evidence that led to the
discovery of seafloor spreading.
• Explain the significance of magnetic patterns
on the seafloor.
• Explain the process of seafloor spreading.
Vocabulary
– magnetometer
– paleomagnetism
– magnetic reversal
– isochron
– seafloor spreading
Seafloor Spreading
• Many thought the ocean floor was flat until
the mid-1900s.
• Many thought that oceanic crust was
unchanging and was much older than
continental crust.
• Technology in the 40’s 50’s,showed those
ideas to be wrong.
SONAR- Sound Navigation and Ranging
• Sound waves could find the depth of the ocean
floor.
• Sound waves travel through
water at:
– R = 1482 m/s
– T= was measured
– D= was found
• R×T = D
Magnetometer
• A magnetometer is
a device that can
detect small changes
in magnetic fields.
Ocean Floor Topography
• Maps made from sonar and magnetometers
showed underwater mountain chains called
ocean ridges.
• Deep-sea trenches were found.
• Geologists could not explain why there
were trenches and mountains on the
oceans floor.
Trenches
Mid-Atlantic
Ridge
• mountains
Ocean Rocks and Sediments
Analysis of deep-sea rocks and sediments found
1. Ages of the seafloor rocks differ. The farther
from a ridge the older the rock.
– The oldest part of the seafloor is geologically
young at about 180 million years old.
2. Ocean-floor sediment gets thicker farther from
a ridge
Ocean Rocks and Sediments
• Paleomagnetism is
the study of Earth’s
magnetic record.
• Rocks containing ironbearing minerals provide
a record of Earth’s
magnetic field.
• Basalt, because it is
rich in iron-bearing
minerals, provides an
accurate record of
ancient magnetism.
Magnetism
The Geomagnetic Time Scale
– Studies reveal a pattern of
magnetic reversals over
geologic time.
– A magnetic reversal is a change
in Earth’s magnetic field.
• A magnetic field that is the same as
the present has normal polarity.
• A magnetic field that is opposite to
the present has reversed polarity.
The Geomagnetic Time Scale
– Towing magnetometers behind ships revealed
an interesting magnetic pattern.
• In places where the magnetic readings of
the ocean floor matched Earth’s present
field, a stronger-than-normal reading (+) was
recorded.
• In places where the magnetic data were
reversed in relation to Earth’s present
magnetic field, a lower-than-normal reading
(–) was recorded.
The Geomagnetic Time Scale
Magnetic Symmetry
– The positive and negative
areas of the seafloor form a
series of stripes that were
parallel to ocean ridges.
– The magnetic pattern on
one side of the ridge is a
mirror image of the pattern
on the other side of the
ridge.
Magnetic Symmetry
– The magnetic data matched the pattern that
had been found in basalt flows on land.
– This allowed scientist to determine the age of
the ocean floor. And make isochron maps.
– An isochron is a line on a map that connects
points that have the same age.
Seafloor Spreading
• An American scientist named Harry Hess
proposed the theory of seafloor spreading.
• Seafloor spreading
states that new ocean
crust is formed at ocean
ridges and destroyed at
deep-sea trenches.
– Magma is forced toward
the crust along an ocean
ridge and fills the gap that
is created.
Seafloor Spreading
– When the magma hardens,
a small amount of new
ocean floor is added to
Earth’s surface.
– Each cycle of spreading
and the intrusion of
magma results in the
formation of another
small section of ocean
floor, which slowly moves
away from the ridge.
The Missing Link
– Seafloor spreading was the missing link to
complete his model of continental drift.
– Continents are not pushing through ocean
crust, as Wegener proposed; they ride with
ocean crust as it slowly moves away from
ocean ridges.
Section Assessment
1. Match the following terms with their
definitions.
A. a device that can detect small
changes in magnetic fields
A magnetometer
___
C
B.
___paleomagnetism
B isochron
___
C.
D seafloor spreading
___
a line on a map that connects
points that have the same age
the study of Earth’s magnetic
record
D. a theory that states that new
ocean crust is formed at ocean
ridges and destroyed at deepsea trenches
Section Assessment
2. How does the distribution of ocean-floor
sediments support the theory of seafloor
spreading?
The thickness of ocean-floor sediments increases
with distance from an ocean ridge which indicates
that the seafloor is older with distance.