igneous rocks and intrusive igneous activity

Igneous Rocks and Intrusive Igneous Activity

This mass of granitic rock in Yosemite National Park in California is calledEl Capitan, meaning “The Chief.” It is part of the Sierra Nevadabatholith, a huge pluton mesuring 640 km long and 110 km wide. Thisnear-vertical cliff rises more than 900 m above the valley floor, making itthe highest unbroken cliff in the world.

4 a With few exceptions, magma is com-posed of silicon and oxygen withlesser amounts of several otherchemical elements.

a Temperature and especially compo-sition are the most important con-trols on the mobility of magma andlava.

a Most magma originates withinEarth’s upper mantle or lower crustat or near divergent and convergentplate boundaries.

a Several processes bring about chem-ical changes in magma, so magmamay evolve from one kind intoanother.

a All igneous rocks form when magmaor lava cools and crystallizes or bythe consolidation of pyroclasticmaterials ejected during explosiveeruptions.

a Geologists use texture and composi-tion to classify igneous rocks.

a Intrusive igneous bodies called plu-tons form when magma cools belowEarth’s surface. The origin of thelargest plutons is not fully understood.

1 Introduction

1 The Properties and Behavior of Magmaand Lava

1 How Does Magma Originate and Change?

1 Igneous Rocks—Their Characteristics andClassification

1 Plutons—Their Characteristics and Origins

GEO-FOCUS 4.1: Some RemarkableVolcanic Necks

1 How Are Batholiths Intruded into Earth’sCrust?

1 Geo-Recap

C H A P T E R 4

O U T L I N EO B J E C T I V E S

At the end of this chapter, you will have learned that:

87

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88 CHAPTER 4 igneous rocks and intrusive igneous activity

Introductione mentioned that the term rockapplies to a solid aggregate of one ormore minerals as well as to mineral-like matter such as natural glass and

solid masses of organic matter such as coal. Furthermore, inChapter 1 we briefly discussed the three main families of rocks:igneous, sedimentary, and metamorphic. Recall that igneousrocks form when molten rock material known as magma or lavacools and crystallizes to form a variety of minerals, or when par-ticulate matter called pyroclastic materials become consoli-dated. We are most familiar with igneous rocks formed fromlava flows and pyroclastic materials because they are easilyobserved at Earth’s surface, but you should be aware that mostmagma never reaches the surface. Indeed, much of it coolsand crystallizes far underground and thus forms plutons,igneous bodies of various shapes and sizes.

Granite and several similar-appearing rocks are the mostcommon rocks in the larger plutons such as those in theSierra Nevada of California (see the chapter opening photo)and in Acadia National Park in Maine (■ Figure 4.1a). The im-ages of Presidents Lincoln, Roosevelt, Jefferson, and Washing-ton at Mount Rushmore National Memorial in South Dakota(Figure 4.1b) as well as the nearby Crazy Horse Memorial (underconstruction) are in the 1.7-billion-year-old Harney Peak Gran-

ite, which consists of a number of plutons. These huge plutonsformed far below the surface, but subsequent uplift and deeperosion exposed them in their present form.

Some granite and related rocks are quite attractive, es-pecially when sawed and polished. They are used for tomb-stones, mantlepieces, kitchen counters, facing stones onbuildings, pedestals for statues, and statuary itself. More im-portant, though, is the fact that fluids emanating from plu-tons account for many ore minerals of important metals, suchas copper in adjacent rocks.

The origin of plutons, or intrusive igneous activity, andvolcanism involving the eruption of lava flows, gases, andpyroclastic materials are closely related topics even thoughwe discuss them in separate chapters. The same kinds ofmagmas are involved in both processes, but magma variesin its mobility, which explains why only some reaches thesurface. Furthermore, plutons typically lie beneath areas ofvolcanism and, in fact, are the source of the overlying lavaflows and pyroclastic materials. Plutons and most volcanoesare found at or near divergent and convergent plate bound-aries, so the presence of igneous rocks is one criterion forrecognizing ancient plate boundaries; igneous rocks alsohelp us unravel the complexities of mountain-buildingepisodes (see Chapter 10).

W

(a)

(b)

■ Figure 4.1

(a) Light-colored granitic rocks exposed along the shoreline inAcadia National Park in Maine. The dark rock is basalt that formedwhen magma intruded along a fracture in the granitic rock. (b) Thepresidents’ images at Mount Rushmore National Memorial in SouthDakota are in the Harney Peak Granite.

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One important reason to study igneous rocks and intru-sive igneous activity is that igneous rocks are one of the threemain families of rocks. In addition, igneous rocks make uplarge parts of all the continents and nearly all of the oceaniccrust, which is formed continuously by igneous activity at di-vergent plate boundaries. And, as already mentioned, impor-tant mineral deposits are found adjacent to many plutons.

89the properties and behavior of magma and lava

In this chapter, our main concerns are (1) the origin,composition, textures, and classification of igneous rocks,and (2) the origin, significance, and types of plutons. InChapter 5, we will consider volcanism, volcanoes, and asso-ciated phenomena that result from magma reaching Earth’ssurface. Remember, though, that the origin of plutons andvolcanism are related topics.

THE PROPERTIES AND BEHAVIOR OFMAGMA AND LAVA

n Chapter 3 we noted that one process that ac-counts for the origin of minerals, and thus rocks, iscooling and crystallization of magma and lava.

Magma is molten rock below Earth’s surface. Anymagma is less dense than the rock from which it wasderived, so it tends to move upward, but much of itcools and solidifies deep underground, thus accountingfor intrusive igneous bodies known as plutons. However,some magma does rise to the surface where it issuesforth as lava flows, or it is forcefully ejected into the at-mosphere as particulate matter known as pyroclasticmaterials (from the Greek pyro, “fire,” and klastos, “bro-ken”). Certainly lava flows and eruptions of pyroclasticmaterials are the most impressive manifestations of allprocesses related to magma, but they result from only asmall percentage of all magma that forms.

All igneous rocks derive from magma, but two sep-arate processes account for them. They form when(1) magma or lava cools and crystallizes to form miner-als, or (2) pyroclastic materials are consolidated to forma solid aggregate from the previously loose particles. Ig-neous rocks that result from the cooling of lava flowsand the consolidation of pyroclastic materials are vol-canic rocks or extrusive igneous rocks—that is, ig-neous rocks that form from materials erupted on the

surface. In contrast, magma that cools below the surfaceforms plutonic rocks or intrusive igneous rocks.

Composition of MagmaIn Chapter 3 we noted that by far the most abundantminerals in Earth’s crust are silicates such as quartz,various feldspars, and several ferromagnesian silicates,all made up of silicon and oxygen, and other elementsshown in Figure 3.9. As a result, melting of the crustyields mostly silica-rich magmas that also contain con-siderable aluminum, calcium, sodium, iron, magne-sium, and potassium, and several other elements inlesser quantities. Another source of magma is Earth’supper mantle, which is composed of rocks that containmostly ferromagnesian silicates. Thus magma from thissource contains comparatively less silicon and oxygen(silica) and more iron and magnesium.

Although there are a few exceptions, the primaryconstituent of magma is silica, which varies enough todistinguish magmas classified as felsic, intermediate,and mafic.* Felsic magma, with more than 65% silica,is silica-rich and contains considerable sodium, potas-sium, and aluminum but little calcium, iron, and mag-nesium. In contrast, mafic magma, with less than 52%silica, is silica-poor and contains proportionately morecalcium, iron, and magnesium. And as you would expect,intermediate magma has a composition between felsicand mafic magma (Table 4.1).

How Hot Are Magma and Lava?Everyone knows that lava is veryhot, but how hot is hot? Eruptinglava generally has a temperaturein the range of 1000° to 1200°C,although a temperature of1350°C was recorded above alava lake in Hawaii where vol-canic gases reacted with the at-mosphere. Magma must be evenhotter than lava, but no direct

I

Table 4.1

The Most Common Types of Magmas and Their Characteristics

Sodium, Calcium,Type of Silica Potassium, Iron, andMagma Content (%) and Aluminum Magnesium

Ultramafic �45 Increase

Mafic 45–52

Intermediate 53–65

Felsic �65 Increase

*Lava from some volcanoes in Africa coolsto form carbonitite, an igneous rock with atleast 50% carbonate minerals, mostly calciteand dolomite.

measurements of magma temperatures have ever beenmade.

