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Chapter 5-Igneous Rocks

Chapter 5-Igneous Rocks. Lecture notes- to be put in notebook. What are igneous rocks?. Temperature generally increases with depth in Earth’s crust. Pressure also increases with depth, a result of the weight of overlying rock. What are igneous rocks?.

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Chapter 5-Igneous Rocks

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  1. Chapter 5-Igneous Rocks Lecture notes- to be put in notebook

  2. What are igneous rocks? • Temperature generally increases with depth in Earth’s crust. • Pressure also increases with depth, a result of the weight of overlying rock.

  3. What are igneous rocks? • As pressure on rock increases, its melting point also increases. • Different minerals have different melting points, not all parts of a rock melt at the same time.

  4. Partial melting -Some minerals melt at low temperatures while other minerals remain solid. • As each group of minerals melts, different elements are added to the magma “stew”

  5. Fractional crystallization-When magma cools, it crystallizes in the reverse order of partial melting. • As each group of minerals crystallizes, it removes elements from the remaining magma instead of adding new elements.

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

  7. Granite

  8. Mineral composition- • Mafic rocks such as gabbro are dark-colored, have lower silica contents, and are rich in iron and magnesium. • contain plagiocide, biotite, amphibole, pyroxene, and olivine.

  9. Gabbro

  10. Diorite is a good example of an intermediate rock with moderate amounts of biotite, amphibole, and pyroxene.

  11. Diorite

  12. Ultramafic rocks-have low silica contents and very high levels of iron and magnesium • Peridotite and dunite • Formed by the crystallization of olivine and pyroxene

  13. Peridotite & Dunite

  14. Cooling rates- • Extrusive igneous rocks such as obsidian have no visible mineral grains (cooled quickly) • Intrusive igneous rocks such as gabbro may have crystals larger than 1 cm (cooled slowly)

  15. Cooling rates- Continued • Geologists make thin sections of minerals so that they can see the shapes of the grains • Interlocking edges form when the grains crystallize and grow together • During fractional crystallization, the minerals that form early in the process float in a liquid and have space to grow distinct shapes

  16. Porphyritic rocks have grains of two different sizes • Caused by slowly cooling magma which suddenly begins to cool rapidly

  17. The interlocking grain textures of igneous rocks help to give them strength. • Many of the minerals found in igneous rocks are resistant to weathering. • Granite is among the most durable of igneous rocks.

  18. Ore Deposits • Valuable ore deposits are often associated with igneous intrusions. • These ore deposits are found within igneous rock, such as the layered intrusions.

  19. Ore Deposits • Veins-Metallic elements are releasedat the end of magma crystallization. • Veins of extremely large-grained minerals are calledpegmatites.

  20. Diamonds are found in ultramafic rocks known as kimberlites. • Diamond can only form under very high pressures.

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