M04 TARB6927 09 SE C04 - The Eye · the origin of magma later in this chapter. Once formed, a magma body buoyantly rises toward the surface because it is less dense than the surrounding

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4Igneous Rocks

101

Two climbers onthe summit of aspire in the SierraNevada. (Photo byBrian Bailey/GettyImages)

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I gneous rocks and metamorphic rocks derived from igneous “parents” make up about95 percent of Earth’s crust. Furthermore, the mantle, which accounts for more than82 percent of Earth’s volume, is composed entirely of igneous rock. Thus, Earth can be

described as a huge mass of igneous rock covered with a thin veneer of sedimentary rockand having a relatively small iron-rich core.

tacular volcanic eruption. Magma that reaches Earth’s sur-face is called lava. Sometimes lava is emitted as fountainsthat are produced when escaping gasses propel molten rockfrom a magma chamber. On other occasions, magma is ex-plosively ejected from a vent, producing a catastrophiceruption. However, not all eruptions are violent; manyvolcanoes emit quiet outpourings of very fluid lava (Fig-ure 4.1).

Igneous rocks that form when molten rock solidifies atthe surface are classified as extrusive (ex = out, trudere =thrust) or volcanic (after the fire god Vulcan). Extrusive ig-neous rocks are abundant in western portions of theAmericas, including the volcanic cones of the CascadeRange and the extensive lava flows of the ColumbiaPlateau. In addition, many oceanic islands, typified by theHawaiian chain, are composed almost entirely of volcanicigneous rocks.

FIGURE 4.1 Fluid basaltic lava emitted from Hawaii’s Kilauea Volcano. (Photo by Philip Rosenberg/Pacific Stock)

Magma: The Parent Material of Igneous Rock

Igneous Rocks� Introduction

In our discussion of the rock cycle, it was pointed out thatigneous rocks form as molten rock cools andsolidifies. Abundant evidence supports the fact that the par-ent material for igneous rocks, called magma, is formed by aprocess called partial melting. Partial melting occurs at vari-ous levels within Earth’s crust and upper mantle to depthsof perhaps 250 kilometers (about 150 miles). We will explorethe origin of magma later in this chapter.

Once formed, a magma body buoyantly rises toward thesurface because it is less dense than the surrounding rocks.Occasionally molten rock breaks through, producing a spec-

1ignis = fire2


Magma: The Parent Material of Igneous Rock 103

Students Sometimes Ask . . .Are lava and magma the same thing?

No, but their composition might be similar. Both are terms thatdescribe molten or liquid rock: Magma exists beneath Earth’ssurface, and lava is molten rock that has reached the surface.That’s the reason why they can be similar in composition. Lavais produced from magma, but it generally has lost materials thatescape as a gas, such as water vapor.

Magma that loses its mobility before reaching the surfaceeventually crystallizes at depth. Igneous rocks that form atdepth are termed intrusive orplutonic (after Pluto, the god of the lower world in classicalmythology). Intrusive igneous rocks would never outcrop atthe surface if portions of the crust were not uplifted and theoverlying rocks stripped away by erosion. (When a mass ofcrustal rock is exposed—not covered with soil—it is calledan outcrop.) Exposures of intrusive igneous rocks occur inmany places, including Mount Washington, New Hamp-shire; Stone Mountain, Georgia; the Black Hills of SouthDakota; and Yosemite National Park, California (Figure 4.2).

The Nature of MagmaMagma is completely or partly molten material, which oncooling solidifies to form an igneous rock. Most magmasconsist of three distinct parts—a liquid component, a solidcomponent, and a gaseous phase.

The liquid portion, called melt, is composed of mobileions of those elements commonly found in Earth’s crust.Melt is made up mostly of ions of silicon and oxygen, alongwith lesser amounts of aluminum, potassium, calcium, sodi-um, iron, and magnesium.

The solid components (if any) in magma are silicate min-erals that have crystallized from the melt. As a magma bodycools, the size and number of crystals increases. During thelast stage of cooling, a magma body is mostly a crystallinesolid with only minor amounts of melt.

The gaseous components of magma, called volatiles, arematerials that will vaporize (form a gas) at surface pres-sures. The most common volatiles found in magma arewater vapor carbon dioxide and sulfur diox-ide which are confined by the immense pressure ex-erted by the overlying rocks. These gases tend to separatefrom magma as it moves toward the surface (low-pressureenvironment), where they may generate a steam eruption.Further, when deeply buried magma bodies crystallize, theremaining volatiles form hot, water-rich fluids that migratethrough the surrounding rocks. These hot fluids play an im-

1SO22,1CO22,1H2O2,

1in = into, trudere = thrust2

portant role in metamorphism and will be considered inChapter 8.

From Magma to Crystalline RockWhen magma is at its hottest, ions and groups of ions jointogether and break apart constantly. Then, as magma cools,the ions begin to move more slowly and eventually join to-gether into orderly crystalline structures. This process,called crystallization, generates various silicate mineralsthat reside within the remaining melt.

Before we examine how magma crystallizes, let us firstexamine how a simple crystalline solid melts. In any crys-talline solid, the ions are arranged in a closely packed regu-lar pattern. However, they are not without some motion.They exhibit a sort of restricted vibration about fixed points.As the temperature rises, the ions vibrate more rapidly andconsequently collide with ever-increasing vigor with theirneighbors. Thus, heating causes the ions to occupy morespace, which in turn causes the solid to expand. When theions are vibrating rapidly enough to overcome the force ofthe chemical bonds, the solid begins to melt. At this stagethe ions are able to slide past one another, and their orderlycrystalline structure disintegrates. Thus, melting converts asolid consisting of tight, uniformly packed ions into a liquidcomposed of unordered ions moving randomly about.

In the process of crystallization, cooling reverses theevents of melting. As the temperature of the liquid drops,the ions pack closer and closer together as they slow theirrate of movement. When cooled sufficiently, the forces of thechemical bonds will again confine the ions to an orderlycrystalline arrangement.

When magma cools, it is generally the silicon and oxygenatoms that link together first to form silicon–oxygen tetrahe-dra, the basic building blocks of the silicate minerals. As amagma continues to lose heat to its surroundings, the tetra-hedra join with each other and with other ions to form em-bryonic crystal nuclei. Slowly each nucleus grows as ionslose their mobility and join the crystalline network.

The earliest formed minerals have space to grow andtend to have better-developed crystal faces than do the laterones that fill the remaining space. Eventually all of the meltis transformed into a solid mass of interlocking silicate min-erals that we call an igneous rock (Figure 4.3).

FIGURE 4.2 Mount Rushmore National Memorial, located in the BlackHills of South Dakota, is carved from intrusive igneous rocks. (Photo byMarc Muench)


As you will see later, the crystallization of magma ismuch more complex than just described. Whereas a singlecompound, such as water, crystallizes at a specific tempera-ture, solidification of magma with its diverse chemistryspans a temperature range of 200°C, or more. During crys-tallization, the composition of the melt continually changesas ions are selectively removed and incorporated into theearliest formed minerals. If the melt should separate fromthe earliest formed minerals, its composition will be differ-ent from that of the original magma. Thus, a single magmamay generate rocks with widely differing compositions. Asa consequence, a great variety of igneous rocks exist. Wewill return to this important idea later in the chapter.

Although the crystallization of magma is complex, it isnevertheless possible to classify igneous rocks based ontheir mineral composition and the conditions under whichthey formed. Their environment during crystallization canbe roughly inferred from the size and arrangement of themineral grains, a property called texture. Consequently,igneous rocks are most often classified by their texture and miner-al composition. We will consider these two rock characteris-tics in the following sections.

Igneous Textures

Igneous Rocks� Igneous Textures

The term texture, when applied to an igneous rock, is usedto describe the overall appearance of the rock based on thesize, shape, and arrangement of its interlocking crystals(Figure 4.4). Texture is an important characteristic because itreveals a great deal about the environment in which the rockformed. This fact allows geologists to make inferences abouta rock’s origin while working in the field where sophisticat-ed equipment is not available.

Factors Affecting Crystal SizeThree 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 gener-alizations, this one has exceptions.

As a magma body loses heat to its surroundings, the mo-bility of its ions decreases. A very large magma body locatedat great depth will cool over a period of perhaps tens orhundreds of thousands of years. Initially, relatively fewcrystal nuclei form. Slow cooling permits ions to migratefreely until they eventually join one of the existing crys-talline structures. Consequently, slow cooling promotes thegrowth of fewer but larger crystals.

On the other hand, when cooling occurs more rapidly—for example, in a thin lava flow—the ions quickly lose theirmobility and readily combine to form crystals. This resultsin the development of numerous embryonic nuclei, all ofwhich compete for the available ions. The result is a solidmass of small intergrown crystals.