Most lava temperatures are taken at volcanoes thatshow little or no explosive activity, so our best informa-tion comes from mafic lava flows such as those in Hawaii(■ Figure 4.2). In contrast, eruptions of felsic lava are notas common, and the volcanoes that these flows issue fromtend to be explosive and thus cannot be approached safely.Nevertheless, the temperatures of some bulbous massesof felsic lava in lava domes have been measured at a dis-tance with an optical pyrometer. The surfaces of theselava domes are as hot as 900°C, but their interiors mustsurely be even hotter.

When Mount St. Helens erupted in 1980 in Wash-ington State, it ejected felsic magma as particulate mat-ter in pyroclastic flows. Two weeks later, these flows hadtemperatures between 300° and 420°C, and a steam ex-plosion took place more than a year later when water en-countered some of the still-hot pyroclastic materials.The reason that lava and magma retain heat so well isthat rock conducts heat so poorly. Accordingly, the inte-riors of thick lava flows and pyroclastic flow depositsmay remain hot for months or years, whereas plutons,depending on their size and depth, may not cool com-pletely for thousands to millions of years.

Viscosity—Resistance to FlowAll liquids have the property of viscosity, or resistanceto flow. For liquids such as water, viscosity is very lowso they are highly fluid and flow readily. For other liq-uids, though, viscosity is so high that they flow muchmore slowly. Good examples are cold motor oil and

syrup, both of which are quiteviscous and thus flow onlywith difficulty. But when theseliquids are heated, their vis-cosity is much lower and theyflow more easily; that is, theybecome more fluid with in-creasing temperature. Accord-ingly, you might suspect thattemperature controls the vis-cosity of magma and lava, andthis inference is partly correct.We can generalize and say thathot magma or lava moves morereadily than cooler magma orlava, but we must qualify thisstatement by noting that tem-perature is not the only controlof viscosity.

Silica content stronglycontrols magma and lava vis-cosity. With increasing silicacontent, numerous networksof silica tetrahedra form and

retard flow because for flow to take place, the strongbonds of the networks must be ruptured. Mafic magmaand lava with 45–52% silica have fewer silica tetrahedranetworks and as a result are more mobile than felsicmagma and lava flows. One mafic flow in 1783 in Ice-land flowed about 80 km, and geologists traced ancientflows in Washington State for more than 500 km. Felsicmagma, in contrast, because of its higher viscosity, doesnot reach the surface as commonly as mafic magma. Andwhen felsic lava flows do occur, they tend to be slowmoving and thick and to move only short distances. Athick, pasty lava flow that erupted in 1915 from LassenPeak in California flowed only about 300 m before itceased moving.

HOW DOES MAGMAORIGINATE AND CHANGE?

ost of us have not witnessed a volcaniceruption, but we have nevertheless seennews reports or documentaries showing

magma issuing forth as lava flows or pyroclastic materi-als. In any case, we are familiar with some aspects of ig-neous activity, but most people are unaware of how andwhere magma originates, how it rises from its place oforigin, and how it might change. Indeed, many believethe misconception that lava comes from a continuouslayer of molten rock beneath the crust or that it comesfrom Earth’s molten outer core.


■ Figure 4.2

A geologist using a thermocouple to determine the temperature of a lava flow in Hawaii.

P. M

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First, let us address how and where magma origi-nates. We know that the atoms in a solid are in constantmotion and that, when a solid is heated, the energy ofmotion exceeds the binding forces and the solid melts.We are all familiar with this phenomenon, and we arealso aware that not all solids melt at the same tempera-ture. Once magma forms, it tends to rise because it isless dense than the rock that melted, and some actuallymakes it to the surface.

Magma may come from 100 to 300 km deep, butmost forms at much shallower depths in the uppermantle or lower crust and accumulates in reservoirsknown as magma chambers. Beneath spreadingridges, where the crust is thin, magma chambers existat a depth of only a few kilometers, but along conver-gent plate boundaries, magma chambers are com-monly a few tens of kilometers deep. The volume of amagma chamber ranges from a few to many hundredsof cubic kilometers of molten rock within the other-wise solid lithosphere. Some simply cools and crystal-lizes within Earth’s crust, thusaccounting for the origin ofvarious plutons, whereassome rises to the surface andis erupted as lava flows or py-roclastic materials.

Bowen’s ReactionSeriesDuring the early part of thelast century, N. L. Bowen hy-pothesized that mafic, inter-mediate, and felsic magmascould all derive from a parentmafic magma. He knew thatminerals do not all crystallizesimultaneously from coolingmagma, but rather crystallizein a predictable sequence.Based on his observationsand laboratory experiments,Bowen proposed a mecha-nism, now called Bowen’s re-action series, to account forthe derivation of intermediateand felsic magmas from maficmagma. Bowen’s reaction se-ries consists of two branches:a discontinuous branch and acontinuous branch (■ Figure4.3). As the temperature ofmagma decreases, mineralscrystallize along both branchessimultaneously, but for conve-nience we will discuss themseparately.

In the discontinuous branch, which contains onlyferromagnesian silicates, one mineral changes to an-other over specific temperature ranges (Figure 4.3). Asthe temperature decreases, a temperature range isreached in which a given mineral begins to crystallize.A previously formed mineral reacts with the remainingliquid magma (the melt) so that it forms the next min-eral in the sequence. For instance, olivine[(Mg,Fe)2SiO4] is the first ferromagnesian silicate tocrystallize. As the magma continues to cool, it reachesthe temperature range at which pyroxene is stable; areaction occurs between the olivine and the remainingmelt, and pyroxene forms.

With continued cooling, a similar reaction takesplace between pyroxene and the melt, and the pyroxenestructure is rearranged to form amphibole. Further cool-ing causes a reaction between the amphibole and themelt, and its structure is rearranged so that the sheetstructure of biotite mica forms. Although the reactionsjust described tend to convert one mineral to the next in

91how does magma originate and change?

■ Figure 4.3

Bowen’s reaction series consists of a discontinuous branch along which a succession offerromagnesian silicates crystallize as the magma’s temperature decreases, and a continuousbranch along which plagioclase feldspars with increasing amounts of sodium crystallize. Noticealso that the composition of the initial mafic magma changes as crystallization takes place alongthe two branches.

Muscovitemica

Potassiumfeldspar

Quartz

Biotitemica

Sodium-richplagioclase

Amphibole(hornblende)

Pyroxene(augite)

Olivine

Reaction

Reaction

Reaction

Calcium-richplagioclase

Dec

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tem

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Plag

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Discontinuous branch

Types ofmagma

Mafic(45–52% silica)

Intermediate(53–65% silica)

Felsic(>65% silica)

the series, the reactions are not always complete.Olivine, for example, might have a rim of pyroxene, indi-cating an incomplete reaction. If magma cools rapidlyenough, the early-formed minerals do not have time toreact with the melt, and thus all the ferromagnesian sili-cates in the discontinuous branch can be in one rock. Inany case, by the time biotite has crystallized, essentiallyall the magnesium and iron present in the originalmagma have been used up.

Plagioclase feldspars, which are nonferromagne-sian silicates, are the only minerals in the continuousbranch of Bowen’s reaction series (Figure 4.3).Calcium-rich plagioclase crystallizes first. As themagma continues to cool, calcium-rich plagioclase re-acts with the melt, and plagioclase containing propor-tionately more sodium crystallizes until all of thecalcium and sodium are used up. In many cases, cool-ing is too rapid for a complete transformation fromcalcium-rich to sodium-rich plagioclase to take place.Plagioclase forming under these conditions is zoned,meaning that it has a calcium-rich core surrounded byzones progressively richer in sodium.

