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.
– magnetometer– paleomagnetism– magnetic reversal
– isochron– seafloor spreading
Vocabulary
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.
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
• Rocks containing iron-bearing minerals provide a record of Earth’s magnetic field.
• Paleomagnetism is the study of Earth’s magnetic record.
• Basalt, because it is rich in iron-bearing minerals, provides an accurate record of ancient magnetism.
• 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.
MagnetismThe Geomagnetic Time Scale
– Studies reveal a pattern of magnetic reversals over geologic time.
– A magnetic reversal is a change in Earth’s magnetic field.
• 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
– Towing magnetometers behind ships revealed an interesting magnetic pattern.
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.
– Magma is forced toward the crust along an ocean ridge and fills the gap that is created.
• Seafloor spreading states that new ocean crust is formed at ocean ridges and destroyed at deep-sea trenches.
Seafloor Spreading
– 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.
– When the magma hardens, a small amount of new ocean floor is added to Earth’s surface.
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.___ magnetometer
___ paleomagnetism
___ isochron
___ seafloor spreading
A. a device that can detect small changes in magnetic fields
B. a line on a map that connects points that have the same age
C. the study of Earth’s magnetic record
D. a theory that states that new ocean crust is formed at ocean ridges and destroyed at deep-sea trenches
A
C
B
D
Section Assessment2. 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.