1. Warm Up
If erosion stripped off the top of a dome, what would be found?
a. The oldest rocks are exposed in the center.
b. The oldest rocks are exposed along the edges of the dome.
c. The youngest rocks are exposed in the center.
d. The rocks in the center are the same age as those along the edges.
The major forces that form folded mountains are ____.
a. volcanic in nature c. shear stresses
b. tensional stresses d. compressional stresses
Compressional stresses can result in the formation of ____.
a. rift valleys c. normal faults
b. thrust faults d. horsts and grabens
Answers: 1) a. 2) d. 3) b.

2. Mountain Formation
Chapter 11, Section 3

3. Mountain Building at Convergent Boundaries
With the original development of the theory of plate tectonics, an accepted model for orogenesis emerged
Most mountain building occurs at convergent plate boundaries
Colliding plates provide the compressional forces that fold, fault, and metamorphose the thick layers of sediments deposited at the edges of landmasses

4. Ocean-Ocean Convergence
Ocean-ocean convergence mainly produces volcanic mountains
These features tend to be long-lived
The accumulation of sediment scraped off of the subducting plate increases the volume of the crust
Examples: Aleutian Islands (Alaska) and Japan

5. Development of Mature Island Arc

6. The Japanese Islands

7. Ocean-Continental Convergence
The convergence of the continental block and the subduction of the oceanic plate leads to deformation and metamorphism of the continental margin
Partial melting of mantle rock above the subducting plate generates magma that migrates upward
Accretionary wedge - the accumulation of different sedimentary and metamorphic rocks with some scraps of ocean crust that gathers on the landward side of a trench
The types of mountains formed by ocean-continental convergence are volcanic mountains and folded mountains
Examples: Andes (South America) and Cascades

8. Ocean-Continental Convergence 1

9. Ocean-Continental Convergence 2

10. Ocean-Continental Convergence 3

11. Andes Mountains

12. Continent-Continent Convergence
Continental crust floats too much to be subducted
At a convergent boundary between two plates carrying continental crust, a collision between the continental fragments will result and form folded mountains
Most of the oceanic crust that separated landmasses, before a collision, is subducted, but some gets caught up in a collision zone and raised above sea level
Examples: Himalayas (Asia), Urals (Russia), and Appalachians

13. Continent-Continent Convergence

14. Concept Check Why can’t continental crust be subducted?
It is too bouyant (not dense enough)

15. Mountain Building at Divergent Boundaries
Most mountains are formed at convergent boundaries, but some are formed at divergent boundaries
The mountains that form along ocean ridges at divergent plate boundaries are fault-block type mountains
Example: the 65,000 kilometer long mid-ocean ridge system

16. Mid-Atlantic Ridge

17. Concept Check
Where is the longest mountain range?
Mid-Ocean Ridge

18. Major Mountainous Landforms of the Western U.S.

19. Continental Accretion
Accretion – the process by which fragments collide with a continental plate and become stuck or embedded into the continent
Terrane – any crustal fragment that has a geologic history distinct from the adjoining terranes
Terranes can contain anything from sediment off the ocean floor to volcanic island arcs
The newly added material increases the width and thickness of the continental crust
The accretion of larger crustal fragments, such as a mature island arc (Hawaii), may result in a mountain range
Because of its buoyancy, an island arc will not subduct beneath the continental plate

20. Submerged Crustal Fragments and Oceanic Plateaus

21. Collision of Volcanic Island Arc with a Continent

22. Accreted Terranes

23. Concept Check What is a terrane?
Any crustal fragment that has a geologic history distinct from the adjoining terranes

24. Isostasy
Isostasy – concept of a floating crust in gravitational balance
Many mountain belts stand high above the surface because they have buoyant (less dense) crustal “roots” that extend deep into the mantle
Isostatic adjustment – the processes of establishing a new level of gravitational equilibrium
As weight is added to the top of a mountain, the crust subsides, and as weight is lifted, the crust will rebound
Because of isostasy, deformed and thickened crust will undergo regional uplift both during mountain building and for a long period afterward

25. Isostasy

26. Erosion and Isostatic Adjustment

27. Isostasy in Action – Iceberg

28. Assignment Read Chapter 11, Section 3 (pg. 317-324)
Do Chapter 11 Assessment #1-36 (pg )
For Section 3: Do #’s 7, 9, 19-21, 23, 25, 26, 28, 35, & 36
Study for the Unit 2 Test (chapters 8-11)
Collect Notes (Total of 15 Sections, including this one!)