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Convergent Boundaries, Mountain Building, and Evolution of Continents

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1 Convergent Boundaries, Mountain Building, and Evolution of Continents
Chapter 14 Convergent Boundaries, Mountain Building, and Evolution of Continents

2 Orogenesis – the processes that collectively produce mountain belts
Some made of lavas & volcanic debris, along with massive amounts of intrusive igneous rocks Most show evidence of compressional forces as well as metamorphic & igneous activity

3 Figure 14.2

4 Figure 14.3

5 An early idea on mountain formation – produced as Earth contracted as it cooled; early hypotheses did not withstand scrutiny

6 Theory of plate tectonics – model that explained many things
Most mountain building occurs at convergent boundaries Subduction of plates causes partial melting of rock, the source of intrusions Collision of plates provides the forces needed to fold, fault, & metamorphose sediments

7 Convergence & subducting plates
Sites of plate destruction – oceanic lithosphere bends & plunges back into the mantle Higher temperatures & pressures cause the plates to eventually reassimilate into the mantle

8 Features of subduction zones
Four major regions Deep-ocean trench (where a slab descends) Volcanic arc (built on the overlying plate) Forearc region (between the trench & volcanic arc) Backarc region (on the side of the volcanic arc opposite the trench)

9 Two types of subduction zone
Oceanic crust subducts beneath another oceanic slab Oceanic crust subducts beneath a continent

10 Figure 14.4A

11 Figure 14.4B

12 Backarc spreading Cold slabs descend rather vertically
Causes the trench to retreat, pulling the upper plate toward the retreating trench May lead to formation of a backarc basin

13 Figure 14.6

14 Island arc mountain belts
Simplest mountain belts From volcanic activity, emplacement of plutonic bodies at depth, and scraping of sediment from subducting plate

15 Andean-type margins Begins as a passive continental margin
Not a plate boundary, but part of the same plate as the adjoining oceanic crust East Coast of U.S. is an example

16 Figure 14.7A

17 Creation of volcanic arc
At some point, oceanic crust breaks from the continental plate & starts to descend Water in subducting crust causes partial melting Partial melting leads to differentiation, causing magma to change composition Magmas become andesitic or rhyolitic in composition

18 Thicker continental crust impedes upward movement of magma
That which does not reach the surface crystallizes to form plutons; large accumulations form batholiths (core of Sierra Nevada; Peruvian Andes) In western N. Am., batholiths are granodiorite to diorite, with some granite In core of Appalachians, much granite

19 Figure 14.7B

20 Accretionary Wedges Sediments carried on subducting plate, & fragments of oceanic crust, may be scraped off and “plastered” onto the overriding plate The process causes deformation & thrust-faulting of the material As the wedge grows, sediments cannot move to the trench and begin to collect Relatively undeformed layers of sediment, called a forearc basin

21 Figure 14.7C

22 Continental collisions
Continental crust is too buoyant to subduct Causes compressional mountains; crust is shortened & thickened Crustal thickening through folding & faulting – fold-and-thrust belts Himalayas & Appalachians

23 Figure 14.9

24 The Himalayas Youngest collision mountains, still rising
Began about 45 mya, when India began to collide with Asia The deformable materials on the seaward edges of both landmasses were highly folded and faulted, and also uplifted Lower layers experienced elevated temperatures & pressures, causing melting Uplift also led to raising of the Tibetan Plateau

25 Asian part highly deformed, Indian part not so
India composed of shield rocks – old, “cold”, crystalline rocks Asia was more recent, still “warm & weak”

26 Figure 14.11

27 The Appalachians Eastern N. Am. From Alabama to Newfoundland
Extension of them in the British Isles, Scandinavia, NW Africa, & Greenland (Caledonian Mountains) This all started a mere 750 mya, ended about 250 mya

28 A three-fold event Eastern N. America collided with Europe, NW Africa, and some things in between

29 Terranes Crustal fragments having a geologic history different from the adjoining areas Often result of collision & merger small crustal fragments to a continent Island arcs Microcontinents Submerged crustal fragments

30 Present-day oceanic plateaus & other submerged crustal fragments
Figure 14.14 Present-day oceanic plateaus & other submerged crustal fragments

31 Figure 14.15 Collision & accretion of an island arc to a continental margin

32 Figure 14.16 Terranes added to western N. Am. Over past 200 m.y.

33 Fault-Block Mountains
Produced by continental rifting (tensional forces) Bounded by high-angle normal faults that flatten with depth Response to broad uplifting Mtns along East African rift valley, Sierra Nevada, Grand Tetons

34 Basin & Range Province Nevada & portions of surrounding states, parts of southern Canada & western Mexico Upper crust broken into hundreds of fault blocks, yielding nearly parallel mtn ranges that rise above adjacent basins Began about 20 mya Crust has been stretched to about twice its original width High heat flow and volcanic episodes suggest mantle upwelling as the cause

35 Figure 14.18

36 Isostasy Less-dense crustal rocks floating on denser, more deformable mantle rocks Gravitational balance

37 Figure 14.20

38 Figure 14.21

39 Figure 14.22

40 Growth of continents Earth about 4.5 by old
Oldest known rocks about 4 by old Oldest mineral grains 4.2 by old Oldest rocks similar to present continental crust are 3.8 to 3.5 by old Actual process by which continents grew not known for certain Possibly by accretion of smaller island arcs along with magmatic differentiation

41 Figure 14.23

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