F7 I..-* .a &. :* -,. ., h .,,,..: sb.:

. : E ' ' . ..,. h..

igneous processes that take place undergrbund. However, you will learn early in this chapter how volcanic as well as intrusive rocks are classified based on their grain size and mineral content. The chapter continues with an explana- tion of the structural relationships between bodies of intrusive rock and other

rocks in the earth's crust. This is followed by a discussion of how magmas form and are altered. We conclude by discussing various hypotheses that relate

igneous activity to plate tectonic theory.

If you go to the island of Hawaii, you might observe red hot lava flowing over the land, and, u it cools, solidifying into the fine-grained, black rock we call basalt. Basalt is an igneous rock, rock that has solidified from magma. Magma is molten rock, usually rich in silica and containing dissolved gasses. (Lava is magma on the earth's surface.) Igneous rocks may be either extrusive if they form at the earth's surface (e.g., basalt) or intrusive if magma solidifies underground. Grpnitc, a coarse-grained rock composed predominantly of feldspar and quartz, is an intrusive rock. In fact, granite is the most abun- dant intrusive rock found in the continents.

Unlike the volcanic rock in Hawaii, nobody has ever seen magma solidify into intrusive rock. So what evidence suggests that bodies of granite (and other intrusive rocks) solidified underground from magma? (1) Mineralogically and chemi- cally, intrusive rocks are essentially identical to volcanic rocks. (2) Volcanic rocks are fine-grained (or glass) due to their rapid solidification; intrusive rocks are generally coarse-grained, which is inferred to mean that the magma crystallized slowly underground. (3) Experiments have confirmed that most of the minerals in these rocks can form only at high tempera- tures. Other experiments indicate that some of the minerals could have formed only under high pressures, implying they were deeply buried. More evidence comes from examining inm*rivc contacts, such as shown in figures 11.1 and 11.2. (A contact is a surface separating different rock types. Other types of contacts are described elsewhere in this book.) (4) Preexisting solid rock, counny rock, appears to have been forcibly broken by an intruding liquid, with the magma flow- ing into the fractures that developed. Country rock, inciden-

/ 'Chill zone " B a W zone

Figure 1 I .1 Igneous rock apparently intruded preexisting rock (country rock) as a liauid.

rlgum i i .s Granite (light-colored rock) solidifled from magma that intruded dark-colored country rock. Torres del Paine, Chile (see also this book's cover photo). Photo by Kay Kepler

tally, is an accepted term for any older rock into which igneous body intruded. (5) Close examination of the coun rock immediately adjacent to the intrusive rock usually in cates that it appears "baked (metamorphosed is the corr term) close to the contact with the intrusive ro types of the country rock often match xenoliths, fragments rock that are distinct from the body of igneous rocks in whi they are enclosed. (7) In the intrusive rock adjacent to co tacts with country rock are chill zones, finer-grained roc that indicate magma solidified more quickly here because the rapid loss of heat to cooler rock.

Although laboratory experiments have our understanding of how igneous rocks form, g not been able to create in the laboratory an a tical to granite. Only very fine-grained rocks c minerals of granite have been made from artifici "melts." The temperature and pressure at which gr ently forms can be duplicated in the laborato time element. According to calculations, magma requires over a million years to solidify This very gradual cooling causes the coarse-grained t most intrusive rocks. Chemical processes involving sil known to be exceedingly slow. Yet another probl to apply experimental procedures to real rocks is the role of water vapor and other gases in the crystallization rocks such as granite. Only a small amount of gases retained in rock crystallized underground from a magma, but large amounts of gas (especially water vapor) are released dur-

. ing volcanic eruptions. No one has seen an intrusive rock forming; hence we can only speculate about the role these gases might have played before they escaped. One example shows why gases are important. Laboratory studies have shown that granite can melt at temperatures as low as 650°C if water

Chapter 11

oarse-grained texture characteristic of plutonic rock. Plagioclase is white and quartz is gray; potassium feldspar has been stained ow. Small gradations on ruler are millimeters. (6) A similar rock seen through a polarizing microscope. Note the interlocking crystal ins of individual minerals.