As minerals crystallize simultaneously along the twobranches of Bowen’s reaction series, iron and magne-sium are depleted because they are used in ferromagne-sian silicates, whereas calcium and sodium are used upin plagioclase feldspars. At this point, any leftovermagma is enriched in potassium, aluminum, and silicon,which combine to form orthoclase (KAlSi3O8), a potas-sium feldspar, and if water pressure is high, the sheetsilicate muscovite forms. Any remaining magma is en-riched in silicon and oxygen (silica) and forms the min-eral quartz (SiO2). The crystallization of orthoclase andquartz is not a true reaction series because they form in-dependently rather than by a reaction of orthoclase withthe melt.

The Origin of Magma at Spreading RidgesOne fundamental observation regarding the origin ofmagma is that Earth’s temperature increases withdepth. Known as the geothermal gradient, this tempera-

ture increase averages about 25°C/km. Accordingly,rocks at depth are hot but remain solid because theirmelting temperature rises with increasing pressure(■ Figure 4.4a). However, beneath spreading ridges thetemperature locally exceeds the melting temperature, atleast in part because pressure decreases. That is, plate sep-aration at ridges probably causes a decrease in pressure onthe already hot rocks at depth, thus initiating melting (Fig-ure 4.4a). In addition, the presence of water decreases themelting temperature beneath spreading ridges becausewater aids thermal energy in breaking the chemical bondsin minerals (Figure 4.4b).

Localized, cylindrical plumes of hot mantle mater-ial, called mantle plumes, rise beneath spreading ridgesand elsewhere, and as they rise, pressure decreases andmelting begins, thus yielding magma. Magma formed be-neath spreading ridges is invariably mafic (45–52% sil-ica). But the upper mantle rocks from which this magmais derived are characterized as ultramafic (�45% silica),consisting largely of ferromagnesian silicates and lesseramounts of nonferromagnesian silicates. To explain howmafic magma originates from ultramafic rock, geologistspropose that the magma forms from source rock thatonly partially melts. This phenomenon of partial meltingtakes place because not all of the minerals in rocks meltat the same temperature.

Recall the sequence of minerals in Bowen’s reactionseries (Figure 4.3). The order in which these minerals meltis the opposite of their order of crystallization. Accordingly,rocks made up of quartz, potassium feldspar, and sodium-rich plagioclase begin melting at lower temperatures thanthose composed of ferromagnesian silicates and the calcicvarieties of plagioclase. So when ultramafic rock starts tomelt, the minerals richest in silica melt first, followed by


?What Would You DoYou are a high school science teacher interested in de-veloping experiments to show your students that(1) composition and temperature affect the viscosity ofa lava flow, and (2) when magma or lava cools, someminerals crystallize before others. Describe the experi-ments you might devise to illustrate these points.

Temperature

(a)

Pre

ssur

e Solid

Liquid

Melting curve

Solid

Liquid

Wet meltingcurve

Dry meltingcurve

Temperature

(b)

Pre

ssur

e

■ Figure 4.4

The effects of pressure and temperature on melting. (a) As pressuredecreases, even when temperature remains constant, melting takesplace. The black circle represents rock at high temperature. Thesame rock (open circle) melts at lower pressure. (b) If water ispresent, the melting curve shifts to the left because water providesan additional agent to break chemical bonds. Accordingly, rocksmelt at a lower temperature (green melting curve) if water ispresent.

those containing less silica. Therefore, if melting is notcomplete, mafic magma containing proportionately moresilica than the source rock results.

Subduction Zones and the Origin of MagmaAnother fundamental observation regarding magmais that, where an oceanic plate is subducted beneatheither a continental plate or another oceanic plate, abelt of volcanoes and plutons is found near the lead-ing edge of the overriding plate (■ Figure 4.5). Itwould seem, then, that subduction and the origin ofmagma must be related in some way, and indeed theyare. Furthermore, magma at these convergent plateboundaries is mostly intermediate (53–65% silica) orfelsic (�65% silica).

Once again, geologists invoke the phenomenon ofpartial melting to explain the origin and composition ofmagma at subduction zones. As a subducted plate de-scends toward the asthenosphere, it eventually reachesthe depth where the temperature is high enough to initi-ate partial melting. In addition, the oceanic crust de-scends to a depth at which dewatering of hydrousminerals takes place, and as the water rises into the over-lying mantle, it enhances melting and magma forms(Figure 4.4b).

Recall that partial melting of ultramafic rock atspreading ridges yields mafic magma. Similarly, partial

melting of mafic rocks of the oceanic crust yields inter-mediate (53–65% silica) and felsic (�65% silica) mag-mas, both of which are richer in silica than the sourcerock. Moreover, some of the silica-rich sediments andsedimentary rocks of continental margins are probablycarried downward with the subducted plate and con-tribute their silica to the magma. Also, mafic magma ris-ing through the lower continental crust must becontaminated with silica-rich materials, which changesits composition.

Processes That Bring AboutCompositional Changes in MagmaOnce magma forms, its composition may change bycrystal settling, which involves the physical separationof minerals by crystallization and gravitational settling(■ Figure 4.6). Olivine, the first ferromagnesian silicate toform in the discontinuous branch of Bowen’s reaction se-ries, has a density greater than the remaining magma andtends to sink. Accordingly, the remaining magma becomesricher in silica, sodium, and potassium because much ofthe iron and magnesium were removed as olivine and per-haps pyroxene minerals crystallized.

Although crystal settling does take place, it does notdo so on a scale that would yield very much felsic magmafrom mafic magma. In some thick, sheetlike plutonscalled sills, the first-formed ferromagnesian silicates areindeed concentrated in their lower parts, thus making

93how does magma originate and change?

MagmaAsthenosphere Asthenosphere

Upper mantle

Magma

Uppermantle

Oceaniccrust

Oceanic crust

Continental crust

Volcanoes

Oceanictrench

Pluton

Continentalplate

■ Figure 4.5

Both intrusive and extrusive igneous activity take place at divergent plate boundaries (spreading ridges) and where plates are subducted atconvergent plate boundaries. Oceanic crust is composed largely of plutons and dark igneous rocks that cooled from submarine lava flows.Magma forms where an oceanic plate is subducted beneath another oceanic plate or beneath a continental plate as shown here. Much ofthe magma forms plutons, but some is erupted to form volcanoes (see Chapter 5).

their upper parts less mafic. But even in these plutons,crystal settling has yielded very little felsic magma.

If felsic magma could be derived on a large scalefrom mafic magma, there should be far more maficmagma than felsic magma. To yield a particular volumeof granite (a felsic igneous rock), about 10 times asmuch mafic magma would have to be present initiallyfor crystal settling to yield the volume of granite in ques-tion. If this were so, then mafic intrusive igneous rocksshould be much more common than felsic ones. How-ever, just the opposite is the case, so it appears thatmechanisms other than crystal settling must account forthe large volume of felsic magma. Partial melting ofmafic oceanic crust and silica-rich sediments of conti-nental margins during subduction yields magma richerin silica than the source rock. Furthermore, magma ris-ing through the continental crust absorbs some felsicmaterials and becomes more enriched in silica.

The composition of magma also changes by assim-ilation, a process by which magma reacts with preex-isting rock, called country rock, with which it comesin contact (Figure 4.6). The walls of a volcanic conduitor magma chamber are, of course, heated by the adjacentmagma, which may reach temperatures of 1300°C. Someof these rocks partly or completely melt, provided theirmelting temperature is lower than that of the magma.Because the assimilated rocks seldom have the samecomposition as the magma, the composition of themagma changes.

The fact that assimilation occurs is indicated by in-clusions, incompletely melted pieces of rock that arefairly common in igneous rocks. Many inclusions weresimply wedged loose from the country rock as magmaforced its way into preexisting fractures (Figure 4.6). Noone doubts that assimilation takes place, but its effect onthe bulk composition of magma must be slight. The rea-son is that the heat for melting comes from the magmaitself, and this has the effect of cooling the magma. Onlya limited amount of rock can be assimilated by magma,and that amount is insufficient to bring about a majorcompositional change.

Neither crystal settling nor assimilation can producea significant amount of felsic magma from a mafic one.But both processes, if operating concurrently, can bringabout greater changes than either process acting alone.Some geologists think that this is one way that interme-diate magma forms where oceanic lithosphere is sub-ducted beneath continental lithosphere.