When molten material is quenched quickly, there maynot be sufficient time for the ions to arrange into an orderedcrystalline network. Rocks that consist of unordered ions arereferred to as glass.

Types of Igneous TexturesAs you saw, the effect of cooling on rock textures isfairly straightforward. Slow cooling promotes the growth oflarge crystals, whereas rapid cooling tends to generate small-er crystals. We will consider the other two factors affectingcrystal growth as we examine the major textural types.

Aphanitic (fine-grained) Texture Igneous rocks that form atthe surface or as small masses within the upper crust wherecooling is relatively rapid possess a very fine-grained tex-ture termed aphanitic By defi-nition, the crystals that make up aphanitic rocks are so smallthat individual minerals can only be distinguished with theaid of a microscope (Figure 4.4A). Because mineral identifi-cation is not possible, we commonly characterize fine-grained rocks as being light, intermediate, or dark in color.Using this system of grouping, light-colored aphanitic rocksare those containing primarily light-colored nonferromag-nesian silicate materials and so forth (see the section titled“Common Silicate Minerals” in Chapter 3).

Commonly seen in many aphanitic rocks are the voidsleft by gas bubbles that escape as lava solidifies. Thesespherical or elongated openings are called vesicles, and therocks that contain them are said to have a vesicular texture.Rocks that exhibit a vesicular texture usually form in theupper zone of a lava flow, where cooling occurs rapidlyenough to “freeze” the lava, thereby preserving the open-ings produced by the expanding gas bubbles (Figure 4.5).

Phaneritic (Coarse-Grained) Texture When large masses ofmagma slowly solidify far below the surface, they formigneous rocks that exhibit a coarse-grained texture described

1a = not, phaner = visible2.

A.

B.

FIGURE 4.3 A. Close-up of interlocking crystals in a coarse-grainedigneous rock. The largest crystals are about 2 centimeters in length. B. Photomicrograph of interlocking crystals in a coarse-grained igneousrock. (Photos by E. J. Tarbuck)

104


Igneous Textures 105

A. Aphanitic (fine-grained) C. Glassy (pumice)

B. Phaneritic (coarse-grained) D. Porphyritic

Intrusiveigneousrocks

Extrusiveigneousrocks

FIGURE 4.4 Igneous rock textures. A. Igneous rocks that form at or near Earth’s surface coolquickly and often exhibit a fine-grained (aphanitic) texture. B. Coarse-grained (phaneritic) igneousrocks form when magma slowly crystallizes at depth. C. During a volcanic eruption in which silica-rich lava is ejected into the atmosphere, a frothy glass called pumice may form. D. A porphyritictexture results when magma that already contains some large crystals migrates to a new locationwhere the rate of cooling increases. The resulting rock consists of larger crystals (phenocrysts)embedded within a matrix of smaller crystals (groundmass).(Photos courtesy of E. J. Tarbuck)

as phaneritic These coarse-grained rocksconsist of a mass of intergrown crystals, which are roughlyequal in size and large enough so that the individualminerals can be identified without the aid of a microscope(Figure 4.4B). (Geologists often use a small magnifyinglens to aid in identifying coarse-grained minerals.) Because

1phaner = visible2. phaneritic rocks form deep within Earth’s crust, their expo-sure at Earth’s surface results only after erosion removesthe overlying rocks that once surrounded the magmachamber.

Porphyritic Texture A large mass of magma located atdepth may require tens to hundreds of thousands of yearsto solidify. Because different minerals crystallize at differ-ent temperatures (as well as at differing rates), it is possi-ble for some crystals to become quite large before otherseven begin to form. If magma containing some large crys-tals should change environments—for example, by erupt-ing at the surface—the remaining liquid portion of the lavawould cool relatively quickly. The resulting rock, whichhas large crystals embedded in a matrix of smaller crystals,is said to have a porphyritic texture (Figure 4.4D). Thelarge crystals in such a rock are referred to as phenocrysts

whereas the matrix ofsmaller crystals is called groundmass. A rock with such atexture is termed a porphyry.

Glassy Texture During some volcanic eruptions, moltenrock is ejected into the atmosphere, where it is quenchedquickly. Rapid cooling of this type may generate rocks hav-ing a glassy texture (Figure 4.4C). As we indicated earlier,

1pheno = show, cryst = crystal2

FIGURE 4.5 Vesicular texture displayed on a freshly broken surface ofthe volcanic rock scoria. Vesicles are small holes left by escaping gasbubbles. (Photo by Michael Collier)


106 C H A P T E R 4 Igneous Rocks

glass results when unordered ions are “frozen” before theyare able to unite into an orderly crystalline structure.Obsidian, a common type of natural glass, is similar in ap-pearance to a dark chunk of manufactured glass (Figure4.6). Because of its excellent conchodial fracture and abilityto hold a sharp, hard edge, obsidian was a prized materialfrom which Native Americans chipped arrowheads and cut-ting tools (Figure 4.6 inset). Today, scalpels made from ob-sidian are being used for delicate plastic surgery becausethey leave less scarring than those made of steel.

Lava flows composed of obsidian a few hundred feetthick occur in some places (Figure 4.7). Thus, rapid coolingis not the only mechanism by which a glassy texture canform. As a general rule, magmas with a high silica contenttend to form long, chainlike structures before crystallizationis complete. These structures in turn impede ionic transportand increase the magma’s viscosity. (Viscosity is a measureof a fluid’s resistance to flow.)

Granitic magma, which is rich in silica, may be extruded asan extremely viscous mass that eventually solidifies to formobsidian. By contrast, basaltic magma, which is low in silica,forms very fluid lavas that upon cooling usually generate fine-grained crystalline rocks. However, the surface of basaltic lavamay be quenched rapidly enough to form a thin, glassy skin.Moreover, Hawaiian volcanoes sometimes generate lavafountains, which spray basaltic lava tens of meters into the air.Such activity can produce strands of volcanic glass calledPele’s hair, after the Hawaiian goddess of volcanoes.

Pyroclastic (Fragmental) Texture Some igneous rocks areformed from the consolidation of individual rock fragmentsthat are ejected during a violent volcanic eruption (Figure4.8). The ejected particles might be very fine ash, moltenblobs, or large angular blocks torn from the walls of the ventduring the eruption. Igneous rocks composed of these rock

fragments are said to have a pyroclastic or fragmental tex-ture (Figure 4.8 inset).

A common type of pyroclastic rock called welded tuff iscomposed of fine fragments of glass that remained hotenough during their flight to fuse together upon impact.Other pyroclastic rocks are composed of fragments that

FIGURE 4.6 Obsidian, a natural glass, was used by Native Americansfor making arrowheads and cutting tools. (Inset photo by Jeffrey Scovil)

FIGURE 4.7 This obsidian flow was extruded from a vent along thesouth wall of Newberry Caldera, Oregon. Note the road for scale. (Photoby Marli Miller)

FIGURE 4.8 Rocks that exhibit a pyroclastic texture are a result of theconsolidation of rock fragments that were ejected during a violentvolcanic eruption. (Photo by Steve Kaufman / DRK)


FIGURE 4.9 A granite pegmatite composed mainly of quartz andfeldspar (salmon color). The elongated, dark quartz crystal on the right isabout the size of a person’s index finger. (Photo by Colin Keates)

solidified before impact and became cemented together atsome later time. Because pyroclastic rocks are made of indi-vidual particles or fragments rather than interlocking crys-tals, their textures often appear to be more similar tosedimentary rocks than to other igneous rocks.

Pegmatitic Texture Under special conditions, exceptional-ly coarse-grained igneous rocks, called pegmatites, mayform. These rocks, which are composed of interlocking crys-tals all larger than a centimeter in diameter, are said to havea pegmatitic texture (Figure 4.9). Most pegmatites are foundaround the margins of large plutons as small masses or thinveins that commonly extend into the adjacent host rock.

Pegmatites form in the late stages of crystallization, whenwater and other volatiles, such as chlorine, fluorine, and sulfur,make up an unusually high percentage of the melt. Becauseion migration is enhanced in these fluid-rich environments, thecrystals that form are abnormally large. Thus, the large crystalsin pegmatites are not the result of inordinately long coolinghistories; rather, they are the consequence of the fluid-rich en-vironment that enhances crystallization.

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

Igneous Compositions

Igneous Rocks� Igneous Compositions

Igneous rocks are mainly composed of silicate minerals (seeBox 4.1). Furthermore, the mineral makeup of a particular ig-neous rock is ultimately determined by the chemical compo-sition of the magma from which it crystallizes. Recall thatmagma is composed largely of the eight elements that are the

major constituents of the silicate minerals. Chemical analysisshows that silicon and oxygen (usually expressed as the sili-ca content of a magma) are by far the most abundantconstituents of igneous rocks. These two elements, plus ionsof aluminum (Al), calcium (Ca), sodium (Na), potassium (K),magnesium (Mg), and iron (Fe), make up roughly 98 percentby weight of most magmas. In addition, magma containssmall amounts of many other elements, including titaniumand manganese, and trace amounts of much rarer elementssuch as gold, silver, and uranium.

