Chapter 21 Convergent Plate Boundaries

Convergent Boundaries

•  Zones where lithospheric plates collide – Three major types

• Ocean - Ocean • Ocean - Continent • Continent – Continent


•  Convergent boundaries may form subduction zones – Occurs in oceanic crust – Associated with trench & forearc,

magmatic arc, and backarc basin – Associated earthquakes range from

shallow to deep


•  Crustal deformation is common – Melange (chaotic mixture of rocks of

different type, age, and origin) produced at subduction zone

– Extension & compression in backarc basin

– Continental collisions involve strong horizontal compression


•  Magma is generated – Subduction and partial melting of oceanic

crust, sediments and surrounding mantle • Produces andesitic magma

– Continental convergence produces silicic magmas from melting of lower portions of thickened continental crust

Convergent Boundaries •  Metamorphism occurs in broad belts

– Metamorphism is associated with high pressure from horizontal compression

– High temperature metamorphism may occur in association with magmas

•  Continents (continental crust) grow by addition; Oceanic crust is destroyed

Ocean-Ocean Convergence

•  One plate thrust under to form subduction zone (the colder, denser, older oceanic crust is subducted) – Subducted plate is heated, magma

generated – Andesitic volcanism forms island arc – Broad belts of crustal warping occur – Ex.: Tonga Islands in western Pacific

Island Arc Magmatism

•  Volcanic islands form arcuate chain – High heat flow & magma production – Build large composite volcanoes

• Andesite with some rhyolite – Volcanoes built on oceanic crust &

metamorphic rocks – Volcanoes tend to be evenly spaced

Fig. 21.2. Ocean-Ocean convergence

Ocean-Continent Convergence •  Oceanic plate thrust under to form

subduction zone – Subducted plate is heated, magma generated – Andesitic volcanism forms continental arc – Broad belts of crustal warping occur including

folded mountain belts – Ex.: Andes of S. Am; Cascade Range of OR &

WA; older ex. is Rocky Mt. 50-150 mya

Continental Arc Magmatism •  Volcanic islands form arcuate chain

– Build large composite volcanoes • Andesite with some rhyolite

– Plutons of granite & diorite – Volcanoes built on older igneous &

metamorphic rocks – Volcanoes tend to be evenly spaced

Fig. 21.3. Ocean-Continent convergence

Continent-Continent Convergence •  One plate thrust over the other

– No subduction zone & associate warping occurs

– Folded mountain belt forms at suture of two continental masses

– Regional metamorphism occurs with generation of granitic magmas

– Ex.: Indian & Eurasian Plate produced Himalaya Mts. & Tibetan Plateau

Fig. 21.4. Continent-Continent convergence

Earthquakes - Subduction Zones

•  Subducting slab forms inclined seismic zone – Reaches depths of > 600 km (deep) – Shallow quakes in broad zone from

shearing of two plates – Intermediate quakes also occur within

slab

Compression at Subduction Zones

•  Unconsolidated ocean sediments form accretionary wedge – Sediments scraped off of subducting plate

sometimes forming a melange – Folds of various sizes formed

• Fold axes parallel to trench

– Thrust faulting & metamorphism occur

Compression at Subduction Zones

•  Orogenic belts (mountain belts) are created at ocean - continent margins – Pronounced folding and thrust faulting – Granitic plutons develop, add to

deformation – Rapid uplift creates abundant erosion

Fig. 21.13. Structure of western NA

Compression in Continent- Continent Collisions

•  Accretionary wedge and magmatic arc remnants included in orogenic belt

•  Continental collision thickens crust – Tight folds and thrust faulting – Possible intrusion of granitic plutons

•  Substantial uplift associated with erosion

Fig. 21.15. Formation of Himalaya Mtns

Extension at Convergent Boundaries •  Extension may be common at

convergent boundaries – Warping of crust can create tensional

stress – Extreme extension can create rifting and

formation of new oceanic crust – Influenced by angle of subduction &

motion of overriding plate

Cascade Range of OR & WA

End of Chapter 21

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Chapter 21 Convergent Plate Boundaries