A single volcano can erupt lavas of different compo-sition, indicating that magmas of differing compositionare present. It seems likely that some of these magmaswould come into contact and mix with one another. Ifthis is the case, we would expect that the composition ofthe magma resulting from magma mixing would be amodified version of the parent magmas. Suppose risingmafic magma mixes with felsic magma of about the samevolume (■ Figure 4.7). The resulting “new” magma wouldhave a more intermediate composition.


Magma chamber

(a)

(b)

Crystalsettling

Country rock

Inclusion

Assimilated piecesof country rock

■ Figure 4.6

(a) Early-formed ferromagnesian silicates are denser than themagma and settle and accumulate in the magma chamber.Fragments of rock dislodged by upward-moving magma maymelt and be incorporated into the magma, or they may remainas inclusions. (b) Dark inclusions in granitic rock.

Sue

Mon

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IGNEOUS ROCKS—THEIRCHARACTERISTICSAND CLASSIFICATION

e have already defined plutonic or intru-sive igneous rocks and volcanic or extru-sive igneous rocks. Here we will have

considerably more to say about the texture, composi-tion, and classification of these rocks, which constituteone of the three major rock families depicted in the rockcycle (see Figure 1.12).

Igneous Rock TexturesThe term texture refers to the size, shape, and arrange-ment of the minerals that make up igneous rocks. Sizeis the most important because mineral crystal size is re-lated to the cooling history of magma or lava and gener-ally indicates whether an igneous rock is volcanic orplutonic. The atoms in magma and lava are in constantmotion, but when cooling begins, some atoms bond toform small nuclei. As other atoms in the liquid chemi-cally bond to these nuclei, they do so in an orderly geo-metric arrangement and the nuclei grow into crystallinemineral grains, the individual particles that make up ig-neous rocks.

During rapid cooling, as takes place in lava flows,the rate at which mineral nuclei form exceeds the rateof growth and an aggregate of many small mineralgrains is formed. The result is a fine-grained oraphanitic texture, in which individual minerals are toosmall to be seen without magnification (■ Figure 4.8a, b).With slow cooling, the rate of growth exceeds the rate of

nuclei formation, and relatively large mineral grains form,thus yielding a coarse-grained or phaneritic texture, inwhich minerals are clearly visible (Figure 4.8c, d).Aphanitic textures generally indicate an extrusive origin,whereas rocks with phaneritic textures are usually intru-sive. However, shallow plutons might have an aphanitictexture, and the rocks that form in the interiors of thicklava flows might be phaneritic.

Another common texture in igneous rocks is onetermed porphyritic, in which minerals of markedly dif-ferent size are present in the same rock. The larger min-erals are phenocrysts and the smaller ones collectivelymake up the groundmass, which is simply the grains be-tween phenocrysts (Figure 4.8e, f). The groundmass canbe either aphanitic or phaneritic; the only requirementfor a porphyritic texture is that the phenocrysts be con-siderably larger than the minerals in the groundmass.Igneous rocks with porphyritic textures are designatedporphyry, as in basalt porphyry. These rocks have morecomplex cooling histories than those with aphanitic orphaneritic textures that might involve, for example,magma partly cooling beneath the surface followed byits eruption and rapid cooling at the surface.

Lava may cool so rapidly that its constituent atomsdo not have time to become arranged in the ordered,three-dimensional frameworks of minerals. As a conse-quence natural glass such as obsidian forms (Figure4.8g). Even though obsidian with its glassy texture is notcomposed of minerals, geologists nevertheless classify itas an igneous rock.

Some magmas contain large amounts of water vaporand other gases. These gases may be trapped in coolinglava where they form numerous small holes or cavitiesknown as vesicles; rocks with many vesicles are termedvesicular, as in vesicular basalt (Figure 4.8h).

A pyroclastic or fragmental texture characterizesigneous rocks formed by explosive volcanic activity (Fig-ure 4.8i). For example, ash discharged high into the at-mosphere eventually settles to the surface where itaccumulates; if consolidated, it forms pyroclastic ig-neous rock.

Composition of Igneous RocksMost igneous rocks, like the magma from which theyoriginate, are characterized as mafic (45–52% silica),intermediate (53–65% silica), or felsic (�65% silica). Afew are referred to as ultramafic (�45% silica), butthese are probably derived from mafic magma by aprocess discussed later. The parent magma plays an im-portant role in determining the mineral composition ofigneous rocks, yet it is possible for the same magma toyield a variety of igneous rocks because its compositioncan change as a result of the sequence in which miner-als crystallize, or by crystal settling, assimilation, andmagma mixing (Figures 4.3, 4.6, and 4.7).

95igneous rocks—their characteristics and classification

Maficmagma

Felsic magma

Country rock

■ Figure 4.7

Magma mixing. Two magmas mix and produce magma with acomposition different from either of the parent magmas. In thiscase, the resulting magma would have an intermediatecomposition.

W

Classifying Igneous RocksGeologists use texture and composition to classify mostigneous rocks. Notice in ■ Figure 4.9 that all rocks ex-cept peridotite are in pairs; the members of a pair have thesame composition but different textures. Basalt and gab-bro, andesite and diorite, and rhyolite and granite are com-positional (mineralogical) pairs, but basalt, andesite, andrhyolite are aphanitic and most commonly extrusive (vol-canic), whereas gabbro, diorite, and granite are phaneritic

and mostly intrusive (plutonic). The extrusive and intru-sive members of each pair can usually be distinguished bytexture, but remember that rocks in some shallow plutonsmay be aphanitic and rocks that formed in thick lava flowsmay be phaneritic. In other words, all of these rocks existin a textural continuum.

The igneous rocks in Figure 4.9 are also differenti-ated by composition—that is, by their mineral content.Reading across the chart from rhyolite to andesite tobasalt, for example, we see that the proportions of non-


Rapid cooling

Slow cooling

(c) (d)

(f)(e)

(a) (b)

(g) (h) (i)

Phenocrysts

■ Figure 4.8

The various textures of igneous rocks. Texture is one criterion used to classify igneous rocks. (a, b) Rapid cooling as in lava flows results inmany small minerals and an aphanitic (fine-grained) texture. (c, d) Slower cooling in plutons yields a phaneritic texture. (e, f ) Theseporphyritic textures indicate a complex cooling history. (g) Obsidian has a glassy texture because magma cooled too quickly for mineralcrystals to form. (h) Gases expand in lava and yield a vesicular texture. (i) Microscopic view of an igneous rock with a fragmental texture. Thecolorless, angular objects are pieces of volcanic glass measuring up to 2 mm.

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ferromagnesian and ferromagnesian silicates change.The differences in composition, however, are gradualalong a compositional continuum. In other words,there are rocks with compositions that correspond tothe lines between granite and diorite, basalt and an-desite, and so on.

Ultramafic Rocks Ultramafic rocks (�45% silica) arecomposed largely of ferromagnesian silicates. The ultra-mafic rock peridotite contains mostly olivine, lesseramounts of pyroxene, and usually a little plagioclasefeldspar (Figures 4.9 and ■ 4.10). Pyroxenite, anotherultramafic rock, is composed predominately of pyroxene.Because these minerals are dark, the rocks are generallyblack or dark green. Peridotite is probably the rock typethat makes up the upper mantle (see Chapter 9). Ultra-mafic rocks in Earth’s crust probably originate by concen-tration of the early-formed ferromagnesian minerals thatseparated from mafic magmas.

Ultramafic lava flows are known inrocks older than 2.5 billion years, butyounger ones are rare or absent. Thereason is that to erupt, ultramafic lavamust have a near-surface temperatureof about 1600°C; the surface tem-peratures of present-day mafic lavaflows are between 1000° and1200°C. During early Earth history,though, more radioactive decay heatedthe mantle to as much as 300°C hotter

than now and ultramafic lavas could erupt onto the sur-face. Because the amount of heat has decreased overtime, Earth has cooled, and eruptions of ultramafic lavaflows ceased.