As magma cools and solidifies, these elements combineto form two major groups of silicate minerals. The dark (orferromagnesian) silicates are rich in iron and/or magnesiumand comparatively low in silica. Olivine, pyroxene, amphibole,and biotite mica are the common dark silicate minerals ofEarth’s crust. By contrast, the light (or nonferromagnesian)silicates contain greater amounts of potassium, sodium, andcalcium rather than iron and magnesium. As a group, theseminerals are richer in silica than the dark silicates. The lightsilicates include quartz, muscovite mica, and the most abun-dant mineral group, the feldspars. The feldspars make up atleast 40 percent of most igneous rocks. Thus, in addition tofeldspar, igneous rocks contain some combination of theother light and/or dark silicates listed above.

Granitic (Felsic) versus Basaltic (Mafic)CompositionsDespite their great compositional diversity, igneous rocks(and the magmas from which they form) can be divided intobroad groups according to their proportions of light anddark minerals (Figure 4.10). Near one end of the continuumare rocks composed almost entirely of light-colored sili-cates—quartz and feldspar. Igneous rocks in which these arethe dominant minerals have a granitic composition. Geolo-gists also refer to granitic rocks as being felsic, a term de-rived from feldspar and silica (quartz). In addition to quartzand feldspar, most granitic rocks contain about 10 percentdark silicate minerals, usually biotite mica and amphibole.Granitic rocks are rich in silica (about 70 percent) and aremajor constituents of the continental crust.

Rocks that contain substantial dark silicate minerals andcalcium-rich plagioclase feldspar (but no quartz) are said tohave a basaltic composition (Figure 4.10). Because basalticrocks contain a high percentage of ferromagnesian minerals,geologists also refer to them as mafic (from magnesium andferrum, the Latin name for iron). Because of their iron content,mafic rocks are typically darker and denser than graniticrocks. Basaltic rocks make up the ocean floor as well as manyof the volcanic islands located within the ocean basins. Basaltalso forms extensive lava flows on the continents.

Other Compositional GroupsAs you can see in Figure 4.10, rocks with a composition be-tween granitic and basaltic rocks are said to have an inter-mediate, or andesitic composition after the common volcanic

[SiO2]

Igneous Compositions 107



Granite/Rhyolite

Potassiumfeldspar

Plagioclasefeldspar

Peridotite/Komatiite

Percentby

volume

80

60

40

20

100

Quartz

Sodium-rich

Calcium-rich

Pyroxene

OlivineBiotite

Amphibole

Muscovite

Composition

Rock types Gabbro/BasaltDiorite/Andesite

Felsic(Granitic)

Intermediate(Andesitic) Ultramafic Mafic

(Basaltic)

Increasing silica (SiO2)

Increasing iron, magnesium and calcium

Temperature at which melting begins

Increasing potassium and sodium

75% 40%

1200°C700°C

FIGURE 4.10 Mineralogy of common igneous rocks and the magmas from which they form. (AfterDietrich, Daily, and Larsen)

rock andesite. Intermediate rocks contain at least 25 percentdark silicate minerals, mainly amphibole, pyroxene, and bi-otite mica with the other dominant mineral being plagio-clase feldspar. This important category of igneous rocks isassociated with volcanic activity that is typically confined tothe margins of the continents.

Another important igneous rock, peridotite, containsmostly olivine and pyroxene and thus falls on the oppositeside of the compositional spectrum from granitic rocks(Figure 4.10). Because peridotite is composed almost entire-ly of ferromagnesian minerals, its chemical composition isreferred to as ultramafic. Although ultramafic rocks are rareat Earth’s surface, peridotite is believed to be the main con-stituent of the upper mantle.

Silica Content As an Indicator of CompositionAn important aspect of the chemical composition of igneousrocks is their silica content. Recall that silicon andoxygen are the two most abundant elements in igneousrocks. Typically, the silica content of crustal rocks rangesfrom a low of about 45 percent in ultramafic rocks to a high

1SiO22

of over 70 percent in granitic rocks (Figure 4.10). The per-centage of silica in igneous rocks actually varies in a system-atic manner that parallels the abundance of other elements.For example, rocks comparatively low in silica contain largeamounts of iron, magnesium, and calcium. By contrast,rocks high in silica contain very small amounts of those ele-ments but are enriched instead in sodium and potassium.Consequently, the chemical makeup of an igneous rock canbe inferred directly from its silica content.

Further, the amount of silica present in magma stronglyinfluences its behavior. Granitic magma, which has a highsilica content, is quite viscous (thick) and exists as a liquid attemperatures as low as 700°C. On the other hand, basalticmagmas are low in silica and are generally more fluid. Fur-ther, basaltic magmas crystallize at higher temperaturesthan granitic magmas and are completely solid when cooledto about 1000°C.

In summary, igneous rocks can be divided into broadgroups according to the proportions of light and dark min-erals they contain. Granitic (felsic) rocks, which are almostentirely composed of the lightcolored minerals quartz andfeldspar, are at one end of the compositional spectrum(Figure 4.10). Basaltic (mafic) rocks, which contain abundantdark silicate minerals in addition to plagioclase feldspar,


Igneous Compositions 109

Igneous rocks are classified on the basis oftheir mineral composition and texture.When analyzing specimens, geologists ex-amine them closely to identify the mineralspresent and to determine the size andarrangement of the interlocking crystals.When out in the field, geologists usemegascopic techniques to study rocks. Themegascopic characteristics of rocks are thosefeatures that can be determined with theunaided eye or by using a low-magnifica-tion (10X) hand lens. When practical to doso, geologists collect hand samples that canbe taken back to the laboratory where mi-croscopic, or high-magnification, methodscan be employed. Microscopic examina-tion is important to identify trace minerals,as well as those textural features that aretoo small to be visible to the unaided eye.

Because most rocks are not transparent,microscopic work requires the preparationof a very thin slice of rock known as a thinsection (Figure 4.A, part B). First, a saw con-taining diamonds embedded in its blade isused to cut a narrow slab from the sample.Next, one side of the slab is polished usinggrinding powder and then cemented to amicroscope slide. Once the mounted sam-ple is firmly in place, the other side of it isground to a thickness of about 0.03 mil-limeter. When a slice of rock is that thin, itis usually transparent. Nevertheless, somemetallic minerals, such as pyrite and mag-netite, remain opaque.

Once produced, thin sections are exam-ined under a specially designed micro-scope called a polarizing microscope. Such aninstrument has a light source beneath thestage so that light can be transmitted up-ward through the thin section. Becauseminerals have crystalline structures thatinfluence polarized light in a measurableway, this procedure allows for the identifi-cation of even the smallest components ofa rock. Part C of Figure 4.A is a photomi-crograph (photo taken through a micro-scope) of a thin section of granite shownunder polarized light. The mineral con-stituents are identified by their unique op-tical properties. In addition to assisting inthe study of igneous rocks, microscopictechniques are used with great success inanalyzing sedimentary and metamorphicrocks as well.

BOX 4.1 � UNDERSTANDING EARTH

Thin Sections and Rock Identification

A. Hand sample of granite

11 cm

B. Thin section

Quartz

Biotite

Feldspar

C. Photomicrograph taken with polarizedlight magnified about 27 times.

FIGURE 4.A Thin sections are very useful in identifying the mineral constituents inrocks. A. A slice of rock is cut from a hand sample using a diamond saw. B. This slice iscemented to a microscope slide and ground until it is transparent to light (about 0.03 mil-limeter thick). This very thin slice of rock is called a thin section. C. A thin section ofgranite viewed under polarized light. (Photos by E. J. Tarbuck)



make up the other major igneous rock group of Earth’scrust. Between these groups are rocks with an intermediate(andesitic) composition, while ultramafic rocks, which total-ly lack light-colored minerals, lie at the other end of thecompositional spectrum from granitic rocks.

Naming Igneous Rocks

Igneous Rocks� Naming Igneous Rocks

As was stated previously, igneous rocks are most often classi-fied, or grouped, on the basis of their texture and mineralcomposition (Figure 4.11). The various igneous textures resultmainly from different cooling histories, whereas the mineralcomposition of an igneous rock is the consequence of thechemical makeup of its parent magma. Because igneous rocksare classified on the basis of their mineral composition andtexture, two rocks may have similar mineral constituents buthave different textures and hence different names. For exam-ple, granite, a coarse-grained plutonic rock, has a fine-grainedvolcanic equivalent called rhyolite. Although these rocks aremineralogically the same, they have different cooling histo-ries and do not look at all alike (Figure 4.12).

