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Igneous Rocks

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

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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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