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Geosciences LibreTexts

8: Igneous Rocks

  • Page ID
    5591
  • Learning Objectives

    After completing this chapter, you should be able to:

    • Classify igneous rock types based on color, texture, and mafic color index.
    • Identify, when possible, the minerals present in an igneous rock.
    • Determine the cooling history of the igneous rock.

    • 8.1: Introduction
      All rocks found on the Earth are classified into one of three groups: igneous, sedimentary, or metamorphic. This rock classification is based on the origin of each of these rock types, or if you prefer, based on the rock-forming process that formed the rock. The focus of this chapter will be on igneous rocks, which are the only rocks that form from what was once a molten or liquid state. Therefore, igneous rocks are defined as those rock types that form by the cooling of magma or lava.
    • 8.2: Igneous Rock Origin
      It seems like a bad joke, but before any igneous rock can form, there must be molten material known as magma produced, which means that you must first have a rock to melt to make magma in order for it to cool and become an igneous rock. Which brings more questions: what rock melted to form the magma? Was there more than one rock type that melted to form that magma? Did the rocks completely melt? These are just a few questions that you should consider when studying the origin of igneous rocks.
    • 8.3: Igneous Rock Composition
      Often added to the Bowen’s reaction series diagram are the igneous rock classifications as well as example igneous rock names that are entirely dependent on the minerals that are found in them. For example, you can expect to find abundant olivine, and maybe a little pyroxene and a little Ca-rich plagioclase, in an ultramafic rock called peridotite or komatiite, or that pyroxene, plagioclase, and possibly some olivine or amphibole may be present in a mafic rock such as gabbro or basalt.
    • 8.4: Lab Exercise (Part A)
      Before attempting to answer the following questions, remove the eight rock samples from the Igneous Rocks bag in your HOL rock kit (samples I1 – I8) and place them on a clean sheet of white paper. Your samples should look identical to the samples in Figure 8.5.
    • 8.5: Igneous Rock Texture
      The classification of igneous rocks is based not just on composition, but also on texture. As mentioned earlier, texture refers to the features that we see in the rock such as the mineral sizes or the presence of glass, fragmented material, or vesicles (holes) in the igneous rock. We will cover mineral crystal sizes and vesicles in this section.
    • 8.6: Igneous Rock Formation: Plutonic vs Volcanic
      The different crystal sizes and the presence or absence of glass in an igneous rock are primarily controlled by the rate of magma cooling. Magmas that cool below the surface of the earth tend to cool slowly, as the surrounding rock acts as an insulator, not unlike a coffee thermos. Most of us are aware that even the most expensive coffee thermos does not prevent your coffee from cooling; it just slows the rate of cooling.
    • 8.7: Lab Exercise (Part B and C)
      For this exercise, you will determine the texture and rate of cooling for the igneous rock samples in your HOL kit. Refer to Figures 8.6 through 8.9 for guidance.
    • 8.8: Student Responses
      The following is a summary of the questions in this lab for ease in submitting answers online.

    Thumbnail: Granite is a common type of igneous rock and Half Dome at the eastern end of Yosemite Valley in Yosemite National Park, California is nearly fully composed of granite. Half Dome as viewed from Glacier Point, Yosemite National Park, California, United States. (CC BY-SA 3.0; David Iliff via Wikipedia).