Felsic (Granitic) Igneous RocksGranite Granite is perhaps the best known of all igneousrocks (Figure 4.12). This is partly because of its natural beau-ty, which is enhanced when it is polished, and partly because

0% to 25% 25% to 45% 45% to 85%Rock Color

(based on % of dark minerals)

Phaneritic(coarse-grained)

Aphanitic(fine-grained)

Porphyritic

Glassy

TEXTURE

“Porphyritic” precedes any of the above names whenever there areappreciable phenocrysts

Obsidian (compact glass)Pumice (frothy glass)

Peridotite

Uncommon

Diorite

Andesite

Granite

Rhyolite

QuartzPotassium feldspar

Sodium-richplagioclase feldspar

Gabbro

Basalt

ChemicalComposition

DominantMinerals

85% to 100%

Ultramafic

AmphiboleSodium- andcalcium-rich

plagioclase feldspar

PyroxeneCalcium-rich

plagioclase feldspar

OlivinePyroxene

Komatiite(rare)

AccessoryMinerals

Calcium-richplagioclase feldspar

PyroxeneBiotite

AmphiboleOlivine

AmphiboleMuscovite

Biotite

Pyroclastic(fragmental)

Tuff (fragments less than 2 mm)Volcanic Breccia (fragments greater than 2 mm)

Felsic(Granitic)

Intermediate(Andesitic)

Mafic(Basaltic)

FIGURE 4.11 Classification of the major igneous rock groups based on mineral composition andtexture. Coarse-grained rocks are plutonic, solidifying deep underground. Fine-grained rocks arevolcanic, or solidify as shallow, thin plutons. Ultramafic rocks are dark, dense rocks, composedalmost entirely of minerals containing iron and magnesium. Although relatively rare on Earth’ssurface, these rocks are major constituents of the upper mantle.

Students Sometimes Ask . . .I’ve heard some igneous rocks described as “granitic.” Is allgranitic rock really granite?

Technically, no. True granite is a coarse-grained intrusive rockwith a certain percentage of key minerals, mostly light-coloredquartz and feldspar, with other accessory dark minerals. How-ever, it has become common practice among geologists to applythe term granite to any coarse-grained intrusive rock composedpredominantly of light-colored silicate minerals. Further, somerocks are polished and sold as granites for use as countertops orfloor tile that, in addition to not being granite, are not even ig-neous rocks!


Phaneritic(course-grained)

Diorite

Andesite

Granite

Rhyolite

Gabbro

Basalt

Andesite porphyryGranite porphyry Basalt porphyry

Felsic(Granitic)

Intermediate(Andesitic)

Mafic(Basaltic)

Aphanitic(fine-grained)

Porphyritic

Texture Composition

FIGURE 4.12 Common igneous rocks. (Photos by E.J. Tarbuck)

of its abundance in the continental crust. Slabs of polishedgranite are commonly used for tombstones and monumentsand as building stones. Well-known areas in the UnitedStates where granite is quarried include Barre, Vermont;Mount Airy, North Carolina; and St. Cloud, Minnesota.

Granite is a phaneritic rock composed of about 25 percentquartz and roughly 65 percent feldspar, mostly potassium-and sodium-rich varieties. Quartz crystals, which are rough-ly spherical in shape, are often glassy and clear to light grayin color. By contrast, feldspar crystals are not as glassy, aregenerally white to gray or salmon pink in color, and exhibita rectangular rather than spherical shape (see Figure 4.3A).Other minor constituents of granite include muscovite andsome dark silicates, particularly biotite and amphibole. Al-though the dark components generally make up less than 10percent of most granites, dark minerals appear to be moreprominent than their percentage would indicate.

When potassium feldspar is dominant and dark pink incolor, granite appears reddish (Figure 4.12). This variety ofgranite is popular for monuments and building stone. How-ever, the feldspar grains are more often white to gray, so whenthey are mixed with lesser amounts of dark silicates, graniteappears light gray in color (Figure 4.13). In addition, somegranites have a porphyritic texture. These specimens containelongated feldspar crystals a few centimeters in length that arescattered among smaller crystals of quartz and amphibole.

Granite and other related crystalline rocks are often theby-products of mountain building. Because granite is veryresistant to weathering, it frequently forms the core of erod-ed mountains. For example, Pikes Peak in the Rockies,Mount Rushmore in the Black Hills, the White Mountains ofNew Hampshire, Stone Mountain in Georgia, and YosemiteNational Park in the Sierra Nevada are all areas where largequantities of granite are exposed at the surface (Figure 4.13).

Naming Igneous Rocks 111


FIGURE 4.13 Rocks contain information about the processes that produce them. This massivegranitic monolith (El Capitan) located in Yosemite National Park, California, was once a molten massfound deep within Earth. (Photo by Tim Fitzharris/Minden Pictures)

Granite is a very abundant rock. However, it has becomecommon practice among geologists to apply the term graniteto any coarse-grained intrusive rock composed predomi-nantly of light silicate materials that contain quartz. We willfollow this practice for the sake of simplicity. You shouldkeep in mind that this use of the term granite covers rockshaving a wide range of mineral compositions.

Rhyolite Rhyolite is the extrusive equivalent of graniteand, like granite, is composed essentially of the light-col-ored silicates (Figure 4.12). This fact accounts for its color,which is usually buff to pink or occasionally very light gray.Rhyolite is aphanitic and frequently contains glass frag-ments and voids, indicating rapid cooling in a surface envi-ronment. When rhyolite contains phenocrysts, they are

A. Large block of obsidian B. Hand sample of obsidian

FIGURE 4.14 Obsidian is a dark-colored, glassy rock formed from silica-rich lava. The scene inpart A shows an obsidian lava flow at Newberry Caldera, Oregon. (Photos courtesy of E. J. Tarbuck)

112


Naming Igneous Rocks 113

2 cm

2 cm

FIGURE 4.15 Pumice, a glassy rock, is very lightweight because itcontains numerous vesicles. (Inset photo by Chip Clark)

Intermediate (Andesitic) Igneous RocksAndesite Andesite is a medium-gray, fine-grained rock ofvolcanic origin. Its name comes from South America’s AndesMountains, where numerous volcanoes are composed of thisrock type. In addition to the volcanoes of the Andes, many ofthe volcanic structures occupying the continental marginsthat surround the Pacific Ocean are of andesitic composition.Andesite commonly exhibits a porphyritic texture (Figure4.12). When this is the case, the phenocrysts are often light,rectangular crystals of plagioclase feldspar or black, elongat-ed amphibole crystals. Andesite often resembles rhyolite, sotheir identification usually requires microscopic examinationto verify mineral makeup.

Diorite Diorite is the plutonic equivalent of andesite. It is acoarse-grained intrusive rock that looks somewhat similar togray granite. However, it can be distinguished from graniteby the absence of visible quartz crystals and because it con-tains a higher percentage of dark silicate minerals. The min-eral makeup of diorite is primarily sodium-rich plagioclasefeldspar and amphibole, with lesser amounts of biotite. Be-cause the light-colored feldspar grains and dark amphibolecrystals appear to be roughly equal in abundance, diorite hasa salt-and-pepper appearance (Figure 4.12).

Mafic (Basaltic) Igneous RocksBasalt Basalt is a very dark green to black fine-grainedvolcanic rock composed primarily of pyroxene and calcium-rich plagioclase feldspar, with lesser amounts of olivine andamphibole present (Figure 4.12). When porphyritic, basaltcommonly contains small light-colored feldspar pheno-crysts or green, glassy-appearing olivine phenocrysts em-bedded in a dark groundmass.

Basalt is the most common extrusive igneous rock. Manyvolcanic islands, such as the Hawaiian Islands and Iceland,are composed mainly of basalt. Further, the upper layers ofthe oceanic crust consist of basalt. In the United States, largeportions of central Oregon and Washington were the sites of

Students Sometimes Ask . . .You mentioned that Native Americans used obsidian formaking arrowheads and cutting tools. Is this the only mate-rial they used?

No. Native Americans used whatever materials were locallyavailable to make tools, including any hard compact rock mate-rial that could be shaped. This includes materials such as themetamorphic rocks slate and quartzite, sedimentary depositsmade of silica called jasper, chert, opal, flint, and even jade.Some of these deposits have a limited geographic distributionand can now help anthropologists to reconstruct trade routesbetween different groups of Indians.

small and composed of either quartz or potassium feldspar.In contrast to granite, which is widely distributed as largeplutonic masses, rhyolite deposits are less common andgenerally less voluminous. Yellowstone Park is one well-known exception. Here, rhyolite lava flows and thick ashdeposits of similar composition are extensive.

