Chapter 14

Convergent Boundaries,

Mountain Building,

and Evolution of Continents

• 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

Figure 14.2

Figure 14.3

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

• 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

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

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)

• Two types of subduction zone– Oceanic crust subducts beneath another

oceanic slab– Oceanic crust subducts beneath a

continent

Figure 14.4A

Figure 14.4B

• 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

Figure 14.6

Island arc mountain belts

• Simplest mountain belts

• From volcanic activity, emplacement of plutonic bodies at depth, and scraping of sediment from subducting plate

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

Figure 14.7A

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

• 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

Figure 14.7B

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

Figure 14.7C

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

Figure 14.9

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

• Asian part highly deformed, Indian part not so– India composed of shield rocks – old, “cold”,

crystalline rocks– Asia was more recent, still “warm & weak”

Figure 14.11

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

• A three-fold event• Eastern N. America collided with Europe, NW

Africa, and some things in between

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

Figure 14.14

Present-day oceanic plateaus & other submerged crustal fragments

Figure 14.15

Collision & accretion of an island arc to a continental margin

Figure 14.16

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

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

• 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

Figure 14.18

Isostasy

• Less-dense crustal rocks floating on denser, more deformable mantle rocks– Gravitational balance

Figure 14.20

Figure 14.21

Figure 14.22

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

Figure 14.23