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.

Mountain Formation Chapter 11, Section 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

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

Development of Mature Island Arc

The Japanese Islands

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

Ocean-Continental Convergence 1

Ocean-Continental Convergence 2

Ocean-Continental Convergence 3

Andes Mountains

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

Continent-Continent Convergence

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

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

Mid-Atlantic Ridge

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

Major Mountainous Landforms of the Western U.S.

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

Submerged Crustal Fragments and Oceanic Plateaus

Collision of Volcanic Island Arc with a Continent

Accreted Terranes

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

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

Isostasy

Erosion and Isostatic Adjustment

Isostasy in Action – Iceberg

Assignment Read Chapter 11, Section 3 (pg. 317-324) Do Chapter 11 Assessment #1-36 (pg. 329-330) 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!)