Igneous Rocks What are igneous rocks? In your textbook, read about the nature of igneous rocks. Use each of the terms below just once to complete the following statements. extrusive igneous rock intrusive lava magma 1. Molten rock inside Earth s crust is called magma. 2. A(n) igneous rock is formed from the crystallization of magma. 3. Magma that flows out onto Earth s surface is called lava. 4. Fine-grained igneous rocks that cool quickly on Earth s surface are called extrusive igneous rocks. 5. Coarse-grained igneous rocks that cool slowly beneath Earth s surface are called intrusive igneous rocks. In your textbook, read about the composition and origins of magma. For each statement below, write true or false. true false true false false true 6. Magma is often a slushy mix of molten rock, gases, and mineral crystals. 7. The elements found in magma are quite different from those found in Earth s crust. 8. Silica is the most abundant compound found in magma. 9. Magmas are classified as intrusive or extrusive. 10. In the laboratory, rocks must be heated from 8000 C to 12 000 C before they melt. 11. Heat in the upper mantle and lower crust may come, in part, from the decay of radioactive elements. Study Guide for Content Mastery Chapter 5 Earth Science: Geology, the Environment, and the Universe 27
What are igneous rocks?, continued In your textbook, read about factors that affect magma formation. Use the diagram to answer the following questions. 0 Melting temperature ( C) 200 400 600 800 1000 1200 1400 1600 Increasing pressure/depth (km) 5 10 15 20 25 30 35 Solid Liquid Melting curve (dry granite) Solid Liquid Melting curve (dry basalt) 12. How does pressure affect the melting point of rock? As the pressure on a rock increases, its melting point increases. 13. Do all minerals have the same melting point? No; different minerals have different melting points. 14. How does temperature change with depth in Earth s crust? Temperature increases with depth in Earth s crust. 15. How does pressure change with depth, and why? Pressure increases with depth from the weight of overlying rock. In your textbook, read about how rocks melt. Use each of the terms below just once to complete the passage. elements fractional crystallization reverse magma melting points partial melting Because different minerals have different (16) melting points,not all parts of a rock melt at the same time. The process whereby some minerals melt at low temperatures while other minerals remain solid is called (17) partial melting.as each group of minerals melts, different (18) elements are added to the magma stew, changing its composition. When the magma cools, it crystallizes in the (19) reverse order of partial melting. The process wherein different minerals form at different temperatures is called (20) fractional.as each group of minerals crystallizes, it removes elements from crystallization the remaining (21) magma instead of adding new elements. 28 Chapter 5 Earth Science: Geology, the Environment, and the Universe Study Guide for Content Mastery
What are igneous rocks?, continued In your textbook, read about Bowen s reaction series. Label the diagram using either continuous reaction series or discontinuous reaction series. discontinuous 22. reaction series Simultaneous Crystallization Calcium-rich continuous 23. reaction series Mafic minerals Feldspar Sodium-rich Answer the following questions. Use the diagram to answer questions 24 and 25. 24. The first feldspars to form are rich in what mineral? calcium 25. The second feldspars to form are rich in what mineral? sodium 26. What causes a zoned crystal? When magma cools rapidly, the calcium-rich cores are unable to react completely with the magma, resulting in a zoned crystal with sodium-rich outer layers and calcium-rich cores. 27. How is quartz formed? At the end of magma crystallization, the remaining melt, which is enriched with silica and oxygen, finally crystallizes, forming quartz. Study Guide for Content Mastery Chapter 5 Earth Science: Geology, the Environment, and the Universe 29
