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Igneous Rocks

Explore the factors causing rocks to melt, including heat, pressure, water content, and geothermal gradients. Learn about decompression and wet melting processes. Discover how plate tectonics influence melting and the classification of igneous rocks based on composition and texture.

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Igneous Rocks

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  1. Igneous Rocks

  2. What causes rocks to melt? • The obvious answer is heat. • at Earth's surface, melting begins around 800oC • Melting temperature is also a function of pressure and water content. • as press. increases, melting temp. increases • as water content increases, melting temp. decreases

  3. What causes rocks to melt? • Geothermal gradient – increase in temperature with depth below surface. • rarely intersects melting line Fig. 4.18

  4. What causes rocks to melt? • Decompression melting – rocks rise rapidly without cooling much. Fig. 4.18

  5. What causes rocks to melt? • Wet melting – added water decreases melting temperature. Fig. 4.18

  6. Melting Animations

  7. Melting and Plate Tectonics

  8. Melting and Plate Tectonics • Divergent plate margins and hot spots – decompression melting of mantle. • Convergent plate margins – wet melting of mantle and crust. • water from dehydration metamorphism • Transform plate margins – little or no melting.

  9. Igneous Rock Classification • First classification criteria – chemical composition • based on cations and silica (SiO2) content Fig. 4.3

  10. Igneous Rock Classification • Felsic rocks • cations – sodium and potassium • high silica content (>65%) Fig. 4.3

  11. Igneous Rock Classification • Intermediate rocks: • cations – sodium, calcium, iron, magnesium • intermediate silica content (between 55-65%) Fig. 4.3

  12. Igneous Rock Classification • Mafic/ultramafic rocks: • cations –calcium, iron, magnesium • low silica content (<55%) Fig. 4.3

  13. Chemistry and Plate Tectonics • Divergent plate margins – mantle (iron/magnesium silicates) melts • mafic magma produced Fig. 12.9

  14. Chemistry and Plate Tectonics • Convergent plate margins – mantle and crust (Na, K, Ca silicates) melt • felsic and intermediate magma produced Fig. 12.21 Fig. 12.21

  15. Igneous Rock Classification • Second classification criteria – texture, determined by cooling history • Plutonic – cool slowly below the surface, large crystals • Volcanic – cool quickly at surface, small/no crystals

  16. Rhyolite – volcanic, felsic rock Igneous Rock Classification • Granite – plutonic, felsic rock

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