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Chapter 3: Igneous Rocks, Intrusive Activity, and the Origin of Igneous Rocks

| Chapter Introduction | Chapter Summary | Student Study Guide Quiz | Boxed Reading Summary | Terms to Remember | Expanding Your Knowledge | Exploring Resources | Interactive Quizzes | Interacting with Journey Through Geology CD-ROM | Journey Through Geology Web Quizzes | Learning Objectives | Matching | Art Labeling Exercises | Internet Exercises | Chapter Web Sites | Chapter Quizzes |

Learning Objectives

Chapter 3: Igneous Rocks, Intrusive Activity, and the Origin of Igneous Rocks

This chapter deals with what geologists call "hard-rock geology," which was for a long time the major focus of geologic science because of the association with mining and the search for precious metals. Now this area of geology includes scientific theories not dreamed of by the old "hard-rockers," who had no need for sophisticated explanations. "Gold is where you find it," they used to say. Modern geologists use complex techniques to search out how intrusive rocks originated, when they were emplaced, and why they occur in such amazing variety. Igneous rocks also yield tantalizing clues about ancient tectonic plate movements. The concepts to understand when studying this chapter are the following:

  1. Differences between intrusive and extrusive rocks
  2. Evidence that plutonic rocks solidified at depth from cooling magma
  3. Silicic or felsic rocks are predominant in continental crust; mafic rocks form most of the oceanic crust.
  4. How to distinguish mineralogically and chemically between granite, diorite, gabbro, and ultramafic rocks
  5. Chemistry of volcanic rocks: mafic rocks (e.g., basalt) relatively deficient in silica but enriched in magnesium, iron, and calcium oxides; silicic or felsic rocks (e.g., rhyolite) high in silica and enriched in potassium and sodium oxides; rocks of intermediate composition (e.g., andesite)
  6. Characteristics of intrusive structures—dikes, sills, plutons, batholiths, stocks
  7.  Sources of heat that may contribute to a rock's melting
  8. Pressure, water pressure, and mixtures of different minerals affect temperatures at which rocks melt or partially melt.
  9. Bowen's reaction series—its relationship to differentiation; how assimilation and partial melting may also account for variations in the composition of igneous rocks
  10. How plate tectonic theory accounts for the distribution of various types of igneous rocks and for the different kinds of intrusive phenomena and volcanic activity
  11. Although no mechanism is universally agreed upon to explain how different magmas evolve, plausible hypotheses include partial melting of basalt, assimilation of crustal rocks by mafic magma, partial melting of the lower crust, heating by magmatic underplating, partial melting of subducted sedimentary rocks, differentiation of the mantle, and combinations of these factors.
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