Obsidian Obsidian is a dark-colored glassy rock that usual-ly forms when silica-rich lava is quenched quickly (Figure4.14). In contrast to the orderly arrangement of ions charac-teristic of minerals, the ions in glass are unordered. Conse-quently, glassy rocks such as obsidian are not composed ofminerals in the same sense as most other rocks.

Although usually black or reddish-brown in color, obsid-ian has a composition that is more akin to light-colored ig-neous rocks such as granite, rather than to dark rocks suchas basalt. Obsidian’s dark color results from small amountsof metallic ions in an otherwise relatively clear, glassy sub-stance. If you examine a thin edge, obsidian will appearnearly transparent (see Figure 4.6).

Pumice Pumice is a volcanic rock with a glassy texture thatforms when large amounts of gas escape through silica-richlava to generate a gray, frothy mass (Figure 4.15). In somesamples, the voids are quite noticeable, whereas in othersthe pumice resembles fine shards of intertwined glass. Be-cause of the large percentage of voids, many samples ofpumice will float when placed in water. Oftentimes flowlines are visible in pumice, indicating that some movementoccurred before solidification was complete. Moreover,pumice and obsidian can often be found in the same rockmass, where they exist in alternating layers.



extensive basaltic outpourings (Figure 4.16). At some loca-tions these once fluid basaltic flows have accumulated tothicknesses approaching 3 kilometers.

Gabbro Gabbro is the intrusive equivalent of basalt (Fig-ure 4.12). Like basalt, it tends to be dark green to black incolor and composed primarily of pyroxene and calcium-richplagioclase feldspar. Although gabbro is not a common con-stituent of the continental crust, it undoubtedly makes up asignificant percentage of the oceanic crust.

Pyroclastic RocksPyroclastic rocks are composed of fragments ejected duringa volcanic eruption. One of the most common pyroclasticrocks, called tuff, is composed mainly of tiny ash-size frag-ments that were later cemented together (Figure 4.17). In

situations where the ash particles remainedhot enough to fuse, the rock is called weldedtuff. Although welded tuff consists mostly oftiny glass shards, it may contain walnut-sizepieces of pumice and other rock fragments.

Welded tuffs blanket vast portions of oncevolcanically active areas of the western UnitedStates. Some of these tuff deposits are hun-dreds of feet thick and extend for tens of milesfrom their source. Most formed millions ofyears ago as volcanic ash spewed from largevolcanic structures (calderas) in an avalanchestyle, spreading laterally at speeds approach-ing 100 kilometers per hour. Early investiga-tors of these deposits incorrectly classifiedthem as rhyolite lava flows. Today we knowthat silica-rich lava is too viscous (thick) toflow more than a few miles from a vent.

Pyroclastic rocks composed mainly of par-ticles larger than ash are called volcanic brec-cia. The particles in volcanic breccia canconsist of streamlined fragments that solidi-fied in air, blocks broken from the walls of thevent, crystals, and glass fragments.

Unlike most igneous rock names, such asgranite and basalt, the terms tuff and volcanic

breccia do not imply mineral composition. Thus, they are fre-quently used with a modifier, as, for example, rhyolite tuff.

Origin of MagmaConsiderable evidence indicates that most magma origi-nates in the uppermost mantle. It is also clear that plate tec-tonics plays a major role in generating most magma.The greatest quantities are produced at divergent plateboundaries in association with seafloor spreading, whereas

Students Sometimes Ask . . .At a store, I saw a barbecue grill with material the clerkcalled “lava rock.”Is this really a volcanic rock?

Not only is “lava rock” at your hardware store, it’s also athome-improvement stores for use as a building and landscap-ing material and is frequently found in aquarium-supply stores.Geologists call this material scoria, which is a red or dark maficrock characterized by a vesicular texture (full of holes). It’s alsocalled volcanic cinder. In gas barbecue grills, lava rock is used toabsorb and reradiate heat to ensure even cooking.

FIGURE 4.17 Outcrop of welded tuff (tan) interbedded with obsidian(black) near Shoshone, California. Tuff is composed mainly of ash-sizedparticles and may contain larger fragments of pumice or other volcanicrocks. (Photo by Breck P. Kent)

FIGURE 4.16 Basalt flows along the Columbia River above The Dalles, Oregon. Note thetrain for scale. (Photo by Michael Collier)


*We will consider the heat sources for the geothermal gradient in Chapter 12.

Temperature (°C)

Dep

th (k

m)

150

200

250

400 800 1200 1600 2000

50

100

0

0

Melting pointcurve

CompletemeltingGeothermal

gradient

Solidrock

Partial m

elting

Temperature (°C)

Dep

th (k

m)

0

10

20

30

400 600 800 1000 1200

Melting curve(wet granite)

Melting curve(dry granite)

Melting curve(dry basalt)

FIGURE 4.19 Idealized melting temperature curves. These curvesportray the minimum temperatures required to melt rock within Earth’scrust. Notice that dry granite and dry basalt melt at higher temperatureswith increasing depth. By contrast, the melting temperature of wet graniteactually decreases as the confining pressure increases.

lesser amounts form at subduction zones, where oceaniclithosphere descends into the mantle. Some igneous activityoccurs far from plate boundaries, indicating that not allmagma is produced in these relatively narrow zones.

Generating Magma from Solid RockBased on available scientific evidence, Earth’s crust and man-tle are composed primarily of solid, not molten, rock. Althoughthe outer core is a fluid, this iron-rich material is very denseand remains deep within Earth. So what is the source ofmagma that produces igneous activity?

Geologists conclude that most magma originates when es-sentially solid rock, located in the crust and upper mantle,melts. The most obvious way to generate magma from solidrock is to raise the temperature above the rock’s melting point.

Role of Heat What source of heat is sufficient to melt rock?Workers in underground mines know that temperatures gethigher as they go deeper. Although the rate of temperaturechange varies from place to place, it averages between 20°C and30°C per kilometer in the upper crust. This increase in temper-ature with depth is known as the geothermal gradient (Figure4.18). Estimates indicate that the temperature at a depth of

100 kilometers ranges between 1200°C and 1400°C*. At thesehigh temperatures, rocks in the upper mantle are near theirmelting points and in some tectonic settings partial meltingmay occur (Figure 4.18).

Role of Pressure If temperature were the only factor thatdetermined whether or not rock melts, our planet would bea molten ball covered with a thin, solid outer shell. This, ofcourse, is not the case. The reason is that pressure also in-creases with depth.

Melting, which is accompanied by an increase in volume,occurs at higher temperatures at depth because of greater con-fining pressure (Figure 4.19). Consequently, an increase inconfining pressure causes an increase in the rock’s meltingtemperature. Conversely, reducing confining pressure low-ers a rock’s melting temperature. When confining pressuredrops enough, decompression melting is triggered. Suchmelting occurs where mantle rock ascends in zones of con-vective upwelling, thereby moving into regions of lowerpressure. Decompression melting is responsible for gener-ating magma along divergent plate boundaries (oceanicridges) where plates are rifting apart (Figure 4.20). Belowthe ridge crest, hot mantle rock rises to replace the materialthat shifted horizontally away from the ridge axis. Decom-pression melting also occurs within ascending mantleplumes, such as the one responsible for the volcanic activitythat created the Hawaiian Islands.

Role of Volatiles Another important factor affecting themelting temperature of rock is its water content. Water andother volatiles act as salt does to melt ice. That is, volatilescause rock to melt at lower temperatures. Further, the effectof volatiles is magnified by increased pressure. Consequent-ly, “wet” rock buried at depth has a much lower melting

Origin of Magma 115

FIGURE 4.18 A highly schematic diagram illustrating a typical geothermalgradient (increase of temperature with depth) for the continental crust andupper mantle. Also illustrated is an idealized curve that depicts the meltingpoint temperatures for mantle rocks. Notice that the geothermal gradientjust crosses the partial melting curve for mantle material between depths ofabout 100 and 200 kilometers. In this zone the mantle is very near or at itsmelting temperature and in some tectonic settings partial melting occurs.At greater and shallower depths, however, the mantle should be completelysolid. Keep in mind that the geothermal gradient varies somewhat from onetectonic setting to another.



temperature than does “dry” rock of the same compositionand under the same confining pressure (Figure 4.19). There-fore, in addition to a rock’s composition, its temperature,depth (confining pressure), and water content determinewhether it exists as a solid or liquid.

Volatiles play an important role in generating magma atconvergent plate boundaries where cool slabs of oceanic lith-osphere descend into the mantle (Figure 4.21). As an oceanicplate sinks, both heat and pressure drive water from the sub-ducting crustal rocks. These volatiles, which are very mobile,migrate into the wedge of hot mantle that lies directly above.This process lowers the melting temperature of mantle rock

sufficiently to generate some melt. Laborato-ry studies have shown that the melting pointof basalt can be lowered by as much as 100°Cby the addition of only 0.1 percent water.

