Classification of Igneous Rocks

mClassification of igneous rocks is based on mineral composition, crystal size, and texture.

Real-World Reading Link Many statues, floors, buildings, and countertops have something in common. Many of them are made of the popular rock type granite—one of the most abundant rocks in Earth's crust.

Mineral Composition of Igneous Rocks

Igneous rocks are broadly classified as intrusive or extrusive. When magma cools and crystallizes below Earth's surface, intrusive rocks form. If the magma is injected into the surrounding rock, it is called an igneous intrusion. Crystals of intrusive rocks are generally large enough to see without magnification. Magma that cools and crystallizes on Earth's surface forms extrusive rocks. These are sometimes referred to as lava flows or flood basalts. The crystals that form in these rocks are small and difficult to see without magnification. Geologists classify these rocks by their mineral compositions. In addition, physical properties such as grain size and texture serve as clues for the identification of various igneous rocks.

Igneous rocks are classified according to their mineral compositions. Basaltic rocks, such as gabbro, are dark-colored, have lower silica contents, and contain mostly plagioclase and pyroxene. Granitic rocks, such as granite, are light-colored, have high silica contents, and contain mostly quartz, potassium feldspar, and plagioclase feldspar. Rocks that have a composition of minerals that is somewhere in between basaltic and granitic are called intermediate rocks. They consist mostly of plagioclase feldspar and hornblende. Diorite is a good example of an intermediate rock. Figure 5.8 shows examples from these three main compositional groups of igneous rocks. A fourth category, called ultrabasic, contains the rock peridotite. These rocks contain only iron-rich minerals such as olivine and pyroxene and are always dark. Figure 5.9 summarizes igneous rock identification.

Figure 5.8 Differences in magma composition can be observed in the rocks that form when the magma cools and crystallizes. Observe Describe the differences you see in these rocks.

Igneous Rocks And Lithium
118 Chapter 5 • Igneous Rocks

(l)Wally Eberhart/Visuals Unlimited, (c)E.R. Degginger/Photo Researchers, (r)Albert Copley/Visuals Unlimited

Igneous Rock Identification

Percent of basic minerals 0 15 45 85

Granitic (light) . Intermediate , Basaltic (dark)

Ultrabasic

Percent of basic minerals 0 15 45 85

Granitic (light) . Intermediate , Basaltic (dark)

Ultrabasic

Distribution Andesite Rock

Origin

Texture

Rock Names

Intrusive

coarse-grained

granite

diorite

gabbro

peridotite

porphyritic

porphyritic rhyolite or granite

porphyritic andesite or diorite

porphyritic basalt or gabbro

Extrusive

fine-grained

rhyolite

andesite

basalt

glassy

obsidian

vesicular

pumice

scoria (vesicular basalt)

Figure 5.9 Rock type can be determined by estimating the relative percentages of minerals in the rocks.

Texture

In addition to differences in their mineral compositions, igneous rocks differ in the sizes of their grains or crystals. Texture refers to the size, shape, and distribution of the crystals or grains that make up a rock. For example, as shown in Figure 5.10, the texture of rhy-olite can be described as fine-grained, while granite can be described as coarse-grained. The difference in crystal size can be explained by the fact that one rock is extrusive and the other is intrusive.

Extrusive Igneous Rocks Olivine

Figure 5.10 Rhyolite, granite, and obsidian have different textures because they formed in different ways.

Figure 5.10 Rhyolite, granite, and obsidian have different textures because they formed in different ways.

Obsidian Texture

Obsidian

Section 2 • Classification of Igneous Rocks 119

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Texture Classifying Rock

Pumice

Figure 5.11 Rock textures provide information about a rock's formation. Evidence of the rate of cooling and the presence or absence of dissolved gases is preserved in the rocks shown here.

Pumice

Figure 5.11 Rock textures provide information about a rock's formation. Evidence of the rate of cooling and the presence or absence of dissolved gases is preserved in the rocks shown here.

120 Chapter 5 • Igneous Rocks

(t)Albert J. Copley/Visuals Unlimited, (c)Jerome Wyckoff/Animals Animals, (b)Breck P. Kent/Animals Animals

Crystal size and cooling rates When lava flows on Earth's surface, it cools quickly and there is not enough time for large crystals to form. The resulting extrusive igneous rocks, such as basalt, which is shown in Figure 5.10, have crystals so small that they are difficult to see without magnification. Sometimes, cooling occurs so quickly that crystals do not form at all. The result is volcanic glass, such as obsidian, also shown in Figure 5.10. In contrast, when magma cools slowly beneath Earth's surface, there is sufficient time for large crystals to form. Thus, intrusive igneous rocks, such as granite, diorite, and gabbro, can have crystals larger than 1 cm.

Porphyritic rocks Look at the textures of the rocks shown in Figure 5.11. The top photo shows a rock with two different crystal sizes. This rock has a porphyritic (por fuh RIH tihk) texture, which is characterized by large, well-formed crystals surrounded by finer-grained crystals of the same mineral or different minerals.