What are igneous rocks?, continued In your textbook, read about the mineral composition of igneous rocks. Complete the table by filling in one of the following terms: felsic, mafic, intermediate,or ultramafic. Description 28. May be formed by fractional crystallization of olivine and pyroxene ultramafic 29. Contains moderate amounts of biotite, amphibole, and pyroxene intermediate 30. Light-colored, high silica content, contains quartz felsic 31. Contains plagioclase, biotite, amphibole, pyroxene, and olivine mafic 32. Peridotite and dunites are examples. ultramafic 33. Dark-colored, low silica content, rich in iron and magnesium mafic 34. Diorite in an example. intermediate 35. Gabbro is an example. mafic 36. Granite is an example. felsic 37. Low silica content, very high iron and magnesium content ultramafic Type of Igneous Rock In your textbook, read about the grain size of igneous rocks. Answer the following questions. 38. Does obsidian, a glassy rock, have a large grain size or a small grain size? Obsidian has a very small grain size. 39. Is obsidian an intrusive or extrusive igneous rock? How do you know? Obsidian is extrusive; its glassy texture without obvious grains probably formed as lava cooled quickly on Earth s surface. 40. How does the texture of gabbro compare to that of obsidian? Gabbro has large crystals, in contrast to obsidian s crystals, which are very small. 41. Is gabbro an intrusive or extrusive igneous rock? How do you know? Gabbro is intrusive; its large grains probably formed as magma cooled slowly below Earth s surface. 30 Chapter 5 Earth Science: Geology, the Environment, and the Universe Study Guide for Content Mastery
SECTION 5.2 Classifying Igneous Rocks In your textbook, read about classifying igneous rocks. For each item in Column A, write the letter of the matching item in Column B. c d b e f a Column A 1. Rock such as peridotite, which has low silica content and very high levels of iron and magnesium 2. Rock with two different-sized grains of the same mineral 3. Rock such as gabbro, which is dark-colored, has low silica content, and is rich in iron and magnesium. 4. Vein of extremely large-grained minerals 5. Rare type of ultramafic rock that can contain diamonds 6. Rock such as granite, which is light-colored and has high silica content Column B a. felsic b. mafic c. ultramafic d. porphyritic e. pegmatite f. kimberlite In your textbook, read about the texture of igneous rocks. Answer the following questions. 7. Why do geologists make thin sections? to observe the shapes of mineral grains 8. How do interlocking edges form in mineral grains? As the grains crystallize from magma, they grow together and form irregular edges. 9. Why can minerals that form early in fractional crystallization grow distinct crystal shapes? They have space in which to grow freely. 10. What does a rock with a porphyritic texture look like? Porphyritic rocks are characterized by large, well-formed crystals surrounded by finer-grained crystals. 11. How do porphyritic textures form? Porphyritic textures can form if a slowly cooling magma suddenly begins cooling rapidly. Study Guide for Content Mastery Chapter 5 Earth Science: Geology, the Environment, and the Universe 31
SECTION 5.2 Classifying Igneous Rocks, continued In your textbook, read about igneous rocks as resources. Circle the letter of the choice that best completes the statement or answers the question. 12. Igneous rocks are strong because of their a. temperature. c. water content. b. color. d. interlocking grain textures. 13. Which of the following is one of the most durable igneous rocks? a. granite c. marble b. sandstone d. limestone 14. Igneous rocks tend to be a. radioactive. c. resistant to weathering. b. full of gold. d. vulnerable to weathering. 15. Igneous intrusions often are associated with valuable a. radioactive elements. c. oil reservoirs. b. ore deposits. d. fossil deposits 16. Ore deposits sometimes are found as a(n) a. layered intrusion. c. obsidian deposit. b. extrusion. d. molten rock. 17. Metal-rich quartz veins are formed at the end of a. volcanic eruptions. c. magma crystallization b. radioactive decay. d. the cooling of Earth s crust. 18. What are pegmatites? a. veins of extremely large-grained minerals c. microscopic, interlocking crystal grains b. magmas of differing densities d. small volcanoes 19. What are kimberlites? a. felsic rocks c. intermediate rocks b. mafic rocks d. ultramafic rocks 20. Diamonds can form only a. under very low pressure. c. above ground. b. under very high pressure. d. near radioactive elements. 32 Chapter 5 Earth Science: Geology, the Environment, and the Universe Study Guide for Content Mastery