B!! tos by C. C Plummer

resent and under high pressure. Without the water, the mperature is several hundred degrees higher. Not

sent during crystallization temperatures difficult and

'D ve rocks are fine-grained rocks, in which most of the are smaller than 1 millimeter. This is because magma

rapidly at the earth's surface and large crystals do not intrusive rocks are also fine-grained; dies that apparently solidified near

e upon intrusion into relatively cold country rock within a couple kilometers of the earth's surface).

andcrite, and rhyolite are the common fine-grained formed at considerable

ral kilometers-are called to, the Roman god of the under-

Characteristically, these rocks are coarse-grained, lidification of magma. For

d rocks are defined as those in larger than 1 millimeter. The

ocks are commonly interlocked

e rocks are porphyn'tic and have larger crystah 'n a much finer-grained matrix (as described more apter 10). If the matrix is fine-grained, extrusive rock

are used. For instance, figure 11.5E shows apophyritic

andcsitc. Porphyritic extrusive rocks are usually interpreted as having begun crystallizing slowly underground followed by eruption and rapid solidification of the remaining magma at the earth's surface.

Identification of Igneous Rocks - Igneous rock names are based on texture (notably grain size) and mineralogical composition (which r e f l a chemical com- position). Mineralogically (and chemically) equivalent rocks are granite-rhyolite, diorite-andrritc, and gabbro-basalt. The relationships between igneous rocks are shown in figure 11.4.

Because of their larger mineral grains, plutonic rocks are easier to identify than extrusive rocks. The physical properties of each mineral in a plutonic rock can be determined more readily. And, of course, knowing what minerals are present makes rock identification a simpler task. For instance, gabbm is formed of coarse-grained ferromagnesian minerals and gray, plagioclase feldspar. One can positively identify the feldspar on the basis of cleavage and, with practice, verify that no quara is present. Gabbro's fine-grained counterpart is b d t , which is also composed of ferromagnesian minerals and plagioclase. However, the individual minerals cannot be identified by the naked eye and one must use the less reliable attribute of color-basalt is usually dark gray to black.

As you can see from figure 11.4, granite and rhyolite are composed predominantly of feldspars (usually white or pink) and quartz. Granite, being coarse-grained, can be pos- itively identified by verifying that quartz is present. Rhyolite

Igneous Rocks, Intrusive Activity, and the Origin ofIprous Rocks

I

. . i> 45% SO* Classification chart for the mod

I common Igneous rocks. Rock ' names based on special textu are not shown. Sodlum-rich piagioclase is associated with siiioic rocks whereas calcium- ?

rich plagloolase is associated with mafic rocks. The names of the particular ferromagnesian minerals indicate the

SlLlClC ULTRA- approximate composition of the, (FELSIC) INTERMEDIATE MAFlC M AFlC rocks in which they are most

I I I I likely to be found.

is usually cream-colored, tan, or pink. Its light color indi- . appearance is intermediate berween light-colored rhyolite cates that ferromagnesian minerals are not abundant. Dio- and dark basalt. \

rite and andcsitc are composed of feldspars and significant Use the chart in figure 11.4 along with table 11.1 to iden- amounts of ferromagnesian minerals (30-50%). The miner- tify common igneous roch. You may also find it helpful to als can be identified and their percentages estimated to indi- turn to Appendix B, which includes a key for identifying com- cate diorite. Andesite, being fine-grained, can usually be mon igneous rocks. (Photos of typical igneous rocks are shown identified by its medium-gray or medium-green color. Its in figure 11.5.)

(porphyrltlc)

Igneous Rocks, Intrusive Am'uirjr and the Origin ofIgntow Rocks

Varieties of Granite Granite and rhyolite occupy a larger area in the classification chart than do the other rocks. This reflects their greater varia- tion in composition. For instance, a granite whose composition corresponds to the right side of the field in figure 11.4 (that is, nearer to dioritelandesite) contains much more plagioclase than potassium feldspar and somewhat more ferromagnesian miner- als than does a rock whose composition plots in the left side of the granite field. Geologists have arbitrarily subdivided the k l d of granite and named each of the varieties; a rock in the right portion of the field, for example, is called granodiorite.