Basalt-Gabbro Basalt and gabbro are the aphaniticand phaneritic rocks, respectively, that crystallize frommafic magma (45–52% silica) (■ Figure 4.11). Thus,both have the same composition—mostly calcium-richplagioclase and pyroxene, with smaller amounts of olivineand amphibole (Figure 4.9). Because they contain a largeproportion of ferromagnesian silicates, basalt and gabbroare dark; those that are porphyritic typically contain cal-cium plagioclase or olivine phenocrysts.

Extensive basalt lava flows cover vast areas in Wash-ington, Oregon, Idaho, and northern California (seeChapter 5). Oceanic islands such as Iceland, the Galá-pagos, the Azores, and the Hawaiian Islands are com-

posed mostly of basalt, and basalt makes up theupper part of the oceanic crust.


Darkness and specific gravity increase

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Texture

Felsic Mafic UltramaficIntermediate■ Figure 4.9

Classification of igneous rocks. Thisdiagram shows the percentages ofminerals as well as the textures ofcommon igneous rocks. For example, anaphanitic (fine-grained) rock of mostlycalcium-rich plagioclase and pyroxene isbasalt.

■ Figure 4.10

This specimen of the ultramafic rockperidotite is made up mostly of olivine.Notice in Figure 4.9 that peridotite is the onlyphaneritic rock that does not have anaphanitic counterpart. Peridotite is rare atEarth’s surface but is very likely the rockmaking up the mantle. Source: Sue Monroe

Gabbro is much less common than basalt, at leastin the continental crust or where it can be easily ob-served. Small intrusive bodies of gabbro are present inthe continental crust, but intermediate to felsic intru-sive rocks are much more common. The lower part ofthe oceanic crust is composed of gabbro, however.

Andesite-Diorite Intermediate composition magma(53–65% silica) crystallizes to form andesite and diorite,which are compositionally equivalent fine- and coarse-grained igneous rocks (■ Figure 4.12). Andesite and dior-ite are composed predominately of plagioclase feldspar,with the typical ferromagnesian component being amphi-bole or biotite (Figure 4.9). Andesite is generally mediumto dark gray, but diorite has a salt-and-pepper appearancebecause of its white to light gray plagioclase and dark fer-romagnesian silicates (Figure 4.12).

Andesite is a common extrusive igneous rockformed from lava erupted in volcanic chains at conver-gent plate boundaries. The volca-noes of the Andes Mountains of

South America and the Cascade Range in westernNorth America are composed in part of andesite. Intru-sive bodies of diorite are fairly common in the continen-tal crust.

Rhyolite-Granite Rhyolite and granite crystallizefrom felsic magma (�65% silica) and are thereforesilica-rich rocks (■ Figure 4.13). They consist largelyof potassium feldspar, sodium-rich plagioclase, andquartz, with perhaps some biotite and rarely amphi-bole (Figure 4.9). Because nonferromagnesian sili-cates predominate, rhyolite and granite are typicallylight colored. Rhyolite is fine grained, although mostoften it contains phenocrysts of potassium feldspar orquartz, and granite is coarse grained. Granite porphyryis also fairly common.

Rhyolite lava flows are much less common than an-desite and basalt flows. Recall that the greatest controlof magma viscosity is silica content. Thus, if felsic

magma rises to the surface, it begins to cool, the pres-sure on it decreases, and gases are released

explosively, usually yielding rhyolitic py-roclastic materials. The rhyolitic lava

flows that do occur are thick andhighly viscous and move only

short distances.Granite is a coarsely

crystalline igneous rock witha composition correspond-ing to that of the field shownin Figure 4.9. Strictly speak-

ing, not all rocks in this fieldare granites. For example, a rock with a

composition close to the line separating granite and dior-ite is called granodiorite. To avoid the confusion thatmight result from introducing more rock names, we willfollow the practice of referring to rocks to the left of thegranite-diorite line in Figure 4.9 as granitic.


■ Figure 4.11

Mafic igneous rocks. (a) Basalt is aphanitic, whereas (b) gabbro is phaneritic. Notice the light reflected from the crystal faces in (b). Bothbasalt and gabbro have the same mineral composition (see Figure 4.9).

(a) Basalt (b) Gabbro

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(a) Andesite

■ Figure 4.12

Intermediate igneous rocks. (a) Andesite has hornblendephenocrysts, so this is andesite porphyry. (b) Diorite has a salt-and-pepper appearance because it contains light-colorednonferromagnesian silicates and dark-colored ferromagnesiansilicates.

(b) Diorite

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Granitic rocks are by far the most common intrusiveigneous rocks, although they are restricted to the conti-nents. Most granitic rocks were intruded at or near con-vergent plate margins during mountain-buildingepisodes. When these mountainous regions are upliftedand eroded, the vast bodies of granitic rocks formingtheir cores are exposed. The granitic rocks of the SierraNevada of California form a composite body measuringabout 640 km long and 110 km wide, and the graniticrocks of the Coast Ranges of British Columbia, Canada,are even more voluminous.

Pegmatite The term pegmatite refers to a particulartexture rather than a specific composition, but most

pegmatites are composed largely of quartz, potassiumfeldspar, and sodium-rich plagioclase, thus correspond-ing closely to granite. A few pegmatites are mafic or in-termediate in composition and are appropriately calledgabbro and diorite pegmatites. The most remarkable fea-ture of pegmatites is the size of their minerals, whichmeasure at least 1 cm across, and in some pegmatitesthey measure tens of centimeters or meters (■ Figure4.14). Many pegmatites are associated with large graniteintrusive bodies and are composed of minerals that formedfrom the water-rich magma that remained after most ofthe granite crystallized.

When magma cools and forms granite, the remain-ing water-rich magma has properties that differ from themagma from which it separated. It has a lower densityand viscosity and commonly invades the adjacent rockswhere minerals crystallize. This water-rich magma alsocontains a number of elements that rarely enter into thecommon minerals that form granite. Pegmatites that areessentially very coarsely crystalline granite are simplepegmatites, whereas those with minerals containing ele-ments such as lithium, beryllium, cesium, boron, andseveral others are complex pegmatites. Some complexpegmatites contain 300 different mineral species, a fewof which are important economically. In addition, sev-eral gem minerals such as emerald and aquamarine,both of which are varieties of the silicate mineral beryl,and tourmaline are found in some pegmatites. Manyrare minerals of lesser value and well-formed crystals ofcommon minerals, such as quartz, are also mined andsold to collectors and museums.

The formation and growth of mineral-crystal nucleiin pegmatites are similar to those processes in othermagmas but with one critical difference: The water-richmagma from which pegmatites crystallize inhibits theformation of nuclei. However, some nuclei do form, andbecause the appropriate ions in the liquid can move eas-ily and attach themselves to a growing crystal, individualminerals have the opportunity to grow very large.

Other Igneous Rocks A few igneous rocks, includ-ing tuff, volcanic breccia, obsidian, pumice, and scoriaare identified primarily by their textures (■ Figure 4.15).Much of the fragmental material erupted by volcanoes isash, a designation for pyroclastic materials measuring lessthan 2.0 mm, most of which consists of broken pieces orshards of volcanic glass (Figure 4.8i). The consolidation ofash forms the pyroclastic rock tuff (■ Figure 4.16a). Mosttuff is silica-rich and light colored and is appropriatelycalled rhyolite tuff. Some ash flows are so hot that as theycome to rest, the ash particles fuse together and form awelded tuff. Consolidated deposits of larger pyroclastic ma-terials, such as cinders, blocks, and bombs, are volcanicbreccia (Figure 4.15).

Both obsidian and pumice are varieties of volcanicglass (Figure 4.16b, c). Obsidian may be black, darkgray, red, or brown, depending on the presence of iron.


■ Figure 4.13

Felsic igneous rocks. (a) Rhyolite and (b) granite are typically lightcolored because they contain mostly nonferromagnesian silicates.The dark spots in the granite are biotite mica. The white and pinkishminerals are feldspars, whereas the glassy minerals are quartz.

(a) Rhyolite

(b) Granite

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Obsidian breaks with the conchoidal (smoothly curved)fracture typical of glass. Analyses of many samples indi-cate that most obsidian has a high silica content and iscompositionally similar to rhyolite.

