GEOLOGI TEKNIK

Modu l 5aIgneous Rocks

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Igneous Rocks Igneous rocks are records of the thermal history of Earth. Their origin is closely associated with the movement of tectonic plates, and they play an important role in the spreading of seafloor, the origin of mountains, and the evolution of continents. The best-known examples of igneous activity are volcanic eruptions, in which liquid rock material works its way to the surface and erupts from volcanic fissures and vents such as those shown above. Less obvious, although just as important, are the enormous volumes of liquid rock that never reach the surface but remain trapped in the crust, where they cool and solidify. Granite is the most common variety of this type of igneous rock and is typically exposed in eroded mountain belts and in the roots of ancient mountain systems now preserved in the shields.

Igneous rocks are found in many parts of the globe, but they are actually formed in a few relatively restricted settings. On the continents, for example, most igneous rocks form at convergent plate margins where intrusions of magma feed overlying volcanoes.

Igneous rocks (ignis = fire) form as molten rock cools and solidifies. Considerable evidence supports the idea that the parent material for igneous rocks, called magma, is formed by partial melting that occurs at various levels within Earths crust and upper mantle to depths of about 250 kilometers.

THE NATURE OF IGNEOUS ROCKS

Igneous rocks form from magmamolten rock material consisting of liquid, gas, and crystals. A wide variety of magma types exists, but important end members are (1) basaltic magma, which is typically very hot (from 900 to 1200C) and highly fluid, and (2) silicic magma, which is cooler (less than 850C) and highly viscous. This is because silicic magmas have lower temperatures and greater amounts of SiO2.

The term magma comes from the Greek word that means kneaded mixture, like a dough or paste. In its geologic application, it refers to hot, partially molten rock material. Most magmas are not entirely liquid but are a combination of liquid, solid, and gas. Crystals may make up a large portion of the mass, so a magma could be thought of more accurately as a slush, a liquid melt mixed with a mass of mineral crystals. Such a mixture has a consistency similar to that of freshly mixed concrete, slushy snow, or thick oatmeal. The movement of most magmas is slow and sluggish.

Like most fluids, magma is less dense than the solid from which it

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forms, and because of buoyancy, it tends to migrate upward through the mantle and crust. Magma can intrude into the overlying rock by injection into fractures, it can dome the overlying rock, or it can melt and assimilate the rock it invades. The rise of the magma may be halted where it comes to density equilibrium with the surrounding rocks or where the roof rocks are too strong to allow the magma to penetrate farther. Magma that solidifies below the surface forms intrusive rock. When magma reaches the surface without completely cooling and flows out over the landscape as lava, it forms extrusive rock.

Fig. 5a.1. Extrusive and Intrusive Igneous Rock

Magma eventually cools and crystallizes to form igneous rocks. Their environment during crystallization can be roughly inferred from the size and arrangement of the mineral grains, a property called texture. Consequently, igneous rocks are most often classified by their texture and mineral composition.

Igneous Rock Textures

Texture refers to a rocks appearance with respect to the size, shape, and arrangement of its grains or other constituents. Most (but not all) igneous rocks are crystalline; that is, they are made of interlocking crystals (of, for instance, quartz and feldspar). The most significant aspect of texture in igneous rocks is grain (or crystal) size.

Three factors contribute to the textures of igneous rocks: (1) the rate at which magma cools; (2) the amount of silica present; and (3) the amount of dissolved gases in the magma. Of these, the rate of cooling is the dominant factor, but like all generalizations, this one has exceptions.

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As a magma body loses heat to its surroundings, the mobility of its ions decreases. A very large magma body located at great depth will cool over a period of perhaps tens or hundreds of thousands of years. Initially, relatively few crystal nuclei form. Slow cooling permits ions to migrate freely until they eventually join one of the existing crystalline structures. Consequently, slow cooling promotes the growth of fewer but larger crystals.

On the other hand, when cooling occurs more rapidly for example, in a thin lava flowthe ions quickly lose their mobility and readily combine to form crystals. This results in the development of numerous embryonic nuclei, all of which compete for the available ions. The result is a solid mass of small intergrown crystals.

When molten material is quenched quickly, there may not be sufficient time for the ions to arrange into an ordered crystalline network. Rocks that consist of unordered ions are referred to as glass.

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Fig. 5a.2. Igneous rock textures.

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Types of Igneous Textures

As you saw, the effect of cooling on rock textures is fairly straightforward. Slow cooling promotes the growth of large crystals, whereas rapid cooling tends to generate smaller crystals. We will consider the other two factors affecting crystal growth as we examine the major textural types.

Aphanitic (fine-grained) Texture . Igneous rocks that form at the surface or as small masses within the upper crust where cooling is relatively rapid possess a very fine-grained texture termed aphanitic By definition, the crystals that make up aphanitic rocks are so small that individual minerals can only be distinguished with the aid of a microscope. Because mineral identification is not possible, we commonly characterize finegrained rocks as being light, intermediate, or dark in color.