Any classification system is, of course, a human device, and for this reason, classification systems differ somewhat among groups of geologists. We define the boundary between granite and diorite by the presence or absence of quartz; but we could just as easily have placed the boundary slightly to the left, so that a rock with 10% or less quartz would be diorite.

Chemistry of Igneous Rocks The chemical composition of the magma determines which minerals and how much of each will crystallize when an igneous rock forms. For instance, the presence of quartz in a rock indicates that the magma was enriched in silica (SiO,).

Chapter 11

The lower part of figure 11.4 shows the relationship of che cal composition to rock type. Chemical analyses of ro reported as weight percentages of oxides (e.g., SiO,, NazO, etc.) rather than as separate elements (e.g., Si, 0, Na). For virtually all igneous rodis, SiO (silica) is the abundant component. The amount of s ib , varies from a 45% to 75% of the total weight of common volcanic roc& The variations between these extremes account for striking difZi ferences in the appearance and mineral content of the rocks. 4

Ma& Rocks 5

Rodts with a silica content close to 50% (by weight) are con4 sidered silica-dpficient, even though SiO, is, by far, the m+ abundant constituent. Chemical analyses show that thq remainder is composed mostly of the oxides of aluminum (Al,03), calcium (CaO), magnesium (MgO), and iron (FeO and Fe,O,): (These oxides generally combine with SiO, to form the s~llcate minerals as described in chapter 9.) Rocks id this group are called mofic-silica-deficient igneous rocks with a relatively high content of magnesium, iron, and calcium. (The term majc comes from magnesium and ferric.) Basalt and gabbro are, of course, maiic rocks.

phase of the plutorr Fluids, n&biy-% dioc.wd&eho Many rare clements are mined from pegmatires. These original magma are left over as well. If no fioaun above dements were not absorbed by the minerals of the main the pluton p m i w the fluids to e s q . Ih arc ecsltd in, pluton and so were concentrated in the residual pegmatitic as in a p s u r e cooker. The watery rcsid 3 magma h a magma, where they crystallized as constituents of unusual low viscosiy which allows appropriate atoms to migrate ' minerals. Minerals containing the element lithium are easily toward growing er).rrplr. The crystals add more and mined from pcgmatites. Lithium becomes part of a sheet

=more atoms te structure to form a pink or purple varietv of mica I # . w . .

atice d e s arc gcnedy quite small. Many q (called Iepidolite). ~ranium'ores, similarly conce;luated in mcturcp, locared either within the upper portion the residual melt of magmas, are also exrracted from peg- te pluton or within the overlying country d matite~.

e concact with granite, the fluid body evidmdy h v - Some pegmatite are mined for gemstones. Emerald and ened into the country rock before sokdifging. Peg- aquamarine, varieties of the mined beryl, occur in peg- dikes are fairly common, especially within gnnite mates that crystallized from a solution containing the ele-

utons, where they apparcndy filled cracks that dcyelopal ment beryllium. A large number of the world's very rare e already solid granite. Some pegmacites form am31 minerals are found only in pegmatites, many of these in along contacts bcnvcen granite and wunny rock, till- only one known pegmarire body. These rare minerals arc

cracks rhat developed as the cooling granite p l u m con- mainly of interest to collectors and museums. Nydrorhermal veins (described in chapter IS) are closely

Most oeematites contain only auara, Mdsoar ywt rru- A & d to pegmatites. Veins of quartz ar; common in coun- 1 s mi;a."~inerals of conside;at;lc commer;ial valuiuc try rock I&& granite. Many of these arc believed to be

in a few pegmatites. Large crystals of muscovite mica awed by water that emolpes from the magma. S 'ned from pegmatite. Thcse crystals are died solved in du wry hot warcr cokes on the walls of cr " because the cleavage flakes (tens of cenrimemn the water cools while traveling +vard. So

) look like pages. Because muscovite is an orcdienc valuable me& such as p k l s k r , lad. dne, .aB tor, the cleavage sheeo are used in electrical devices, arc depodd with jhe q u ~ m in veins.