Pumice is a variety of volcanic glass containing nu-merous vesicles that develop when gas escapesthrough lava and forms a froth (Figure 4.16c). Ifpumice falls into water, it can be carried great dis-tances because it is so porous and light that it floats.Another vesicular rock is scoria. It is more crystallineand denser than pumice, but it has more vesicles thansolid rock (Figure 4.16d).

Log into GeologyNow and select this chapterto work through a Geology Interactive activity on “Rock Labora-tory” (click Rocks and the Rock Cycle→Rock Laboratory).

PLUTONS—THEIRCHARACTERISTICSAND ORIGINS

nlike volcanism and the origin of volcanicrocks, we can study intrusive igneous activityonly indirectly because plutons, intrusive ig-

neous bodies, form when magma cools and crystallizes


(a) (b)

■ Figure 4.14

(a) The light-colored rock is pegmatite exposed in the Black Hills of South Dakota. (b) A closeup view of a pegmatite specimen with mineralsmeasuring 2 to 3 cm across. This is a simple pegmatite that has a composition much like that of granite.

Composition Felsic

Pumice

Obsidian

Tuff/welded tuff

Mafic

ScoriaVesicular

Glassy

Pyroclasticor

fragmental

Volcanic Breccia

Text

ure

■ Figure 4.15

Classification of igneous rocks for which texture is the mainconsideration. Composition is shown, but it is not essential fornaming these rocks.

?What Would You DoAs the only member of your community with any geol-ogy background, you are considered the local expert onminerals and rocks. Suppose one of your friends bringsyou a rock specimen with the following features—com-position: mostly potassium feldspar and plagioclasefeldspar with about 10% quartz and minor amounts ofbiotite; texture: minerals average 3 mm across, but sev-eral potassium feldspars are up to 3 cm. Give the speci-men a rock name and tell your friend as much as youcan about the rock’s history. Why are the minerals solarge? U

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within Earth’s crust (see “Plutons” on pages 102 and103). We can observe them only after erosion has ex-posed them at the surface. Furthermore, geolgists can-not duplicate the conditions under which plutons formexcept in small laboratory experiments. Accordingly, ge-ologists face a greater challenge in interpreting themechanisms whereby plutons form. Magma that coolsto form plutons is emplaced in Earth’s crust mostly atconvergent and divergent plate boundaries, which arealso areas of volcanism.

Geologists recognize several types of plutons basedon their geometry (three-dimensional shape) and rela-tionships to the country rock. In terms of their geome-try, plutons are massive (irregular), tabular, cylindrical,or mushroom-shaped. Plutons are also either concor-dant, meaning they have boundaries that parallel thelayering in the country rock, or discordant, with bound-aries that cut across the country rock’s layering (see“Plutons” on pages 102 and 103).

Dikes and Sills

Dikes and sills are tabular or sheetlike plutons, differ-ing only in that dikes are discordant whereas sills areconcordant (see “Plutons” on pages 102 and 103).Dikes are quite common; most of them are small bodiesmeasuring 1 or 2 m across, but they range from a fewcentimeters to more than 100 m thick. Invariably theyare emplaced within preexisting fractures or where fluidpressure is great enough for them to form their ownfractures.

Erosion of the Hawaiian volcanoes exposes dikes inrift zones, the large fractures that cut across these vol-canoes. The Columbia River basalts in Washington State(discussed in Chapter 5) issued from long fissures, andmagma that cooled in the fissures formed dikes. Someof the large historic fissure eruptions are underlain bydikes; for example, dikes underlie both the Laki fissureeruption of 1783 in Iceland and the Eldgja fissure, also

101plutons—their characteristics and origins

(a) Tuff

(c) Pumice (d) Scoria

(b) Obsidian

■ Figure 4.16

Examples of igneous rocksclassified primarily by theirtexture. (a) Tuff is made up ofpyroclastic materials such asthose shown in Figure 4.8i.(b) The natural glass obsidian.(c) Pumice is glassy andextremely vesicular. (d) Scoriais also vesicular, but it isdarker, denser, and morecrystalline than pumice.

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Intrusive bodies called plutons are common, but we see them at the surface only after deep erosion. Notice that they vary in geometry and their relationships to the country rock.

PlutonsPlutonsPlutons

Block diagram showing various plutons. Some plutons cut across the layering in country rock and are discordant, whereas others parallel the layering and are concordant.

Part of the Sierra Nevada batholith in Yosemite National Park, California. The batholith, consisting of multiple intrusions of granitic rock, is more than 600 km long and up to 110 km wide. To appreciate the scale in this image, the waterfall has a descent of 435 m.

Granitic rocks of a smallstock at Castle Crags State Park, California.

A volcanic neck in Monument Valley Tribal Park, Arizona. This landform is 457 m high. Most of the original volcano was eroded, leaving only this remnant.

Volcanic neck

Stock Batholith

Magma

Laccolith

Cinder cone Lava flow

Sill

Volcanic pipe

Composite volcano

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The dark materials in this image are igneous

rocks, whereas the light layers are

sedimentary. Notice that the sill parallels the layering, so it is

concordant. The dike, though, clearly cuts across the layering

and is discordant. Sills and dikes have

sheetlike geometry, but in this view we can

see them in only two dimensions.

Diagrams showing the evolution of an

eroded laccolith.

Crown Butte in Montana is an eroded laccolith standing about 300 m above the surrounding plain. The magma making up this small pluton was intruded about 50 million years ago.

Sill

Dike

Eroded laccolith

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G E O FOCUS

Some Remarkable Volcanic Necks

e mentioned that asan extinct volcanoweathers anderodes, a remnant of

the original mountain may persist asa volcanic neck. The origin of volcanicnecks is well known, but these iso-lated monoliths rising above other-wise rather flat land are scenic,awe-inspiring, and the subject of leg-ends. They are found in many areasof recently active volcanism. A smallvolcanic neck rising only 79 m abovethe surface in the town of Le Puy,France, is the site of the 11th-centurychapel of Saint Michel d’Aiguilhe (■ Figure 1). It is so steep that materi-als and tools used in its constructionhad to be hauled up in baskets.

Perhaps the most famous vol-canic neck in the United States isShiprock, New Mexico, which risesnearly 550 m above the surroundingplain and is visible from 160 kmaway. Radiating outward from thisconical structure are three verticaldikes that stand like walls above theadjacent countryside (■ Figure 2a).

According to one legend, Shiprock,or Tsae-bidahi, meaning “wingedrock,” represents a giant bird thatbrought the Navajo people fromthe north. The same legend holdsthat the dikes are snakes thatturned to stone.

An absolute age determined forone of the dikes indicates thatShiprock is about 27 million yearsold. When the original volcanoformed, apparently during explo-sive eruptions, rising magma pene-trated various rocks including theMancos Shale, the rock unit nowexposed at the surface adjacent toShiprock. The rock that makes upShiprock itself is tuff-breccia, con-sisting of fragmented volcanic debris as well as pieces of meta-morphic, sedimentary, and igneousrocks.

Geologists agree that Devil’sTower in northeastern Wyomingcooled from a small body ofmagma and that erosion hasexposed it in its present form(■ Figure 2b). However, opinion is

divided on whether it is a volcanicneck or an eroded laccolith. In ei-ther case, the rock that makes upDevil’s Tower is 45 to 50 millionyears old, and President TheodoreRoosevelt designated this impres-sive landform as our first nationalmonument in 1906. At 260 m high,Devil’s Tower is visible from 48 kmaway and served as a landmark forearly travelers in this area. Itachieved further distinction in 1977when it was featured in the filmClose Encounters of the Third Kind.

The Cheyenne and Sioux Indi-ans call Devil’s Tower MateoTepee, meaning “Grizzly BearLodge.” It was also called the“Bad God’s Tower,” and reportedly“Devil’s Tower” is a translationfrom this phrase. The tower’s mostconspicuous features are the near-vertical lines that, according toCheyenne legends, are scratchmarks made by a gigantic grizzlybear. One legend holds that thebear made the scratches whilepursuing a group of children. An-

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in Iceland, where eruptions occurred in A.D. 950 from afissure nearly 30 km long.

