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Igneous Rocks and Plate Tectonics PowerPoint PPT Presentation


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Igneous Rocks and Plate Tectonics . Igneous Petrotectonic Assemblages. Igneous Rocks and Tectonics. Igneous rock formation largely tied to tectonic activity 3 main tectonic settings for making igneous rocks:. Igneous Rocks at Divergent Margins: Mid-Ocean Ridges. Magma origin:

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Igneous Rocks and Plate Tectonics

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Igneous rocks and plate tectonics l.jpg

Igneous Rocks and Plate Tectonics

Igneous Petrotectonic Assemblages


Igneous rocks and tectonics l.jpg

Igneous Rocks and Tectonics

  • Igneous rock formation largely tied to tectonic activity

  • 3 main tectonic settings for making igneous rocks:


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Igneous Rocks at Divergent Margins: Mid-Ocean Ridges

  • Magma origin:

    • Partial melting of ultramafic, fertile mantle rock (lherzolite)

    • Decompression melting due to mantle convection

  • Magmatic processes:

    • Mantle melting produces dominantly “primitive,” mafic magma (basalt); magma may be modified slightly by fractional crystallization

  • Rocks & structures

    • “Ophiolite suite”

    • Cool and crystallize in magma chamber

    • Erupted onto sea floor as effusive lava flows or pillows


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Igneous Rocks at Divergent Margins: Mid-Ocean Ridges


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Igneous Rocks at Divergent Margins: Continental Rifts

  • Magma origin:

    • Partial melting of ultramafic mantle rocks

    • Decompression melting due to rise of material in a hot spot (?)

  • Magmatic processes:

    • Initial mantle melting produces “primitive,” mafic magma (basalt)

    • Magma accumulates in chambers

    • Fractional crystallization, contamination produce intermediate (andesite) and feslic (rhyolite) magmas

  • Rocks & structures

    • Highly varied

    • Dominated by bimodal volcanic activity

      • Early: basalt lava flows, shield volcanoes, cinder cones

      • Later: rhyolite flows and calderas (ignimbrites)

    • Also mafic through felsic plutons


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Ocean-Continent Plate convergence

Continental arcs

Continent-Continent Plate convergence

Igneous Rocks at Convergent Margins

  • Ocean-Ocean Plate convergence

    • Ocean Island arcs

“Recycled” magmas!


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Igneous Rocks at Convergent Margins: Ocean Island Arcs

  • Magma origin:

    • Water released from subducting slab due to T-P increase

    • Flux melting (partial) of overlying ultramafic mantle rocks

  • Magmatic processes:

    • Initial melting produces mafic magma (basalt)

    • Basalt magma rises into crust; fractional crystallization, melting and assimilation of crust modifies composition (intermediate – andesite)

  • Rocks & structures

    • Dominantly andesite stratovolcanoes (“Ring of Fire”)

    • Mafic to intermediate batholiths (Gabbro, diorite)


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Igneous Rocks at Convergent Margins: Continental Arcs

  • Magma origin:

    • Water released from subducting slab due to T-P increase

    • Flux melting (partial) of overlying ultramafic mantle rocks

  • Magmatic processes:

    • Initial melting produces mafic magma (basalt)

    • Basalt magma rises into crust; fractional crystallization, melting and assimilation of crust modifies composition (intermediate to felsic)

  • Rocks & structures

    • Dominantly andesite to rhyolite stratovolcanoes

    • Rhyolite calderas and ignimbrites (tuffs)

    • Vast intermediate to felsic batholiths (granodiorite, granite)


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Magma origin:

Crustal thickening due to thrust faulting during collision

Causes partial meting of crustal rocks (anatexis)

Magmatic processes:

Magma composition depends largely on the source rocks that were melted

Dominantly felsic composition

Igneous Rocks at Convergent Margins: Continental Collisions

  • Rocks & structures

    • Extensive batholiths and plutons of mostly felsic composition (granite)

    • Compression and thick crust prevent eruption – volcanoes rare


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Igneous Rocks at Hot Spots

  • Magma origin:

    • Partial melting of ultramafic mantle rocks

    • Decompression melting due to rise of hot material

  • Magmatic processes:

    • Initial mantle melting produces “primitive,” mafic magma (basalt)

    • Heat from mafic magma can melt overlying crustal rocks, producing felsic magmas

  • Rocks & structures

    • Initial flood basalts erupted from fissures (associated dikes)

    • Oceanic: basalt lava flows, shield volcanoes, cinder cones

    • Continental: bimodal (basalt-rhyolite) volcanic activity; rhyolite calderas and ignimbrites

    • Also mafic through felsic plutons


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