. .

(65% or more of SiO,) all amounts of the oxides of he remaining 25% to 35% of

um oxide (A1,0,) and oxides of ( q O ) . These are called lilicic ous rocks with a relatively high um (the fll part of the name

r, which crystallizes from the potassium, and silicon oxides; si infcLic is for silica). lite and granite are light-colored because ferromagnesian minerals.

with a chemical content between that offelsic and malic

course, no quartz. Most ultrarnafics are composed of coarse- grained pyroxene and/or olivine. Chemically, these ro& con- tain less than 45% silica.

Note that the chart (figure 11.4) does not include a fine- grained counterpart. This is because ultrarnafic extrusive rocks are restricted to the very early history of the earth and are quite rare. For our purposes they can be ignored.

Some Jtcvnafic rocks form from differentiation (explained later in this chapter) of a basaltic magma at very high tempera- tures. Most ultramafic rocks come from the mantle, rather than from the earrh's crust (see box 2.2).

Where we find large bodies of ultramaiic rocks, the usual interpretation is that a part of the mantle has traveled upward as solid rock.

sified as intermediate rocks. Ana'esite, which is usually or medium gray, is the most common intermediate vol- Intrusive Bodies

Intrusions, or intrusive structures, are bodies of intrusive

Roch rock whose names are based on their size and shape, as well as their relationship to surrounding rocks. They are important

rock is composed entirely or almost entirely of aspects of the architecture, or strplcturc, of the earth's crust. The minerals. No feldspars are present and, of various intrusions are named and classified on the basis of rhe

Igncow Rocks, Intrusive Actiui9 and the Origin ofIgneour Rocks

Layered sedimentary rock

B

Flgum 11.8 (A) Ship Rock in New Mexico, which rlses 420 meters (1,400 feet) above the desert floor. (B) Relationship to the former volcano. Photo by Frank M Hanna

276 . .

Chapter 11 hnp//wwu~n ~n/crli

-

ther, and fat more common, intrusive stmc- can also be seen at ShipAock. The low, wall- ridge extending outward &om Ship Rock is roded dike. A dike is a tabular (shaped like a top), discordant, intrusive structure (figure ). Discordant means that the bodv is not

1 to any layering in the country rock. f a dike as cutting across layers of coun- Dikes may form at shallow depths and ained, such as those at Ship Rock, or

ter depths and be coarser-grained. not appear as walls protruding from (figure 11.8). The ones at Ship Rock

ecause they are more resistant to

Dlker (l@ht-colored rmks) in northern Victoria Land, Antarctica. Photo by C. C. Plummer

e country rock is not

ives That CryS&..* Oqdk is a body of magma or ignsub rock d S i l t , q

erable depth within the crust. M a s p k k & b no

particular shape, unlike d ik s and sills. Where plutons are exposed at the earth's surface, they are arbitrarily distinguished by size. A stock is a small discordant pluton with an outcrop area (i.e., the area over which it is exposed to the atmosphere) oflus than 100 square kilometers. If the outcrop area is greater than 100 square kilometers, the body is called a batholith (fig- ure 11.10), a large discordant pluton.

Most batholiths crop out over areas vastly greater than the minimum 100 square kilometers.

Although batholiths ofren contain m&c and intermediate &, they almost always are predominantly composed of gran- ite. Detailed studies of batholiths indicate that they are formed

Ignaw M, inirusim Activirjl and thr Origin ofIpeovr Rmkr

of numerous. coalesced olutons. A~oarentlv.