Commonly seen in many aphanitic rocks are the voids left by gas bubbles that escape as lava solidifies. These spherical or elongated openings are called vesicles, and the rocks that contain them are said to have a vesicular texture.

Rocks that exhibit a vesicular texture usually form in the upper zone of a lava flow, where cooling occurs rapidly enough to freeze the lava, thereby preserving the openings produced by the expanding gas bubbles.

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Fig. 5a.3. Vesicular texture displayed on a freshly broken surface of the volcanic rock

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Phaneritic (Coarse-Grained) Texture . When large masses of magma slowly solidify far below the surface, they form igneous rocks that exhibit a coarse-grained texture described as phaneritic (phaner = visible). These coarse-grained rocks consist of a mass of intergrown crystals, which are roughly equal in size and large enough so that the individual minerals can be identified without the aid of a microscope. (Geologists often use a small magnifying lens to aid in identifying coarse-grained minerals.) Because phaneritic rocks form deep within Earths crust, their exposure at Earths surface results only after erosion removes the overlying rocks that once surrounded the magma chamber.

Porphyritic Texture . A large mass of magma located at depth may require tens to hundreds of thousands of years to solidify. Because different minerals crystallize at different temperatures (as well as at differing rates), it is possible for some crystals to become quite large before others even begin to form. If magma containing some large crystals should change environmentsfor example, by erupting at the surfacethe remaining liquid portion of the lava would cool relatively quickly. The resulting rock, which has large crystals embedded in a matrix of smaller crystals, is said to have a porphyritic texture. The large crystals in such a rock are referred to as phenocrysts whereas the matrix of smaller crystals is called groundmass. A rock with such a texture is termed a porphyry.

Glassy Texture . During some volcanic eruptions, molten rock is ejected into the atmosphere, where it is quenched quickly. Rapid cooling of this type may generate rocks having a glassy texture. As we indicated earlier, Obsidian, a common type of natural glass, is similar in appearance to a dark chunk of manufactured glass. Because of its excellent conchodial fracture and ability to hold a sharp, hard edge, obsidian was a prized material from which prehistoric human chipped arrowheads and cutting tools.

Fig. 5a.4. Obsidian, a natural glass, was used for making

arrowheads and cutting tools.

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Granitic magma, which is rich in silica, may be extruded as an extremely viscous mass that eventually solidifies to form obsidian. By contrast, basaltic magma, which is low in silica, forms very fluid lavas that upon cooling usually generate finegrained crystalline rocks. However, the surface of basaltic lava may be quenched rapidly enough to form a thin, glassy skin.

Pyroclastic (Fragmental) Texture . Some igneous rocks are formed from the consolidation of individual rock fragments that are ejected during a violent volcanic eruption. The ejected particles might be very fine ash, molten blobs, or large angular blocks torn from the walls of the vent during the eruption. Igneous rocks composed of these rock fragments are said to have a pyroclastic or fragmental texture. Because pyroclastic rocks are made of individual particles or fragments rather than interlocking crystals, their textures often appear to be more similar to sedimentary rocks than to other igneous rocks.

Fig. 5a.5. Rocks that exhibit a pyroclastic texture are a result of the consolidation of rock fragments that were ejected during a violent

volcanic eruption.

Pegmatitic Texture . Under special conditions, exceptionally coarse-grained igneous rocks, called pegmatites, may form. These rocks, which are composed of interlocking crystals all larger than a

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centimeter in diameter, are said to have a pegmatitic texture. Most pegmatites are found around the margins of large plutons as small masses or thin veins that commonly extend into the adjacent host rock.

Pegmatites form in the late stages of crystallization, when water and other volatiles, such as chlorine, fluorine, and sulfur, make up an unusually high percentage of the melt. Because ion migration is enhanced in these fluid-rich environments, the crystals that form are abnormally large. Thus, the large crystals in pegmatites are not the result of inordinately long cooling histories; rather, they are the consequence of the fluid-rich environment that enhances crystallization.

The composition of most pegmatites is similar to that of granite. Thus, pegmatites contain large crystals of quartz, feldspar, and muscovite. However, some contain significant quantities of comparatively rare and hence valuable minerals

Igneous Rock Compositions

Igneous rocks are mainly composed of silicate minerals. Furthermore, the mineral makeup of a particular igneous rock is ultimately determined by the chemical composition of the magma from which it crystallizes. Recall that magma is composed largely of the eight elements that are the major constituents of the silicate minerals. Chemical analysis shows that silicon and oxygen (usually expressed as the silica [SiO2] content of a magma) are by far the most abundant constituents of igneous rocks. These two elements, plus ions of aluminum (Al), calcium (Ca), sodium (Na), potassium (K), magnesium (Mg), and iron (Fe), make up roughly 98 percent by weight of most magmas. In addition, magma contains small amounts of many other elements, including titanium and manganese, and trace amounts of much rarer elements such as gold, silver, and uranium.

