Convergent Boundaries

Transcription

1 Question 13 a. Long columns of hot, less dense rock, rising from deep in the mantle which are responsible for the volcanism at mid-ocean ridge spreading zones such as the Mid-Atlantic Ridge b. Long columns of hot, less dense rock, rising from deep in the mantle and usually erupting in the middle of oceanic and continental crust, and occasionally at mid-ocean ridge spreading zones c. Long columns of hot, less dense rock, rising from deep in the mantle and resulting in regional uplift at the Earth s surface d. Long columns of hot, less dense rock, rising from deep in the mantle and responsible for about 10% of the Earth s total heat loss e. B, C and D are all correct

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4 Convergent Boundaries Zones where lithospheric plates collide Three major types Ocean - Ocean Ocean - Continent Continent - Continent Direction and rate of plate motion influence final character

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7 Convergent Boundaries Convergent boundaries may form subduction zones Occurs in oceanic crust Associated with outer swell, trench & forearc, magmatic arc, and backarc basin Associated earthquakes range from shallow to deep

8 Convergent Boundaries Crustal deformation is common Melange produced at subduction zone Continental collisions involve strong horizontal compression Extension & compression in backarc basin and the swell

9 Convergent Boundaries 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

10 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 grow by addition

11 Plate Buoyancy Processes at convergent margins influenced by plate density Sharp contrast in density of oceanic and continental crust Differences in thickness change density Thick oceanic crust forms less dense lithospheric plate Temperature & age also affect density: Older oceanic crust is usually colder

12 1. Ocean-Ocean Convergence One plate (oldest and coldest) thrust under to form subduction zone Subducted plate is heated, magma generated Andesitic volcanism forms island arc Broad belts of crustal warping occur

13 Ocean-Ocean convergence

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15 Island Arc Magmatism Volcanic islands form arcuate chain ~ 100 km from trench 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

16 The Aleutian Island Chain

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18 Seismic activity in the Aleutian Islands

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21 2. 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

22 Ocean-Continent convergence

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27 Continental Arc Magmatism Volcanic islands form arcuate chain ~ km from trench 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

28 Fig Magma production at subduction zones

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30 Magma Generation Characteristically andesite in composition Contain more water and gases than basalt Results in more violent volcanism Water in slab is released, induces melting of overlying mantle Water lowers mineral melting points

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32 Magma Generation Hybrid magma rises & interacts with crust Magma has oceanic crust, sediment mantle and overlying crust components Fractional crystallization enriches the magma is silica

33 Magma Generation Smaller volumes of granitic magma are produced at continental collisions Melting is induced by deep burial of crust Melt forms from partial melting of metamorphic rocks Granites have distinct geochemistry and include several rare minerals

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37 Mt. Vesuvius

38 The people of Pompeii; mummified in 5-8 meters of hot ash in A.D. 79

39 The smoldering city of Pierre, Martinique

40 movies PyroclasticFlow.mov Redflow.mpg

41 Thermal Structure of Subduction Cold slab Cold subducting plate heats very slowly Temperature at 150 km Cold slab ~ 400 o C Surrounding mantle ~ 1200 o C Temperature variation influences slab behavior More brittle & stronger Moves downward as coherent slab

42 Thermal Structure of Subduction Hot Arc Heat flow is elevated beneath volcanic arc Ascending magma carries heat from mantle Subducting plate may cause mixing in the asthenosphere beneath the arc

43 Thermal structure of subduction zone

44 Plate Motion Direction & rate of plate motion are important factors in plate dynamics Head on collisions form large subduction zones with intense compression and igneous activity Oblique angle collisions are less energetic and have smaller subduction zones

45 Earthquakes - Subduction Zones Subducting slab forms inclined seismic zone Angle of plunge between o Reaches depths of > 600 km Shallow quakes in broad zone from shearing of two plates Deeper quakes occur within slab

46 Compression at Subduction Zones Unconsolidated sediments form accretionary wedge Sediments scraped off of subducting plate Folds of various sizes formed Fold axes parallel to trench Thrust faulting & metamorphism occur Growing mass tends to collapse