I rounding rock that is shouldered aside as the

I '2

magma rises. Batholiths occupy large portions of North America, particularly in the west. Over half of California's Sierra Nevada mountains (fig- ure 11.12 and map, figure 1.5) is a batholiFh

I whose individual plutons were emplaced during a period of over 100 million years. An even larger batholith extends almost the entire lenmh of the

I 0

mountain ranges of Canada's west coast and southeastern Alaska-a distance of 1,800 kilo- meters. Smaller batholiths are also found in the Appalachian Mountains in eastern North Amer- ica. (The extent and location of North American = batholiths are shown on the geologic map on the inside cover.)

- .--.- -1.9 Sill (dark rock) in sedimentary layered rock, Grand Canyon, Arizona.

Chapter 11

-Earth's surface

-Country rock

arth's former sulli

Figure 11.10 (A) The first of numerous magma diapirs has worked its way upward and is emplaced in the country rock. (6) Other magma diapirs have intruded, coalesced, and solidified into a solid mass of plutonic rock. (C) After erosion, surface exposures of plutonic rock are a batholith and a stock.

mmer

Figure 11 .ll Diap~rs of magma travel upward from the lower crust and solldlfy in the upper crust.

I Granite is considerably more common than rhyolite, its

volcanic counterpart. Why is this? Silicic magma is much more uircous (that is, more resistant to flow) than m&c magma. Therefore, a silicic magma body will travel upward through the crust more slowly and with more difficulty than malic or inter- mediate magma. Unless it is exceptionally hot a silicic magma will not be able to work its way through the relatively cool and rigid rocks of the upper few kilometers of crust. Instead, it is much more likely to solidiFy slowly into a pluton.

Abundance and Distribution of Plutonic Rocks Granite is the most abundant igneous rock in mountain ranges. It is also the most commonly found igneous rock in the interior lowlands of continents. Throughout the lowlands of much of Canada, very old plutons have intruded even older metamorphic rock. As explained in chapter 5, very old moun- tain ranges have, over time, eroded and become the stable inte- rior of a continent. Metamorphic and plutonic rocks similar in age and complexity to those in Canada are found in the Great Plains of the United States. Here, however, they are mostly covered by a veneer (a kilometer or so) of younger, sedimentary rock. These "basement" rocks are exposed to us in only a few places. In Grand Canyon, Arizona, the Colorado River has eroded through the layers of sedimentary rock to expose the ancient plutonic and metamorphic basement. In the Black Hills of South Dakota, local uplift and subsequent erosion has exposed similar rocks.

Granite, then, is the predominant igneous rock of the continents. As described in chapter 1 10, basalt and gabbro are the predominant rocks underlying the oceans. Andesite (usually along continental margins) is the building material of most young volcanic mountains. Underneath the crust, ultramafic rocks make up the upper mantle.

I -

If a rock is heated sufficiently, it begins melt- ing to form magma. Under ideal conditions, rock can melt and yield a granitic magma at temperatures as low as 625°C. Temperatures

Part of rhe S~erra Nevaaa batnol~tn. Al. Ifght- colored rock shown new (inclxl ng ma1 ~naer the d slant snow-covered mountarns) is granite. P-010 O, c c P . T ~ V

Igneow Rocks, I n m r v e Activrty, and the Orrpn oflgnrow Rocks

Flgum I I .I 3 Geothermal gradients at two parts of the earth's crust.

over 1,000'C are required to create basaltic magma. How- ever, there arc several factors that control the melting tem- perature of rock. Pressure, amount of gas (particularly water) present, and the particular mix of minerals all influence when melting takes place. These factors are discussed later in this chapter.

Heat for Melting Rock Most of the heat that contributes to the generation of magma comes from the very hot earth's core (where temperatures are estimated to be greater than 5,000°C). Heat is conducted toward the earth's surface through the mantle and crust. This is comparable to the way heat is conducted through the wall from a hot room into a cooler room or through the metal of a frying pan. Heat is also brought from the lower mantle when part of the mantle flows upward, either through convection (described in chapter 1) or by hot mantle plumes. The geother- mal gradient, described below, is a manifestation of heat trans- fer in the mantle.