When enough mantle-derived basalticmagma forms, it will buoyantly rise towardthe surface. In a continental setting, basalticmagma may “pond” beneath crustal rocks,which have a lower density and are alreadynear their melting temperature. This may re-sult in some melting of the crust and the for-mation of a secondary, silica-rich magma.

In summary, magma can be generatedunder three sets of conditions: (1) Heat maybe added; for example, a magma body froma deeper source intrudes and melts crustalrock; (2) a decrease in pressure (without theaddition of heat) can result in decompressionmelting; and (3) the introduction of volatiles(principally water) can lower the meltingtemperature of mantle rock sufficiently togenerate magma.

How Magmas EvolveBecause a large variety of igneous rocks exists, it is logical to as-sume that an equally large variety of magmas must also exist.However, geologists have observed that a single volcano mayextrude lavas exhibiting quite different compositions (Figure4.22). Data of this type led them to examine the possibility thatmagma might change (evolve) and thus become the parent to avariety of igneous rocks. To explore this idea, a pioneering in-vestigation into the crystallization of magma was carried outby N. L. Bowen in the first quarter of the 20th century.

Bowen’s Reaction Series and the Composition of Igneous RocksRecall that ice freezes at a single tempera-ture, whereas magma crystallizes through atleast 200°C of cooling. In a laboratory settingBowen and his coworkers demonstrated thatas a basaltic magma cools, minerals tend tocrystallize in a systematic fashion based ontheir melting points. As shown in Figure4.23, the first mineral to crystallize from abasaltic magma is the ferromagnesian min-eral olivine. Further cooling generates cal-cium-rich plagioclase feldspar as well aspyroxene, and so forth down the diagram.

During the crystallization process, thecomposition of the liquid portion of themagma continually changes. For example, atthe stage when about a third of the magma

Waterdriven

from plate

Mantlerock melts

Subducting oceanic lithosphere

Oceanic crust

Trench

Continentalvolcanic arc

Continental crust

Asthenosphere

Continentallithosphere

FIGURE 4.21 As an oceanic plate descends into the mantle, water and other volatiles aredriven from the subducting crustal rocks. These volatiles lower the melting temperature ofmantle rock sufficiently to trigger melting.

Asthenosphere

Crust

Decompressionmelting

Upwellingmantle rocks

Lithosphere

Magmachamber

Ridge

FIGURE 4.20 As hot mantle rock ascends, it continually moves into zones of lowerpressure. This drop in confining pressure can trigger melting, even without additional heat.


has solidified, the melt will be nearly depleted of iron, mag-nesium, and calcium because these elements are constit-uents of the earliest-formed minerals. The removal of theseelements from the melt will cause it to become enriched insodium and potassium. Further, because the originalbasaltic magma contained about 50 percent silica thecrystallization of the earliest-formed mineral, olivine, whichis only about 40 percent silica, leaves the remaining meltricher in Thus, the silica component of the melt be-comes enriched as the magma evolves.

Bowen also demonstrated that if the solid componentsof a magma remain in contact with the remaining melt,they will chemically react and evolve into the next mineralin the sequence shown in Figure 4.23. For this reason, thisarrangement of minerals became known as Bowen’sreaction series (Box 4.2). As you will see, in some naturalsettings the earliest-formed minerals can be separated fromthe melt, thus halting any further chemical reaction.

1SiO22.

1SiO22,

TemperatureRegimes

Composition(rock types)

High temperature(~1200°C)

Low temperature(~750°C)

Olivine

Pyroxene

Amphibole

Biotite mica

Discontinuous Series

of Crystallization

Ultramafic(peridotite/komatiite)Calcium-

rich

Plag

iocl

ase

feld

spar

Con

tinuo

us S

erie

s

of C

ryst

alliz

atio

n

Potassium feldspar

Muscovite mica

Quartz

Sodium-rich

+

+

Mafic(gabbro/basalt)

Intermediate(diorite/andesite)

Felsic(granite/rhyolite)

Coo

ling

mag

ma

Bowen's Reaction Series

Eruptionof

Mt. Mazama

Silica-richrhyoliticmagma

Basalticmagma

rich in darksilicates

Magmachamber

FIGURE 4.23 Bowen’s reaction series shows the sequence in which mineralscrystallize from a magma. Compare this figure to the mineral composition of the rockgroups in Figure 4.11. Note that each rock group consists of minerals that crystallize inthe same temperature range.

FIGURE 4.22 Ash and pumice ejected during a large eruption of MountMazama (Crater Lake). Notice the gradation from light-colored, silica-rich ashnear the base to dark-colored rocks at the top. It is likely that prior to thiseruption the magma began to segregate as the less dense, silica-rich magmamigrated toward the top of the magma chamber. The zonation seen in therocks resulted because a sustained eruption tapped deeper and deeper levelsof the magma chamber. Thus, this rock sequence is an inverted representationof the compositional zonation in the magma body; that is, the magma from thetop of the chamber erupted first and is found at the base of these ash depositsand vice versa. (Photo by E. J. Tarbuck)

How Magmas Evolve 117



The diagram of Bowen’s reaction series in Figure 4.23 de-picts the sequence that minerals crystallize from a magma ofaverage composition under laboratory conditions. Evidencethat this highly idealized crystallization model approxi-mates what can happen in nature comes from the analysis ofigneous rocks. In particular, we find that minerals that formin the same general temperature regime on Bowen’s reac-tion series are found together in the same igneous rocks. Forexample, notice in Figure 4.23 that the minerals quartz,

potassium feldspar, and muscovite, which are located in thesame region of Bowen’s diagram, are typically found to-gether as major constituents of the plutonic igneous rockgranite.

Magmatic Differentiation Bowen demonstrated that min-erals crystallize from magma in a systematic fashion. Buthow do Bowen’s findings account for the great diversity ofigneous rocks? It has been shown that, at one or more stages

Although it is highly idealized, Bowen’sreaction series provides us with a visualrepresentation of the order in which miner-als crystallize from a magma of averagecomposition (see Figure 4.23). This modelassumes that the magma cools slowly atdepth in an otherwise unchanging en-vironment. Notice that Bowen’s reactionseries is divided into two branches—a discontinuous series and a continuousseries.

Discontinuous Reaction Series.The upper left branch of Bowen’s reactionseries indicates that as a magma cools,olivine is the first mineral to crystallize.Once formed, olivine will chemically reactwith the remaining melt to form the miner-al pyroxene (see Figure 4.23). In this re-action, olivine, which is composed ofindividual silicon–oxygen tetrahedra, in-corporates more silica into its structure,thereby linking its tetrahedra into single-chain structures of the mineral pyroxene.(Note: pyroxene has a lower crystallizationtemperature than olivine and is more sta-ble at lower temperatures.) As the magmabody cools further, the pyroxene crystalswill in turn react with the melt to generatethe double-chain structure of amphibole.This reaction will continue until the lastmineral in this series, biotite mica, crystal-lizes. In nature, these reactions do not usu-ally run to completion, so that variousamounts of each of the minerals in the se-ries may exist at any given time, and someminerals such as biotite may never form.

This branch of Bowen’s reaction seriesis called a discontinuous reaction series be-cause at each step a different silicate struc-ture emerges. Olivine, the first mineral inthe sequence, is composed of isolatedtetrahedra, whereas pyroxene is composedof single chains, amphibole of doublechains, and biotite of sheet structures.

Continuous Reaction Series.The right branch of the reaction series,called the continuous reaction series, illus-trates that calcium-rich plagioclase feld-spar crystals react with the sodium ions inthe melt to become progressively moresodium-rich (see Figure 4.23). Here thesodium ions diffuse into the feldspar crys-tals and displace the calcium ions in thecrystal lattice. Often, the rate of cooling oc-curs rapidly enough to prohibit a completereplacement of the calcium ions by sodiumions. In these instances, the feldspar crys-tals will have calcium-rich interiors sur-rounded by zones that are progressivelyricher in sodium (Figure 4.B).

During the last stage of crystallization,after much of the magma has solidified,

potassium feldspar forms. (Muscovite willform in pegmatites and other plutonicigneous rocks that crystallize at consider-able depth.) Finally, if the remaining melthas excess silica, the mineral quartz willform.

Testing Bowen’s Reaction Series.During an eruption of Hawaii’s Kilauea vol-cano in 1965, basaltic lava poured into a pitcrater, forming a lava lake that became a nat-ural laboratory for testing Bowen’s reactionseries. When the surface of the lava lakecooled enough to form a crust, geologistsdrilled into the magma and periodically re-moved samples that were quenched to pre-serve the melt, and minerals that weregrowing within it. By sampling the lava atsuccessive stages of cooling, a history ofcrystallization was recorded.