As magma cools and solidifies, these elements combine to form two major groups of silicate minerals. The dark (or ferromagnesian) silicates are rich in iron and/or magnesium and comparatively low in silica. Olivine, pyroxene, amphibole, and biotite mica are the common dark silicate minerals of Earths crust. By contrast, the light (or nonferromagnesian) silicates contain greater amounts of potassium, sodium, and calcium rather than iron and magnesium. As a group, these minerals are richer in silica than the dark silicates. The light silicates include quartz, muscovite mica, and the most abundant mineral group, the feldspars. The feldspars make up at least 40 percent of most igneous rocks. Thus, in addition to feldspar, igneous

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rocks contain some combination of the other light and/or dark silicates listed above.

Granitic (Felsic) versus Basaltic (Mafic) Compositions

Despite their great compositional diversity, igneous rocks (and the magmas from which they form) can be divided into broad groups according to their proportions of light and dark minerals. Near one end of the continuum are rocks composed almost entirely of light-colored silicatesquartz and feldspar. Igneous rocks in which these are the dominant minerals have a granitic composition. Geologists also refer to granitic rocks as being felsic, a term derived from feldspar and silica (quartz). In addition to quartz and feldspar, most granitic rocks contain about 10 percent dark silicate minerals, usually biotite mica and amphibole.

Granitic rocks are rich in silica (about 70 percent) and are major constituents of the continental crust. Rocks that contain substantial dark silicate minerals and calcium-rich plagioclase feldspar (but no quartz) are said to have a basaltic composition. Because basaltic rocks contain a high percentage of ferromagnesian minerals, geologists also refer to them as mafic (from magnesium and ferrum, the Latin name for iron). Because of their iron content, mafic rocks are typically darker and denser than granitic rocks. Basaltic rocks make up the ocean floor as well as many of the volcanic islands located within the ocean basins. Basalt also forms extensive lava flows on the continents.

Other Compositional Groups

As you can see in Figure 5a.6, rocks with a composition between granitic and basaltic rocks are said to have an intermediate, or andesitic composition after the common volcanic rock andesite. Intermediate rocks contain at least 25 percent dark silicate minerals, mainly amphibole, pyroxene, and biotite mica with the other dominant mineral being plagioclase feldspar. This important category of igneous rocks is associated with volcanic activity that is typically confined to the margins of the continents.

Another important igneous rock, peridotite, contains mostly olivine and pyroxene and thus falls on the opposite side of the compositional spectrum from granitic rocks. Because peridotite is composed almost entirely of ferromagnesian minerals, its chemical composition is referred to as ultramafic. Although ultramafic rocks are rare at Earths surface, peridotite is believed to be the main constituent of the upper mantle.

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Fig. 5a.6. Mineralogy of common igneous rocks and the magmas from which they form

Silica Content As an Indicator of Composition

An important aspect of the chemical composition of igneous rocks is their silica content. Recall that silicon and oxygen are the two most abundant elements in igneous rocks. Typically, the silica content of crustal rocks ranges from a low of about 45 percent in ultramafic rocks to a high of over 70 percent in granitic rocks. The percentage of silica in igneous rocks actually varies in a systematic manner that parallels the abundance of other elements. For example, rocks comparatively low in silica contain large amounts of iron, magnesium, and calcium. By contrast, rocks high in silica contain very small amounts of those elements but are enriched instead in sodium and potassium. Consequently, the chemical makeup of an igneous rock can be inferred directly from its silica content.

In summary, igneous rocks can be divided into broad groups according to the proportions of light and dark minerals they contain. Granitic (felsic) rocks, which are almost entirely composed of the lightcolored minerals quartz and feldspar, are at one end of the

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compositional spectrum. Basaltic (mafic) rocks, which contain abundant dark silicate minerals in addition to plagioclase feldspar make up the other major igneous rock group of Earths crust. Between these groups are rocks with an intermediate (andesitic) composition, while ultramafic rocks, which totally lack light-colored minerals, lie at the other end of the compositional spectrum from granitic rocks.

Fig. 5a.7 Classification of the major igneous rock groups based on mineral composition and texture. Coarse-grained rocks are plutonic,

solidifying deep underground. Fine-grained rocks are volcanic, or solidify as shallow, thin plutons. Ultramafic rocks are dark, dense rocks, composed almost entirely of minerals containing iron and

magnesium. Although relatively rare on Earths surface, these rocks are major constituents of the upper mantle.

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Fig. 5a.8 Common igneous rocks.

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Daftar Bacaan

Carlson, Diane H. (2010). Physical geology: earth revealed. New York: The McGraw-Hill Companies, Inc.

Tarbuck, Edward J. (2008). EARTH An Introduction to Physical Geology. NJ: Pearson Education, Inc.

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