Geothermal Gradient A miner descending a mine shaft notices a rise in tempera- ture. This is due to the geothermal gradient, the rate at which temperature increases with increasing depth beneath the surface. Data show the geothermal gradient, on the aver- age, to be about 3°C for each 100 meters (3O0C/km) of depth in the upper part of the crust. The geothermal gradi- ent is not the same everywhere. Figure 11.13 shows geother- mal gradients for two regions. The curve for the volcanic

region indicates a higher geothermal gradient than that the continental interior. Temperatures high enough to me rock would be expected at a relatively shallow depth the volcanic region. You would have to go deeper in tinental interior to reach the same temperature; however, t rock there does not melt because of the increased pressure at that depth.

One reason for a higher geothermal gradient is tha deeper, and therefore hotter, mantle rock has worked its way upward closer to the earth's surface due either to mantle con- vection or mantle plumes. "Hot spots" in the crust (where the geothermal gradient is locally very high) are believed to caused by hot mantle plumes, which are narrow upwellings hot material within the mantle. Hot mantle plu account for some igneous activity, such as the oceanic erup tions that built up the Hawaiian Islands. Volcanism in the middle of continents may also be attributable to mantle plumes. Yellowstone National Park is a product of silicic emp- tions. The eruptions were much larger and more violent th& any of historical time. Geologists attribute these eruptions to a hot mantle plume that caused melting of the crust beneath thii area.

Factors That Control Melting Temperatures

Pressure 1 The melting point of a mineral generally incwaes with increas- ing pressure. Pressure increases with depth in the earth's crust, just as temperature does. So a rock that melts at a given tem- perature at the surface of the earth requires a higher tempera- ture to melt deep underground. Rock will not melt where the geothermal gradient for the plate interior is applicable, because the melting temperature is always going to be higher than the temperature of rock at any given depth.

Hawaiian volcanic activity attributable to the underly mantle plume illustrates how reduced pressure contributes the creation of magma. Solid rock that was once very deep the mantle (and, therefore, very hot) has worked its upward. Most of its heat has been retained during the up journey. However, the pressure decreases as the rock body els upward. As it approaches 50 kilometers or so fro earth's surface, pressure is sufficiently reduced so that melting takes place.

Water Under 13.essure If enough gas, especially water vapor, is present and under high pressure, a dramatic change occurs in the melting process. Water vapor sealed in under high pressure by overlying rocks helps break down crystal structures. High water pressure can significantly lower the melting points of minerals. (Figure 11.14 shows the relationship between water pressure and the melting temperature of a plagioclase.)

Chapter I I

E*periments have ahown that, uadq moderately high p m r ~ votsr mixed with gmnite lowers tbe melting point o f granite from ovn 9PLI.C (when dry) to as low a 65VC whcfi s a w d with water under pnssun equivalent m that o f 10,000 atmosphere or kn. (P- at depth k usually erp"ed in kilobats; one kilobar is equal m l,Q00 bars.) -

s f ~ i x c c d ~ i t t d ' ' ' ?vc &as in soWcr-wn be mixed in a rado that lowers their mdcing tempeurarre far below that o f the melting points o f

PI.ui. 11.14 MUng tmperature of a mineral relative to water pressure. If no water is present t m l u r e s to the right of the red line are required to melt plagioclase at carreaponding pressures. Note that if water pressure is zero, a temperature of 1,100@ Is required for the mineral to melt, whereas with 2 kllobars of water pressure, the mineral w~ll melt at temperatures just over 800%. After Tuttle and Brown, Geologlc Soclety of Amerlca, 1958

I p m R&, Inmrrivc Activit~ and the Origin ofIpeour Rocks

1500'C -

.. 1300'

1 looo

/Melting begins above this temperature

75% Quartz 75% 58% 25% Potassium

feldspar

Figure 11.15 Melting temperatures for mixtures of quartz and potassium feldspar at atmospheric pressure. Modified from Schairer and Bowen, 1956. V 254. p. 16. American Journal of Science.