47 Compression at Subduction Zones Melange is a complex mixture of rock types Includes metamorphosed sediments and fragments of seamounts & oceanic crust Not all sediment is scraped off 20-60% carried down with subducting slab

48 Compression at Subduction Zones Orogenic belts are created at ocean - continent margins Pronounced folding and thrust faulting Granitic plutons develop, add to deformation Rapid uplift creates abundant erosion

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50 Fig The San Andreas transform fault system

51 Juan de Fuca Plate

52 Oregon/Washington Idaho Montana Cascades/Olympics Rockies Fig Structure of western NA

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63 3. 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 Orogenic metamorphism occurs with generation of granitic magmas

64 Continent-Continent convergence

65 Compression in 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

66 C onv marg.swf

67 Formation of Himalaya Mountains

68 Major units in an ophiolite sequence

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70 Extension at Convergent Boundaries Extension may be common at convergent boundaries Warping of crust creates extensional stress Extreme extension creates rifting and formation of new oceanic crust Influenced by angle of subduction & absolute motion of overriding plate

71 Metamorphism Metamorphism driven by changes in environment Tectonic & magmatic processes at convergent margins create changes in P & T Paired metamorphic belts are commonly associated with subduction zones

72 Fig Paired metamorphic belts

73 Metamorphism Outer metamorphic belt forms in accretionary wedge Blueschist facies metamorphism High P - low T Metamorphosed rocks brought back to surface by faulting Include chunks of oceanic crust and serpentine

74 Metamorphism Inner metamorphic belt forms near magmatic arc High T and varying P conditions Contact metamorphism occurs near magma bodies Orogenic metamorphism occurs in broader area Greenschist and amphibolite grade

75 Regional Metamorphism: Subduction zones..

76 Change in metamorphic grade with depth

77 Formation of Continental Crust Continental crust grows by accretion New material introduced by arc magmatism Older crust is strongly deformed New crust is enriched in silica & is less dense No longer subject to subduction

78 Accreted Terranes Continental margins contain fragments of other crustal blocks Each block is a distinctive terrane with its own geologic history Formation may be unrelated to current associated continent Blocks are separated by faults Mostly strike-slip

79 Accreted terranes along convergent margin

80 Continental Growth Rates Basement ages in NA form concentric rings of outward decreasing age Each province represents of series of mountain building events Rate varies over geologic time Slow rate during early history - some crust may have been swept back into mantle Rapid growth between 3.5 and 1.5 bya Subsequent growth slower

81 North American Craton Shield Western North American Mobile Belt Eastern Nor American Mobile Bel Platform

82 Growth of North American Continent

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87 Volcanism associated with Plumes, i.e., Hot Spots

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89 Hotspots Long, vertical columns of hot magma First evidenced in Hawaii Shield volcanoes not associated with other tectonic activity Age of islands get progressively older Similar trends seen in other linear island chains

90 Hotspot Characteristics Distribution is linear Produces submarine volcanoes Some become islands Lithosphere moves over mantle plume One volcano becomes dormant, a new one develops Magma generation is in the lower mantle At least 700 km, maybe at mantle base

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92 Evidence of Mantle Plumes Evidence is indirect Local zones of high heat flow Hotspots do not drift with plate movement Geochemistry of basalt is distinct Deep mantle source Oceanic islands associated with swells Seismic studies

93 Evolution of Mantle Plumes Plumes are a form of convection Less dense material at base of mantle Less dense material begins to rise Diapirs Starting plume enlarges Large bulbous head grows Narrow tail feeds material upward

94 Fig Plume evolution and geometry

95 Evolution of Mantle Plumes Rising plume swells lithosphere Plume rises and spreads beneath lithosphere Reduced pressure allows magma generation Rifting provides conduits for magma Most of plume head cools Tail may continue to feed new material

96 Fig A starting plume

97 Making Magma Magma is generated by decompression melting Lower pressure allows material to partially melt Similar process at midocean ridges Occurs at ~ 100 km deep Less dense magma continues to rise Source of material controls geochemistry