As Bowen’s reaction series predicts,olivine crystallized early but later ceasedto form, and was partly reabsorbed intothe cooling melt. (In a larger magma bodythat cooled more slowly, we would expectmost, if not all, of the olivine to react withthe melt and change to pyroxene.) Mostimportant, the melt changed compositionthroughout the course of crystallization. Incontrast to the original basaltic lava, whichcontained about 50 percent silica the final melt contained more than 75 per-cent silica and had a composition similar togranite.

Although the lava in this setting cooledrapidly compared to rates experienced in deep magma chambers, it was slowenough to verify that minerals do crystal-lize in a systematic fashion that roughlyparallels Bowen’s reaction series. Further,had the melt been separated at any stage inthe cooling process, it would have formeda rock with a composition much differentfrom the original lava.

1SiO22,

BOX 4.2 � UNDERSTANDING EARTH

A Closer Look at Bowen’s Reaction Series

FIGURE 4.B Photomicrograph of a zonedplagioclase feldspar crystal. After this crystal(composed of calcium-rich feldspar) solidified,further cooling resulted in sodium ions displac-ing calcium ions. Because replacement wasnot complete, this feldspar crystal has a calci-um-rich interior surrounded by zones that areprogressively richer in sodium. (Photo courtesyof E. J. Tarbuck)


How Magmas Evolve 119

during crystallization, a separation of the solid and liquidcomponents of a magma can occur. One example is calledcrystal settling. This process occurs when the earlier-formed minerals are denser (heavier) than the liquid portionand sink toward the bottom of the magma chamber, asshown in Figure 4.24. When the remaining melt solidifies—either in place or in another location if it migrates into frac-tures in the surrounding rocks—it will form a rock with achemical composition much different from the parentmagma (Figure 4.24). The formation of one or more second-ary magmas from a single parent magma is called magmaticdifferentiation.

A classic example of magmatic differentiation is found inthe Palisades Sill, which is a 300-meter-thick tabular mass ofdark igneous rock exposed along the west bank of the lowerHudson River. Because of its great thickness and subsequentslow rate of solidification, crystals of olivine (the first miner-al to form) sank and make up about 25 percent of the lowerportion of the Palisades Sill. By contrast, near the top of thisigneous body, where the last melt crystallized, olivine repre-sents only 1 percent of the rock mass.2

At any stage in the evolution of a magma, the solid and liq-uid components can separate into two chemically distinctunits. Further, magmatic differentiation within the secondarymelt can generate additional chemically distinct fractions.Consequently, magmatic differentiation and separation of thesolid and liquid components at various stages of crystalliza-tion can produce several chemically diverse magmas and ul-timately a variety of igneous rocks (Figure 4.24).

Assimiliation and Magma MixingBowen successfully demonstrated that through magmaticdifferentiation, a parent magma can generate several miner-alogically different igneous rocks. However, more recentwork indicates that magmatic differentiation cannot by it-self account for the entire compositional spectrum of ig-neous rocks.

Once a magma body forms, its composition can changethrough the incorporation of foreign material. For example,as magma migrates upward, it may incorporate some of thesurrounding host rock, a process called assimilation (Figure4.25). This process may operate in a near-surface environ-ment where rocks are brittle. As the magma pushes upward,stress causes numerous cracks in the overlying rock. Theforce of the injected magma is often sufficient to dislodgeblocks of “foreign” rock and incorporate them into themagma body. In deeper environments, the magma may behot enough to simply melt and assimilate some of the sur-rounding host rock, which is near its melting temperature.

Another means by which the composition of a magmabody can be altered is called magma mixing. This processoccurs whenever one magma body intrudes another (Figure4.25). Once combined, convective flow may stir the two

2Recent studies indicate that this igneous body was produced by multiple injections ofmagma and represents more than just a simple case of crystal settling.

Hostrock

Magmabody

A.

B.

C.

Crystallizationand settling

Crystallizationand settling

Time

Igneous activityproduces rocks havinga composition of the

initial magma

Crystallization andsettling changes thecomposition of the

remaining melt

Further magmaticdifferentiation results

in a more highlyevolved melt

FIGURE 4.24 Illustration of how a magma evolves as the earliest-formed minerals (those richer in iron, magnesium, and calcium) crystallizeand settle to the bottom of the magma chamber, leaving the remainingmelt richer in sodium, potassium, and silica A. Emplacement of amagma body and associated igneous activity generates rocks having acomposition similar to that of the initial magma. B. After a period of time,crystallization and settling change the composition of the melt, whilegenerating rocks having a composition quite different than the originalmagma. C. Further magmatic differentiation results in another more highlyevolved melt with its associated rock types.

1SiO22



series, the minerals have progressively high-er melting temperatures and that olivine,which is found at the top, has the highestmelting point. When a rock undergoes partialmelting, it will form a melt that is enriched inions from minerals with the lowest meltingtemperatures. The unmelted crystals arethose of minerals with higher melting tem-peratures. Separation of these two fractionswould yield a melt with a chemical composi-tion that is richer in silica and nearer to thegranitic end of the spectrum than the rockfrom which it was derived.

Formation of a Basaltic MagmaMost basaltic magmas probably originatefrom partial melting of the ultramafic rockperidotite, the major constituent of the uppermantle. Basaltic magmas that originate fromdirect melting of mantle rocks are calledprimary magmas because they have not yetevolved. Melting to produce these mantle-derived magmas may be triggered by areduction in confining pressure (decompres-sion melting). This can occur, for example,

where mantle rock ascends as part of slow-moving convec-tive flow at mid-ocean ridges (see Figure 4.20). Recall thatbasaltic magmas are also generated at subduction zones,where water driven from the descending slab of oceaniccrust promotes partial melting of mantle rocks (see Fig-ure 4.21).

Because most basaltic magma forms between about 50 and250 kilometers (30 and 150 miles) below the surface, we mightexpect that this material would cool and crystallize at depth.However, as basaltic magma migrates upward, the confiningpressure steadily diminishes and reduces its melting tempera-ture. As you will see in the next chapter, environments existwhere basaltic magmas ascend rapidly enough that the heatloss to the surrounding environment is offset by the drop inthe melting temperature. Consequently, large outpourings ofbasaltic lavas are common at Earth’s surface. In some situa-tions, however, basaltic magmas that are comparatively densewill pond beneath crustal rocks and crystallize at depth.

Formation of Andesitic and Granitic MagmasIf partial melting of mantle rocks generates basaltic mag-mas, what is the source of the magma that generates an-desitic and granitic rocks? Recall that intermediate andfelsic magmas are not erupted from volcanoes in the deep-ocean basins; rather, they are found only within, or adjacentto, the continental margins (Figure 4.26). This is strong evi-dence that interactions between mantle-derived basalticmagmas and more silica-rich components of Earth’s crustgenerate these magmas. For example, as a basaltic magma

Hostrock

Host rocksincorporatedinto magma

Earliest-formedminerals settles

Youngermagma body

intrudesolder one

Magma

A. Assimilation of country rock

C. Magma mixing

B. Crystallization and settling

FIGURE 4.25 This illustration shows three ways that the composition of a magma body maybe altered: magma mixing, assimilation of host rock, and crystallization and settling (magmaticdifferentiation).

magmas and generate a fluid with an intermediate composi-tion. Magma mixing may occur during the ascent of twochemically distinct magma bodies as the more buoyantmass overtakes the slower moving mass.

In summary, Bowen successfully demonstrated thatthrough magmatic differentiation, a single parent magma cangenerate several mineralogically different igneous rocks.Thus, this process, in concert with magma mixing and con-tamination by crustal rocks, accounts in part for the great di-versity of magmas and igneous rocks. We will next look atanother important process, partial melting, which also gen-erates magmas having varying compositions.

Partial Melting and MagmaCompositionRecall that the crystallization of a magma occurs over a tem-perature range of at least 200°C. As you might expect, melt-ing, the reverse process, spans a similar temperature range.As rock begins to melt, those minerals with the lowest melt-ing temperatures are the first to melt. Should melting con-tinue, minerals with higher melting points begin to melt andthe composition of the magma steadily approaches the over-all composition of the rock from which it was derived. Mostoften, however, melting is not complete. The incompletemelting of rocks is known as partial melting, a process thatproduces most, if not all, magma.

Notice in Figure 4.23 that rocks with a granitic composi-tion are composed of minerals with the lowest melting (crys-tallization) temperatures—namely, quartz and potassiumfeldspar. Also note that as we move up Bowen’s reaction


migrates upward, it may melt and assimilate some of thecrustal rocks through which it ascends. The result is the for-mation of a more silica-rich magma of andesitic composition(intermediate between basaltic and granitic).

