Laboratory Manual for Introductory Geology 4e

crystalline rocks. Those that are shiny and contain no grains are called glasses.
Sponge-like masses are said to be porous, and those that appear to have pieces
cemented together are said to be fragmental. Each of these textures indicates a
unique cooling history—if you understand how to “read” the textural information.
Exercise 5.2 will help you start this process.
EXERCISE 5.2
A First Look at the Textures of Igneous Rocks
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We look first at igneous rocks that have the same chemical composition so that their different textures could only be
caused by the different ways in which they cooled. For example, the rocks in Figure 5.2 contain the same minerals and thus
have similar compositions. Separate the light-colored igneous rocks in your set. Describe their textures, paying careful
attention to the sizes and shapes of the grains and the relationships among adjacent grains. Use everyday language—the
appropriate geologic terms will be introduced later.
Specimen
Textural description
5.2.1 Grain Size in Crystalline Igneous Rock
Grain size is the key to understanding the cooling history of most igneous rocks that
solidified underground or on the surface (you will experiment with these processes
in Exercises 5.3 and 5.4). When magma or lava begins to cool, small crystal seeds
form (a process called nucleation), and crystals grow outward from each seed until
they interfere with one another. The result is the three-dimensional interlocking
texture found in most igneous rocks. But why do some igneous rocks have coarser
(larger) grains than others?
Crystals grow as ions migrate through magma to crystal seeds, so anything that
assists ionic migration increases grain size in an igneous rock. Cooling rate is the
most important factor controlling grain size. The slower a magma cools, the more
time ions have to migrate, and the larger crystals can grow; the faster it cools, the
less time there is, and the smaller the grains will be. Another factor is a magma’s
viscosity (its resistance to flow). The less viscous a magma is (i.e., the more fluid),
the easier it is for ions to migrate through it, and the larger the crystals can become.
Differences in cooling rate and viscosity cause some igneous rocks to have coarse
grains (sometimes called a phaneritic texture) and others to have fine grains (an
aphanitic texture). Extremely coarse-grained igneous rock, like that in FIGURE 5.2a,
are said to be pegmatitic. FIGURE 5.2a–c illustrates the difference between pegmatitic,
coarse, and fine grains. Some igneous rocks have grains of two different sizes,
one much larger than the other (FIG. 5.3). These rocks are said to have a porphyritic
texture. The larger grains are called phenocrysts and the smaller grains are called, collectively,
the rock’s groundmass.
114 CHAPTER 5 USING IGNEOUS ROCKS TO INTERPRET EARTH HISTORY