98 Fig Decompression melting

99 Making Magma Source of magma appears to be mantle material contaminated with ancient oceanic crust and sediments Cold oceanic crust is metamorphosed during subduction Resulting material is very dense Dense material sinks to base of mantle

100 Mantle Composition of different basalts

101 Mantle Plumes - Oceanic Tail plumes create island chains Large shield volcanoes produced over plume tails Quiet flows of basaltic lava Collapse caldera forms at summit Vertical tectonic processes from high heat flow and weight of volcano

102 Mantle Plumes - Oceanic Starting plumes generate flood basalts Broad oceanic plateaus Extremely large volcanic event Oceanic crust increased in thickness by up to 5x No large shield volcanoes Magnetic stripes hidden Eruption rate similar to all of ridge system

103 Hawaiian Plume Best example of still-rising tail plume Hawaii is active portion of chain of islands Remaining islands are extinct volcanoes Most are now below sea level Hawaii has 2 active volcanoes Mauna Loa & Kilauea

104 Fig Formation of island chain

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106 Fig Hawaiian Island chain

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110 Hawaiian Volcanism Volcanism dominated by basalt Partial melting of mantle material Low water and volatiles content compared to subduction zone basalts Few andesites or rhyolites No continental crust component Eruptions commonly form along fissures

111 Fig Evolution of a volcanic island

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114 Size comparison of various volcanic features

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116 Hawaiian Earthquakes Earthquakes are relatively small and infrequent Most are shallow associated with magma movement or slumping Usually magnitude 4.5 or less Some quakes form in the mantle May be larger, up to 6.2 magnitude

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118 Mantle Plumes - Midocean Ridges Plumes may form coincident with midocean ridge system Iceland formed at intersection of ridge and hotspot tail plume Combination produced island Basalt geochemistry shows mixing Rhyolite forms as basalt partially melts Plume may have assisted in initial rifting

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120 Mantle Plumes - Continents Plumes beneath continents create regional uplift and bimodal volcanism Lithosphere gently warps from rising plume Flood basalts erupted Rhyolite forms from melting of crust May initiate continental rifting

121 Yellowstone Plume Yellowstone plume has evolved from head to tail stage Starting plume produced Columbia River flood basalts Uplift created rifting in Nevada NA has moved SW over plume Tail plume forms Yellowstone volcanics and geysers

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123 Fig Cenozoic features of NW U.S.

124 Fig b. Cross section of Yellowstone plume

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127 Continental Rifting Rifting may be initiated by mantle plumes Rising starting plume spreads out beneath continental lithosphere Buoyant plume domes lithosphere Extension may lead to rift development Etendeka and Parana basalt provinces

128 Continental Rifting Plumes do not always cause rifting Major mantle plumes produce continental flood basalts Rifting occurs in an intraplate environment on a plate already in motion Siberian flood basalts - latest Paleozoic Lake Superior - Precambrian Yellowstone

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130 Flood basalts with several thick and thin layers. Each layer represents a separate eruption.

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133 Plumes, Climate & Extinction Mantle plumes may affect Earth s climate and magnetic field Starting plumes create enormous amount of volcanism over short time period May change composition and circulation in ocean and atmosphere Large volumes of volcanic gases produced, including CO 2

134 Plumes, Climate & Extinction Flood basalts may be correlated with climate change and extinction events Ontong-Java Plateau - Cretaceous warming Deccan Plateau - Cretaceous Tertiary boundary Siberian flood basalts - late Paleozoic extinctions

135 Plumes, Climate & Extinction Plume events may correlate with polarity reversals Large number of plume events correlates with decreased polarity changes during Cretaceous Plume events may remove heat from outer core area, slowing convection

136 Evolution of Seamounts & Islands Grow by extrusion of lava at various points of their surface - mostly subaqueous Intrusive features also form Base subsides as volcano grows upward Submarine mass movements, mainly along faults Islands subject to subaerial erosion Subsidence occurs away from hotspot