1. Warm Up 11/3
What hypothesis states that the continents were once joined to form a single supercontinent?
a. continental drift c. plate tectonics
b. seafloor spreading d. paleomagnetism
What kind of plate boundary occurs where two plates grind past each other without destroying or producing lithosphere?
a. convergent boundary c. divergent boundary
b. transitional boundary d. transform fault boundary
The lithospheric plates move an average of ____.
a. 50 inches per year c. 5 centimeters per year
b. 5 inches per year d. 50 centimeters per year
Answers: 1) a. 2) d. 3) c.

2. Actions at Plate Boundaries
Chapter 9, Section 3

3. Divergent Boundaries
Most divergent boundaries are located along the crests of oceanic ridges
These plate boundaries can be thought of as constructive plate boundaries, because it is where new oceanic lithosphere is created

4. Location of Most Divergent Boundaries

5. Oceanic Ridges and Seafloor Spreading
Oceanic Ridge – along well-developed divergent plate boundaries, area where seafloor is elevated
The system of ridges is the longest physical feature on Earth’s surface (70,000 km long)
These features are 1000 to 4000 km wide, not narrow at all
Rift Valley – Deep faulted structures found along the ridge system
Seafloor Spreading – process by which plate tectonics produces new lithosphere
Typically the rates of spreading average about 5 cm/year, fast enough that all of the current oceans have been produced in the last 200 million years

6. Seafloor Spreading

7. Continental Rifts
When spreading centers develop within a continent, the landmass may split into two or more smaller segments
The most widely accepted view is that areas where plumes of hot rock rise from the mantle are weaker then other areas, stretching the areas above it causing faulting and volcanism
In the East African rift valley, the beginning stages of continental breakup may be occurring creating the Red Sea

8. East African Rift Valleys

9. Concept Check How do rifts begin to form?
The lithosphere is stretched and a plume of hot rock from the mantle weakens and then splits the lithosphere

10. Convergent Boundaries
Because lithosphere is being “destroyed” at convergent boundaries, they are also called destructive plate margins
Subduction Zones – destructive plate margins where oceanic crust is being pushed down into the mantle
Trench – Surface feature produced by descending slab, some of the deepest spots on Earth
Convergent boundaries are controlled by the type of crust involved in the convergence

11. Oceanic-Continental Convergence
When a continental plate meets an oceanic plate, the less dense continental plate remains floating as the oceanic plate is pushed underneath
When the descending plate reaches a depth of ~ km, some of the surrounding material will begin to melt, forming magma
Some of this magma may reach the surface and cause volcanic eruptions
Continental Volcanic Arc – mountains produced by the volcanic activity that is caused by the subduction of oceanic lithosphere under continental (Andes Mountains)

12. Oceanic-Continental Convergence

13. Oceanic-Oceanic Convergence
When two oceanic slabs converge, one descends below the other
This causes volcanic activity similar to the oceanic-continental boundary, with the volcanoes forming on the ocean floor instead
It will eventually form a chain of volcanic structures that will become islands
Volcanic Island Arc – newly formed land consisting of an arc-shaped chain of small volcanic islands (Aleutian Islands, Alaska)

14. Oceanic-Oceanic Convergence

15. Continental-Continental Convergence
Eventually, subduction may bring two continents together and a collision occurs
The result of this collision is usually seen in the form of complex mountains such as the Himalayas in South Asia
When the continents collide, after all of an ocean basin between them has been subducted, the collision folds and deforms the sediments along the margin
A mountain chain forms that is composed of deformed and metamorphosed sedimentary rocks
The Himalayas, Alps, Appalachians, and Urals were all formed by this process

16. Continental-Continental Convergence

17. Collision of India and Asia

18. Concept Check What happens when continents collide?
Mountains form as the lithosphere is forced upward

19. Transform Fault Boundaries
At a transform fault boundary, plates grind past each other without destroying lithosphere
Most transform faults join two segments of mid-ocean ridges, present about every 100 km along the ridge axes
The seafloor produced at one ridge axis moves in the opposite direction as the seafloor produced at an opposing ridge segment, and these segments grind past each other on their ways
Most transform faults are found within the ocean basins, a few cut through the continental crust (San Andreas Fault)

20. Transform Faults

21. The Role of Transform Faults

22. The World’s Oceanic Trenches, Ridge System, and Transform Faults

23. Assignment Read Chapter 9, Section 3 (pg. 258-264)
Do Chapter 9 Assessment #1-30 (pg )
For Section 3: Do #’s 5, 7, 14, 17, 22, 23, 25, 27, & 31