1. Warm Up 11/5
Why are subduction zones not commonly found at convergent continental-continental boundaries?
a. Oceanic lithosphere is too buoyant to be forced down into the mantle.
b. Continental lithosphere is too dense to be forced down into the mantle.
c. Continental lithosphere is too buoyant to be forced down into the mantle.
d. Subduction zones are never found at convergent boundaries.
Which of the following is a geographic example of a transform fault boundary?
a. the Andes Mountains c. the San Andreas Fault
b. the East African Rift valley d. the Mid-Atlantic Ridge
The Red Sea is theorized to be the site of a recently formed ____.
a. convergent boundary c. divergent boundary
b. hot spot d. transform fault boundary
Answers: 1) c. 2) c. 3) c.

2. Testing Plate Tectonics & Mechanisms of Plate Motion
Chapter 9, Sections 4 & 5

3. Paleomagnetism
Paleomagnetism – the study of changes in Earth’s magnetic field, as shown by patterns of magnetism in rocks that have formed over time
Some rocks contain iron-rich minerals that will align with Earth’s magnetic field at the time of their formation, creating a time capsule of the magnetic field at that time in time
When the rock moves, or the magnetic pole changes position, it will still retain its original alignment

4. Apparent Polar Wander

5. Paleomagnetism Cont.
Normal Polarity – the same direction of magnetism as the present day
Reverse Polarity – the reverse direction of the magnetic field today
The discovery of strips of rocks of alternating polarity, which lie as mirror images across the ocean ridges, provided strong evidence of seafloor spreading
Ships towed instruments called magnetometers across the sea floor, revealing alternating high and low magnetism running in parallel bands to the oceanic ridges
As new basalt is added to the ocean floor, it gains the magnetism of the current magnetic field, and when the field changes over time, the basalt records the changes

6. Paleomagnetism

7. Time Scale of Earth’s Polarity

8. Magnetic Reversals in Seafloor Spreading

9. Earthquake Patterns
Scientists found a close link between deep-focus earthquakes and ocean trenches.
Also, the absence of deep-focus earthquakes along the oceanic ridge system was shown to be consistent with the new theory
Scientists have found that intermediate and deep focus earthquakes occur within the subducting plate as it goes into the mantle
Shallow-focus earthquakes are produced as the descending slab reacts with the lithosphere around it

10. Earthquake Distribution

11. Distribution of Earthquake Foci in Japan Trench

12. Ocean Drilling
Some of the best evidence has come from deep-sea drilling into the sediments on the ocean floor
The data on the ages of seafloor sediment confirmed what the seafloor spreading hypothesis predicted
The youngest oceanic crust is at the ridge crest and the oldest oceanic crust is at the continental margins
No sediment older than 180 million years has been found in the ocean basins

13. Hot Spots
Mapping revealed that there was a chain of volcanic structures in the middle of the Pacific Ocean ranging from the Hawaiian Islands to Midway Island and then north to the Aleutian trench
Hot Spot – a rising plume of mantle material that melts as it nears the surface, creating a volcanic area
As the Pacific plate moves over this spot, the island chain gets longer
The age of each volcanic island indicates when that part of the Pacific plate was over the hot spot
Hot spot evidence supports the idea that the plates move over Earth’s surface

14. Hot Spots Trace Plate Motion

15. Directions and Rates of Plates

16. Causes of Plate Motion
Scientists generally agree that convection occurring in the mantle is the basic driving force for plate tectonics
Convective Flow – the motion of matter resulting from convection
The slow movements of the plates and mantle are driven by the unequal distribution of Earth’s heat from the radioactive decay elements

17. Convective Flow

18. Slab-Pull and Ridge-Push
Slab-Pull – old oceanic crust, which is relatively cool and dense, sinks into the asthenosphere and “pulls” the trailing lithosphere along
Slab-pull is thought to be the primary downward arm of convective flow in the mantle
Ridge-Push – causes oceanic lithosphere to slide down the sides of the oceanic ridge as a result of gravity

19. Mantle Convection
Mantle Plumes – hot plumes of rock that are attributed to the upward flowing arms of mantle convection
Mantle plumes sometimes show themselves on the surface as hot spots and volcanoes
Whole-mantle convection is when slabs of cold oceanic lithosphere descend into the lower mantle, at the same time, hot mantle plumes originating near the mantle-core boundary move heat toward the surface
Another model is called the deep-layer model, and can be likened to a lava lamp, heat from Earth’s interior causes the layers to mix and combine in swirling patterns
The unequal distribution of heat within Earth causes the thermal convection in the mantle that ultimately drives plate motion

20. Whole-Mantle Convection

21. Deep-Layer Model

22. Assignment Read Chapter 9, Sections 4 & 5 (pg. 265-270)
Do Chapter 9 Assessment #1-30 (pg )
For Section 4: Do #’s 9, 13, 26, & 30
For Section 5: Do # 24