Concordant sheetlike plutons are sills; many sillsare a meter or less thick, although some are muchthicker. A well-known sill in the United States is the Pal-isades sill that forms the Palisades along the west side ofthe Hudson River in New York and New Jersey. It is ex-posed for 60 km along the river and is up to 300 m thick.Most sills were intruded into sedimentary rocks, buteroded volcanoes also reveal that sills are commonly in-jected into piles of volcanic rocks. In fact, some infla-tion of volcanoes preceding eruptions may be caused bythe injection of sills (see Chapter 5).

In contrast to dikes, which follow zones of weak-ness, sills are emplaced when the fluid pressure is so

great that the intruding magma actually lifts the overly-ing rocks. Because emplacement requires fluid pressureexceeding the force exerted by the weight of the overly-ing rocks, many sills are shallow intrusive bodies, butsome were emplaced deep in the crust.

LaccolithsLaccoliths are similar to sills in that they are concordant,but instead of being tabular, they have a mushroomlikegeometry (see “Plutons” on pages 102 and 103). Theytend to have a flat floor and are domed up in their cen-tral part. Like sills, laccoliths are rather shallow intrusivebodies that actually lift up the overlying rocks whenmagma is intruded. In this case, however, the rock layers


are arched up over the pluton. Most laccoliths are rathersmall bodies. Well-known laccoliths in the United Statesare in the Henry Mountains of southeastern Utah, andseveral buttes in Montana are eroded laccoliths.

Volcanic Pipes and NecksA volcano has a cylindrical conduit known as a vol-canic pipe that connects its crater with an underlyingmagma chamber. Through this structure magma risesto the surface. When a volcano ceases to erupt, itsslopes are attacked by water, gases, and acids and iterodes, but the magma that solidified in the pipe iscommonly more resistant to alteration and erosion.Consequently, much of the volcano is eroded but the

pipe remains as a remnant called a volcanic neck. Sev-eral volcanic necks are found in the southwesternUnited States, especially in Arizona and New Mexico,and others are recognized elsewhere (see Geo-Focus4.1 and “Plutons” on pages 102–103).

Batholiths and StocksBy definition a batholith, the largest of all plutons, musthave at least 100 km2 of surface area, and most are farlarger. A stock, in contrast, is similar but smaller. Somestocks are simply parts of large plutons that once ex-posed by erosion are batholiths (see “Plutons” on pages102 and 103). Both batholiths and stocks are generallydiscordant, although locally they may be concordant,

105plutons—their characteristics and origins

other tells of six brothers and awoman also pursued by a grizzlybear. One brother carried a rock,and when he sang a song it grewinto Devil’s Tower, safely carryingthe brothers and woman out of thebear’s reach.

Although not nearly as interest-ing as the Cheyenne legends, theorigin for the “scratch marks” is wellunderstood. These lines actuallyformed at the intersections of

columnar joints, fractures that formin response to the cooling and con-traction that occur in some plutonsand lava flows (see Chapter 5). Thecolumns outlined by these fracturesare up to 2.5 m across, and the pileof rubble at the tower’s base is sim-ply an accumulation of collapsedcolumns.

Log into Geology-Now and select this chapter to workthrough a Geology Interactive activityon “Rock Laboratory” (click Rocks andthe Rock Cycle→Rock Laboratory).

■ Figure 2

(a) Shiprock, a volcanic neck in northwestern New Mexico, rises nearly 550 m above thesurrounding plain. One of the dikes radiating from Shiprock is in the foreground. (b) Devil’sTower in northeastern Wyoming. The vertical lines result from intersections of fractures calledcolumnar joints (see Chapter 5).

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■ Figure 1

This volcanic neck in Le Puy, France, rises79 m above the surrounding surface.Workers on the Chapel of Saint Micheld’Aiguilhe had to haul building materialsand tools up in baskets.

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and batholiths, especially, consist of multiple intrusions.In other words, a batholith is a large composite body pro-duced by repeated, voluminous intrusions of magma inthe same region. The coastal batholith of Peru, for in-stance, was emplaced during a period of 60 to 70 mil-lion years and is made up of as many as 800 individualplutons.

The igneous rocks that make up batholiths aremostly granitic, although diorite may also be present.Batholiths and stocks are emplaced mostly near conver-gent plate boundaries during episodes of mountainbuilding. One example is the Sierra Nevada batholith ofCalifornia (see the chapter opening photo), whichformed over millions of years during a mountain-building episode known as the Nevadan orogeny. Lateruplift and erosion exposed this huge composite plutonat the surface. Other large batholiths in North Americainclude the Idaho batholith, the Boulder batholith inMontana, and the Coast Range batholith in British Co-lumbia, Canada.

Mineral resources are found in rocks of batholithsand stocks and in the adjacent country rocks. The cop-per deposits at Butte, Montana, are in rocks near themargins of the granitic rocks of the Boulder batholith.Near Salt Lake City, Utah, copper is mined from themineralized rocks of the Bingham stock, a compositepluton composed of granite and granite porphyry.Granitic rocks also are the primary source of gold, whichforms from mineral-rich solutions moving throughcracks and fractures of the igneous body.

HOW ARE BATHOLITHSINTRUDED INTO EARTH’S CRUST?

eologists realized long ago that the originof batholiths posed a space problem. Whathappened to the rock that was once in the

space now occupied by a batholith? One proposed an-swer was that no displacement had occurred, butrather that batholiths formed in place by alteration ofthe country rock through a process called granitiza-tion. According to this view, granite did not originateas magma but rather from hot, ion-rich solutions thatsimply altered the country rock and transformed it intogranite. Granitization is a solid-state phenomenon, soit is essentially an extreme type of metamorphism (seeChapter 7).

Granitization is no doubt a real phenomenon, butmost granitic rocks show clear evidence of an igneousorigin. For one thing, if granitization had taken place,we would expect the change from country rock to gran-ite to take place gradually over some distance. However,

in almost all cases no such gradual change can be de-tected. In fact, most granitic rocks have what geologistsrefer to as sharp contacts with adjacent rocks. Anotherfeature that indicates an igneous origin for granitic rocksis the alignment of elongate minerals parallel with theircontacts, which must have occurred when magma wasinjected.

A few granitic rocks lack sharp contacts and gradu-ally change in character until they resemble the adjacentcountry rock. These probably did originate by granitiza-tion. In the opinion of most geologists, only small quan-tities of granitic rock could form by this process, so itcannot account for the huge volume of granitic rocks ofbatholiths. Accordingly, geologists conclude that an ig-neous origin for almost all granite is clear, but they stillmust deal with the space problem.

One solution is that these large igneous bodiesmelted their way into the crust. In other words, they sim-ply assimilated the country rock as they moved upward(Figure 4.6). The presence of inclusions, especially nearthe tops of some plutons, indicates that assimilationdoes occur. Nevertheless, as we noted, assimilation is alimited process because magma cools as country rock isassimilated. Calculations indicate that far too little heatis available in magma to assimilate the huge quantitiesof country rock necessary to make room for a batholith.

Geologists now generally agree that batholiths wereemplaced by forceful injection as magma moved upward.Recall that granite is derived from viscous felsic magmaand therefore rises slowly. It appears that the magma de-forms and shoulders aside the country rock, and as itrises farther, some of the country rock fills the space be-neath the magma (■ Figure 4.17a). A somewhat analo-gous situation was discovered in which large masses ofsedimentary rock known as rock salt rise through the over-lying rocks to form salt domes (Figure 4.17b–d).

Salt domes are recognized in several areas of theworld, including the Gulf Coast of the United States. Lay-ers of rock salt exist at some depth, but salt is less densethan most other types of rock materials. When under pres-sure, it rises toward the surface even though it remainssolid, and as it moves up, it pushes aside and deforms thecountry rock. Natural examples of rock salt flowage areknown, and it can easily be demonstrated experimentally.In the arid Middle East, for example, salt moving up in themanner described actually flows out at the surface.