Andesitic magma may also evolve from a basaltic magmaby the process of magmatic differentiation. Recall from ourdiscussion of Bowen’s reaction series that as basaltic magmasolidifies, it is the silica-poor ferromagnesian minerals thatcrystallize first. If these iron-rich components are separatedfrom the liquid by crystal settling, the remaining melt, nowenriched in silica, will have a composition more akin to an-desite. These evolved (changed) magmas are termed sec-ondary magmas.

Granitic rocks are found in much too large a quantity tobe generated solely from the magmatic differentiation ofprimary basaltic magmas. Most likely they are the end prod-uct of the crystallization of an andesitic magma, or the prod-uct of partial melting of silica-rich continental rocks. Theheat to melt crustal rocks often comes from hot mantle-

FIGURE 4.26 1996 eruption of Mount Ruapehu, Tongariro National Park, New Zealand. Volcanoesthat border the Pacific Ocean are fed largely by magmas that have intermediate or felsic compositions.These silica-rich magmas often erupt explosively, generating large plumes of volcanic dust and ash.(Photo by Tui De Roy/Minden Pictures)

derived basaltic magmas that formed above a subductingplate and were then emplaced within the crust.

Granitic melts are higher in silica and thus more viscous(thicker) than other magmas. Therefore, in contrast to ba-saltic magmas that frequently produce vast outpourings oflava, granitic magmas usually lose their mobility beforereaching the surface and tend to produce large plutonicstructures. On those occasions when silica-rich magmas doreach the surface, explosive pyroclastic eruptions, such asthose from Mount St. Helens, are the rule.

In summary, Bowen’s reaction series is a useful simpli-fied guide to understanding the partial melting process. Ingeneral, the low temperature minerals toward the bottom ofBowen’s reaction series melt first and produce magma thatis richer in silica (less mafic) than the parent rock. Thus, par-tial melting of ultramafic rocks in the mantle produces themafic basalts that form the oceanic crust. In addition, partialmelting of basaltic rocks will generate an intermediate (an-desitic) magma commonly associated with volcanic arcs.

Summary

� Igneous rocks form when magma cools and solidifies. Extru-sive, or volcanic, igneous rocks result when lava cools atthe surface. Magma that solidifies at depth produces in-trusive, or plutonic, igneous rocks.

� As magma cools, the ions that compose it arrange them-selves into orderly patterns during a process called crys-tallization. Slow cooling results in the formation of ratherlarge crystals. Conversely, when cooling occurs rapidly,

121



Review Questions

1. What is magma?2. How does lava differ from magma?3. How does the rate of cooling influence the crystalliza-

tion process?4. In addition to the rate of cooling, what two other factors

influence the crystallization process?5. The classification of igneous rocks is based largely on

two criteria. Name these criteria.6. The statements that follow relate to terms describing ig-

neous rock textures. For each statement, identify the ap-propriate term.a. openings produced by escaping gasesb. obsidian exhibits this texturec. a matrix of fine crystals surrounding phenocrystsd. crystals are too small to be seen without a microscopee. a texture characterized by two distinctly different

crystal sizesf. coarse-grained, with crystals of roughly equal sizeg. exceptionally large crystals exceeding 1 centimeter in

diameter

7. Why are the crystals in pegmatites so large?8. What does a porphyritic texture indicate about an ig-

neous rock?9. How are granite and rhyolite different? In what way are

they similar?10. Compare and contrast each of the following pairs of

rocks:a. granite and dioriteb. basalt and gabbroc. andesite and rhyolite

11. How do tuff and volcanic breccia differ from other ig-neous rocks such as granite and basalt?

12. What is the geothermal gradient?13. Describe the three conditions that are thought to cause

rock to melt.14. What is magmatic differentiation? How might this

process lead to the formation of several different ig-neous rocks from a single magma?

15. Relate the classification of igneous rocks to Bowen’s re-action series.

the outcome is a solid mass consisting of tiny intergrowncrystals. When molten material is quenched instantly, amass of unordered atoms, referred to as glass, forms.

� Igneous rocks are most often classified by their textureand mineral composition.

� The texture of an igneous rock refers to the overall ap-pearance of the rock based on the size and arrangementof its interlocking crystals. The most important factor af-fecting texture is the rate at which magma cools. Com-mon igneous rock textures include aphanitic, with grainstoo small to be distinguished without the aid of a mi-croscope; phaneritic, with intergrown crystals that areroughly equal in size and large enough to be identifiedwith the unaided eye; porphyritic, which has large crys-tals (phenocrysts) interbedded in a matrix of smaller crys-tals (groundmass); and glassy.

� The mineral composition of an igneous rock is the conse-quence of the chemical makeup of the parent magmaand the environment of crystallization. Igneous rocks aredivided into broad compositional groups based on thepercentage of dark and light silicate minerals they con-tain. Felsic rocks (e.g., granite and rhyolite) are composedmostly of the light-colored silicate minerals potassiumfeldspar and quartz. Rocks of intermediate composition,(e.g., andesite and diorite) contain plagioclase feldsparand amphibole. Mafic rocks (e.g., basalt and gabbro) con-tain abundant olivine, pyroxene, and calcium feldspar.They are high in iron, magnesium, and calcium, low insilicon, and are dark gray to black in color.

� The mineral makeup of an igneous rock is ultimately de-termined by the chemical composition of the magmafrom which it crystallizes. N. L. Bowen discovered that

as magma cools in the laboratory, those minerals withhigher melting points crystallize before minerals withlower melting points. Bowen’s reaction series illustratesthe sequence of mineral formation within magma.

� During the crystallization of magma, if the earlier-formed minerals are denser than the liquid portion, theywill settle to the bottom of the magma chamber during aprocess called crystal settling. Owing to the fact that crys-tal settling removes the earlier-formed minerals, theremaining melt will form a rock with a chemical compo-sition much different from the parent magma. Theprocess of developing more than one magma type from acommon magma is called magmatic differentiation.

� Once a magma body forms, its composition can changethrough the incorporation of foreign material, a processtermed assimilation, or by magma mixing.

� Magma originates from essentially solid rock of the crustand mantle. In addition to a rock’s composition, its tem-perature, depth (confining pressure), and water contentdetermine whether it exists as a solid or liquid. Thus,magma can be generated by raising a rock’s temperature,as occurs when a hot mantle plume “ponds” beneathcrustal rocks. A decrease in pressure can cause decom-pression melting. Further, the introduction of volatiles(water) can lower a rock’s melting point sufficiently togenerate magma. Because melting is generally not com-plete, a process called partial melting produces a melt madeof the lowest-melting-temperature minerals, which arehigher in silica than the original rock. Thus, magmasgenerated by partial melting are nearer to the felsic endof the compositional spectrum than are the rocks fromwhich they formed.


GEODe: Earth 123

16. What is partial melting?17. How does the composition of a melt produced by partial

melting compare with the composition of the parent rock?18. How are most basaltic magmas generated?

19. Basaltic magma forms at great depth. Why doesn’t mostof it crystallize as it rises through the relatively cool crust?

20. Why are rocks of intermediate (andesitic) and felsic(granitic) composition generally not found in the oceanbasins?

Key Terms

andesitic composition (p. 107)

aphanitic texture (p. 104)assimilation (p. 119)basaltic composition

(p. 107)Bowen’s reaction series

(p. 117)crystallization (p. 103)crystal settling (p. 119)decompression melting

(p. 115)extrusive (p. 102)

felsic (p. 107)fragmental texture (p. 106)geothermal gradient

(p. 115)glass (p. 104)glassy texture (p. 105)groundmass (p. 105)granitic composition

(p. 107)igneous rocks (p. 102)intermediate composition

(p. 107)intrusive (p. 103)

lava (p. 102)mafic (p. 107)magma (p. 102)magma mixing (p. 119)magmatic differentiation

(p. 119)melt (p. 103)partial melting (p. 120)pegmatite (p. 107)pegmatitic texture (p. 107)phaneritic texture (p. 105)phenocryst (p. 105)plutonic (p. 103)

porphyritic texture (p. 105)porphyry (p. 105)pyroclastic texture (p. 106)texture (p. 104)ultramafic (p. 108)vesicular texture (p. 104)volatiles (p. 103)volcanic (p. 102)

Web Resources

The Earth Website uses the resources and flexi-bility of the Internet to aid in your study of thetopics in this chapter. Written and developedby geology instructors, this site will help im-

prove your understanding of geology. Visit http://www.prenhall.com/ tarbuck and click on the cover of Earth 9e tofind:

• Online review quizzes.• Critical thinking exercises.• Links to chapter-specific Web resources.• Internet-wide key-term searches.

http://www.prenhall.com/tarbuck

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M04 TARB6927 09 SE C04 - The Eye · the origin of magma later in this chapter. Once formed, a magma body buoyantly rises toward the surface because it is less dense than the surrounding