If erosion stripped off the top of a dome, what would be found?

1. 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 convergence of two continental plates would produce a. trenches. c. folded mountains. b. rift valleys. d. island arcs. • What type of mountains are produced by ocean-ocean convergence? a. a continental volcanic arc b. mainly volcanic mountains c. parallel belts of uplifted mountains and volcanic mountains d. parallel belts of folded mountains and volcanic mountains

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?

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?

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. 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

24. Isostasy

25. Erosion and Isostatic Adjustment

26. Isostasy in Action – Iceberg

27. Assignment • Read Chapter 11, Section 3 (pg. 317-324) • Do Section 11.3 Assessment #1-7 (pg. 324) • Start Studying for the Unit 2 Test (Chapters 8-11)