Some batholiths do indeed show evidence of hav-ing been emplaced forcefully by shouldering aside anddeforming the country rock. This mechanism probablyoccurs in the deeper parts of the crust where tempera-ture and pressure are high and the country rocks areeasily deformed in the manner described. At shallowerdepths, the crust is more rigid and tends to deform byfracturing. In this environment, batholiths may move up-ward by stoping, a process in which rising magma de-taches and engulfs pieces of country rock (■ Figure 4.18).


G

107how are batholiths intruded into eaeth’s crust?

Salt

Batholith

Countryrock

(a)

(d)

(c)

(b)

■ Figure 4.17

(a) Emplacement of a pluton by forceful injection. As the magma rises, it shoulders aside and deforms the country rock. (b–d) Three stages ina somewhat analogous situation when a salt dome forms by upward migration of the rock under pressure.

(a) (b)

■ Figure 4.18

Emplacement of a batholith by stoping. (a) Magma is injected into fractures and planes between layers in the country rock. (b) Blocks ofcountry rock are detached and engulfed in the magma, thereby making room for the magma to rise farther. Some of the engulfed blocksmight be assimilated, and some may remain as inclusions (Figure 4.6).

According to this concept, magma moves up along frac-tures and the planes separating layers of country rock.Eventually, pieces of country rock detach and settle into

the magma. No new room is created during stoping; themagma simply fills the space formerly occupied by countryrock (Figure 4.18).


a Magma is the term for molten rock below Earth’ssurface, whereas the same material at the surface iscalled lava.

a Silica content distinguishes among mafic (45–52%silica), intermediate (53–65% silica), and felsic(�65% silica) magmas.

a Magma and lava viscosity depends on temperatureand especially on composition: The more silica, thegreater the viscosity.

a Minerals crystallize from magma and lava whensmall crystal nuclei form and grow.

a Rapid cooling accounts for the aphanitic textures ofvolcanic rocks, whereas comparatively slow coolingyields the phaneritic textures of plutonic rocks.Igneous rocks with markedly different sized miner-als are porphyritic.

a Igneous rock composition is determined largely bythe composition of the parent magma, but magmacomposition can change so that the same magmamay yield more than one kind of igneous rock.

a According to Bowen’s reaction series, cooling maficmagma yields a sequence of minerals, each ofwhich is stable within specific temperature ranges.Only ferromagnesian silicates are found in the dis-continuous branch of Bowen’s reaction series. Thecontinuous branch of the reaction series yields onlyplagioclase feldspars that become increasinglyenriched in sodium as cooling occurs.

a A chemical change in magma may take place asearly-formed ferromagnesian silicates form and,because of their density, settle in the magma.

a Compositional changes also take place in magmawhen it assimilates country rock or one magmamixes with another.

a Geologists recognize two broad categories ofigneous rocks: volcanic or extrusive and plutonic orintrusive.

a Texture and composition are the criteria used toclassify igneous rocks, although a few are definedonly by texture.

a Crystallization from water-rich magma results invery large minerals in rocks known as pegmatite.Most pegmatite has an overall composition similarto granite.

a Intrusive igneous bodies known as plutons vary intheir geometry and their relationships to countryrock: Some are concordant, whereas others are dis-cordant.

a The largest plutons, known as batholiths, consist ofmultiple intrusions of magma during long periodsof time.

a Most plutons, including batholiths, are found at ornear divergent and convergent plate boundaries.

G E ORECAP4

Chapter Summary

109review questions

aphanitic texture (p. 95)assimilation (p. 94)batholith (p. 105)Bowen’s reaction series (p. 91)concordant pluton (p. 101)country rock (p. 94)crystal settling (p. 93)dike (p. 101)discordant pluton (p. 101)felsic magma (p. 89)igneous rock (p. 89)intermediate magma (p. 89)

laccolith (p. 104)lava flow (p. 89)mafic magma (p. 89)magma (p. 89)magma chamber (p. 91)magma mixing (p. 94)phaneritic texture (p. 95)pluton (p. 100)plutonic (intrusive igneous) rock(p. 89)

porphyritic texture (p. 95)pyroclastic (fragmental) texture(p. 95)

pyroclastic materials (p. 89)sill (p. 104)stock (p. 105)stoping (p. 106)vesicle (p. 95)viscosity (p. 90)volcanic neck (p. 105)volcanic pipe (p. 105)volcanic (extrusive igneous) rock(p. 89)

Important Terms

1. A dike is a discordant pluton, whereas a_____ is concordant.a._____batholith; b._____volcanic neck;c._____laccolith; d._____stock; e._____ash fall.

2. An aphanitic igneous rock composed mostlyof pyroxenes and calcium-rich plagioclase isa._____granite; b._____obsidian; c._____rhyolite; d._____diorite;e._____basalt.

3. The size of the mineral grains that make upan igneous rock is a useful criterion for deter-mining whether the rock is _____ or _____.a._____volcanic/plutonic;b._____discordant/concordant;c._____vesicular/fragmental;d._____porphyritic/felsic;e._____ultramafic/igneous.

4. Magma characterized as intermediatea._____flows more readily than mafic magma;b._____has between 53% and 65% silica;c._____crystallizes to form granite and rhyo-lite; d._____cools to form rocks that make upmost of the oceanic crust; e._____is one fromwhich ultramafic rocks are derived.

5. The phenomenon by which pieces of countryrock are detached and engulfed by risingmagma is known asa._____stoping; b._____assimilation;c._____magma mixing; d._____Bowen’s reac-tion series; e._____crystal settling.

6. An igneous rock that has minerals largeenough to see without magnification is saidto have a(n) _____ texture and is probably_____.a._____laccolithic/pegmatite;b._____fragmental/felsic;c._____isometric/magmatic; d._____phaneritic/plutonic; e._____fragmental/obsidian.

7. Which one of the following statements aboutbatholiths is false?a._____They consist of multiple voluminousintrusions; b._____They form mostly at con-vergent plate boundaries during mountainbuilding; c._____They consist of a variety ofvolcanic rocks, but especially basalt;d._____They must have at least 100 km2 ofsurface area; e._____Though locally concor-dant, they are mostly discordant.

Review Questions


8. An igneous rock characterized as a porphyryis onea._____that formed by crystal settling and as-similation; b._____possessing minerals ofmarkedly different sizes; c._____made uplargely of potassium feldspar and quartz;d._____that forms when pyroclastic materialsare consolidated; e._____resulting from veryrapid cooling.

9. Which pair of igneous rocks have the sametexture?a._____basalt-andesite; b._____granite-rhyo-lite; c._____pumice-obsidian;d._____tuff-diorite; e._____scoria-lapilli.

10. One process by which magma changes com-position isa._____crystal settling; b._____rapid cooling;c._____explosive volcanism; d._____fracturing; e._____plate convergence.

11. Two aphanitic igneous rocks have the follow-ing compositions: Specimen 1: 15% biotite;15% sodium-rich plagioclase, 60% potassiumfeldspar, and 10% quartz. Specimen 2: 10%olivine, 55% pyroxene, 5% hornblende, and30% calcium-rich plagioclase. Use Figure 4.9to classify these rocks. Which would be thedarkest and most dense?

12. How does a sill differ from a dike? (A diagramwould be helpful.)

13. How do crystal settling and assimilation bringabout compositional changes in magma?Also, give evidence that these processes actu-ally take place.

14. Describe or diagram the sequence of eventsthat lead to the origin of a volcanic neck.

15. Describe a porphyritic texture, and explainhow it might originate.

16. Why are felsic lava flows so much more vis-cous than mafic ones?

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17. How does a pegmatite form, and why aretheir mineral crystals so large?

18. Compare the continuous and discontinuousbranches of Bowen’s reaction series. Why arepotassium feldspar and quartz not part of ei-ther branch?

19. You analyze the mineral composition of athick sill and find that it has a mafic lowerpart but its upper part is more intermediate.

Given that it all came from a single magmainjected at one time, how can you account forits compositional differences?

20. What kind(s) of evidence would you look forto determine whether the granite in abatholith crystallized from magma or origi-nated by granitization?

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igneous rocks and intrusive igneous activity

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