What causes minerals to form both large and small crystals in the same rock? Porphyritic textures indicate a complex cooling history during which a slowly cooling magma suddenly began cooling rapidly. Imagine a magma body cooling slowly, deep in Earth's crust. As it cools, the first crystals to form grow large. If this magma were to be suddenly moved higher in the crust, or if it erupted onto Earth's surface, the remaining magma would cool quickly and form smaller crystals.

Vesicular rocks Magma contains dissolved gases that escape when the pressure on the magma lessens. If the lava is thick enough to prevent the gas bubbles from escaping, holes called vesicles are left behind. The rock that forms looks spongy. This spongy appearance is called vesicular texture. Pumice and vesicular basalt are examples shown in Figure 5.11.

^p Reading Check Explain what causes holes to form in igneous rocks.

Thin Sections

It is usually easier to observe the sizes of mineral grains than it is to identify the mineral. To identify minerals, geologists examine samples that are called thin sections. A thin section is a slice of rock, generally 2 cm X 4 cm and only 0.03 mm thick. Because it is so thin, light is able to pass through it.

Igneous Rock Classification

When viewed through a special microscope, called a petro-graphic microscope, mineral grains exhibit distinct properties. These properties allow geologists to identify the minerals present in the rock. For example, feldspar grains often show a distinct banding called twinning. Quartz grains might appear wavy as the microscope stage is rotated. Calcite crystals become dark, or extinguish, as the stage is rotated. Figure 5.12 shows the appearance of a thin section of granite under a petrographic microscope.

Igneous Rocks as Resources

^¡¡t The cooling and crystallization history of igneous rocks sometimes results in the formation of unusual but useful minerals. These minerals can be used in many fields, including construction, energy production, and jewelry making. Some of these uses are described in the following paragraphs.

Veins As you learned in Chapter 4, ores are minerals that contain a useful material that can be mined for a profit. Valuable ore deposits often occur within igneous intrusions. At other times, ore minerals are found in the rocks surrounding intrusions. These types of deposits sometimes occur as veins. Recall from Bowen's reaction series that the fluid left during magma crystallization contains high levels of silica and water. This fluid also contains any leftover elements that were not incorporated into the common igneous minerals. Some important metallic elements that are not included in common minerals are gold, silver, lead, and copper. These elements, along with the dissolved silica, are released at the end of magma crystallization in a hot, mineral-rich fluid that fills cracks and voids in the surrounding rock. This fluid solidifies to form metal-rich quartz veins, such as the gold-bearing veins in the Sierra Nevada. An example of gold formed in a quartz vein is shown in Figure 5.13.

^p Reading Check Explain why veins have high amounts of quartz.

Figure 5.13 Gold and quartz are extracted from mines together. The two are later separated. Infer What can you determine from this photo about the melting temperature of gold?

Figure 5.13 Gold and quartz are extracted from mines together. The two are later separated. Infer What can you determine from this photo about the melting temperature of gold?

Quartz Vein With Silver

Section 2 • Classification of Igneous Rocks 121

(tl)E. R. Degginger/Photo Researchers, (tr)Alfred Pasieka/Photo Researchers, (br)Breck P. Kent/Animals Animals

Section 2 • Classification of Igneous Rocks 121

(tl)E. R. Degginger/Photo Researchers, (tr)Alfred Pasieka/Photo Researchers, (br)Breck P. Kent/Animals Animals

Pegmatite Mount Rushmore
Figure 5.14 Pegmatite veins cut through much of the rock from which Mount Rushmore National Memorial is carved. You can see the veins running across Thomas Jefferson's face.

Pegmatites Vein deposits can contain other valuable resources in addition to metals. Veins of extremely large-grained minerals are called pegmatites. Ores of rare elements, such as lithium (Li) and beryllium (Be), form in pegmatites. In addition to ores, pegmatites can produce beautiful crystals. Because these veins fill cavities and fractures in rock, minerals grow into voids and retain their shapes. Some of the world's most beautiful minerals have been found in pegmatites. A famous pegmatite is the rock source for the Mount Rushmore National Memorial located near Keystone, South Dakota. A close-up view of President Thomas Jefferson, shown in Figure 5.14, reveals the huge mineral veins that run through the rock.

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Responses

  • peony
    What is the basis grain size scheme determine in thin section study of igneous rock?
    8 years ago
  • Venla
    What types of igneous rocks are at mount rushmore?
    8 years ago
  • august
    What rocks and minerals formed in mt Rushmore called?
    8 years ago
  • semira
    How to find the percentage of minerals in igneous rocks?
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  • jarkko
    What causes igneous rocks to form?
    8 years ago
  • alem
    How to describe an igneous rock as observed under microscopes?
    8 years ago
  • beatrice
    Can you walk though mount rushmore?
    7 years ago

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