1. 17.2 Seafloor Spreading

2. Objectives Vocabulary
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

3. 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.

4. 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

5. Magnetometer
A magnetometer is a device that can detect small changes in magnetic fields.

6. 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.

7. Trenches

8. Mid-Atlantic Ridge
mountains

13. 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

15. Ocean Rocks and Sediments

16. Paleomagnetism is the study of Earth’s magnetic record.
Rocks containing iron-bearing 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.

17. 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.

18. 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.

19. The Geomagnetic Time Scale

20. 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.

22. 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.

24. 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.

25. 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.

26. 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.

27. 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

28. 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.