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Igneous Rocks

Igneous Rocks. Text ref. Ch5 (pg. 98). Contents. What are igneous rocks? Classifying igneous rocks. What are igneous rocks?. Text ref. ch.5.1 (pg 99-106). Objectives. Compare and contrast intrusive and extrusive igneous rocks Describe the composition of magma

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Igneous Rocks

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  1. Igneous Rocks Text ref. Ch5 (pg. 98)

  2. Contents • What are igneous rocks? • Classifying igneous rocks.

  3. What are igneous rocks? Text ref. ch.5.1 (pg 99-106)

  4. Objectives • Compare and contrast intrusive and extrusive igneous rocks • Describe the composition of magma • Discuss the factors that effect how rocks melt and crystallize

  5. Igneous Rocks • Formed from the crystallization of magma • Lava is magma the flows out onto the Earth’s surface. • Extrusive igneous rocks are fine grained and cool quickly on Earth’s surface (magma) • Intrusive igneous rocks are coarse grained and cool slowly beneath the Earth’s surface. (lava flows)

  6. Composition of Magma • Magma is often a slushy mix of molten rock, gases, and mineral crystals. • Magma is classified on its SiO2 content

  7. Factors that Affect Magma Formation • Temperature • Pressure • Water content • Mineral composition

  8. Temperature • Temperature gradient compares the internal temperature of the Earth with depth.

  9. Pressure • Pressure also increases with depth. • In general, as pressure on a rock increases, so does its melting temperature. • Ex. Rock sample may melt at 1000°C at Earth’s surface, but 1400°C at a depth of 100m.

  10. Water content • Also affects magma formation. • As water content increases, melting decreases

  11. Mineral content • Magma containing minerals such as olivine, calcium feldspar, and pyroxene melt at higher temperatures • In general oceanic crust tends to melt at higher temperatures than continental crust, due to its Fe content.

  12. How Rocks Melt • Rocks melt depending on their composition • Partial Melting • Fractional Crystallization

  13. Partial Melting • The process whereby some minerals melt at low temperatures while other minerals remain solid is called partial melting.

  14. Fractional Crystallization • The process wherein different minerals form at different temperatures is called fractional crystallization.

  15. Fe-Mg minerals

  16. Iron-rich minerals • Left branch of Bowen’s reaction series represents iron-rich minerals. • Undergo abrupt changes during fractional crystallization.

  17. Layered Intrusions • Minerals sometimes form in layers according to Bowen’s reaction series. • Known a layered intrusion • Important formations in mining: largest sources of platinum in U.S. found in layered intrusions in Montana

  18. Classifying Igneous Rocks Text ref. Ch.5.2, 107-113

  19. Objectives • Classify different types and textures of igneous rocks. • Recognize the effects of cooling rates on the grain sizes of igneous rocks. • Describe some uses of igneous rocks.

  20. Mineral Composition • Felsic rocks such as granite are light-colored, have high silica contents, and contain quartz and the feldspars orthoclase and plagioclase.

  21. Mineral Composition • Intermediate rocks such as diorite contain moderate amounts of biotite, amphibole, and pyroxene

  22. Mineral Composition • Mafic rocks such as gabbro are dark-colored, have lower silica contents, and are rich in iron and magnesium.

  23. Mineral Composition • Ultramafic rocks are unusual, containing peridotite, dunite, have low silica contents and very high levels of iron and magnesium.

  24. Grain Size • Igneous rocks differ in the size of the grains that compose them • Cooling rates will affect grain size • Fast cooling rocks will form smaller grains • Slow cooling rocks will form larger grains

  25. Texture • Distinct crystal shapes caused by fractional crystallization can be used to indentify igneous rocks • Porphyritic texture is characterized by large crystals surrounded by smaller crystals of the same or other minerals

  26. Igneous Rocks as Resources • Used as building materials • Granite

  27. Ore Deposits • Veins of gold, platinum, silver, and other precious metals can be found in igneous rock deposits.

  28. Pegmatites are veins that contain extremely large-grained crystals Pegmatite containing Amethyst

  29. Kimberlites • Diamond is found naturally occurring ultramafic rocks known as kimberlites. • Named after Kimberly, South Africa where they were first discovered • Form 150-300 km; intense pressures required for kimberlites to form

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