the pure metals. Minerals behave similarly. Experiments have shown that in some cases mixed fragments of two min- erals melt at a lower temperature than either mineral alone. Figure 11.15 shows the melting temperatures for quartz and potassium feldspar mixed in various propor- tions. If the mixture is 42% quartz (58% potas- sium feldspar), melting takes place at just above 1,00O0C. On the other hand, 1,50O0C is needed to liquify a mixture of 75% quartz and 25% potassium feldspar (corresponding to the right edge of the diagram). Pure quartz requires even higher temperatures to melt. I How Magmas of Different I A major topic of investigation for geologists is why igneous rocks are so varied in composition. On a global scale magma composition is clearly

often show considerable variation-in rock type. For instance. individual olutons micallv disolav

amounts of gabbm or diorite. In chis section, we d processes that result in differences in composition of m The following section relates these processes to plate tecto for the larger view of igneous activity.

Differentiation and Bowen's Reaction Theory Differentiation is the process by which different ingredien separate from an originally homogenous mixture. An ex ple is the separation of whole milk into cream and n milk. In the early part of the twentieth century, N. L. B conducted a series of laboratory experiments demonstratln that differentiation is a plausible way for silicic and m rocks to form from a single parent magma.

Bowen'a reaction series, shown in figure 11.16, is th sequence in which minerals crystallize from a magma, as demonstrated by Bowen's laboratory ments. In simplest terms, Bowen's reaction series shows those minerals with the highest melting temperatures crys tallize from the cooling magma before those with lowe melting points. However, the concept is a bit more compli cated than that.

Crystallization begins along two branches, the discontin uous branch and the continuous branch. In the discontinuo branch, one mineral changes to another at discrete temp tures during cooling and solidification of the mag Changes in the continuous branch occur gradation through a range in temperatures and affect only the on

,. , L , a considerable range ofcdmpositions, mostly vari- Fieum I I .IS eties of granite, but many also will contain minor Bowen's reaction series.

Chapter I I

mineral, plagioclase. Although crystallization takes place simultaneously along both branches, we must explain each separately. i Discontinuow Branch All the minerals in the discontinuous branch are ferromagne- sian. In this branch, as the completely liquid magma slowly cools, it reaches the temperature at which olivine begins to crysrallize from the magma. Olivine is a mineral with an excep- tionally high proportion (2:l) of iron and magnesium to

chains of tetrahe- ne, with a formula of

ount of silicon rela- 1 to 1. After all of

to form pyroxene, the ase and pyroxene will

basaltic, all of the liquid re all of the olivine has reacted

rock formed would have only nesian minerals, which

t crystallized simultaneously in be a basalt. If, on the other

d, the original melt were more silicic or if early formed fer- sian minerals were removed from the melt, there

till be melt left after pyroxenc crystallized and the next (amphibok) could crystallize.

ning when the crystallization ched, pyroxene reacts with nges into amphibole's dou- drons. More of the silicon um, calcium, and minor ted into the newly devel-

phibole has formed, on further cool- the melt to produce biotite (which is he last of the ferromagnesian miner-

a remaining after biotite has fin- ry little iron or magnesium.

o n t i n u o w ~rancib

feldspars, combine with calcium and sodium to form plagio- clase. Calcium-rich plagioclase will crystallize first and, upon slow cooling, increasingly more sodic plagioclase will crystallize. If a basaltic melt, which is enriched in calcium relative to sodium, is cooled slowly, a very calcium-rich pla- gioclase will crystallize first. With progressive cooling, the plagioclase crystals react with the melt and grow larger. The growing plagioclase crystals will have an increasingly higher amount of sodium relative to calcium. Crystallization will stop when the plagioclase crystals have the same calcium-to- sodium ratio as did the original magma. In the case of a basaltic magma this will be at a fairly high temperature (approximately the temperature at which pyroxene crystal- lizes in the discontinuous branch). If there is a lower ratio of calcium to sodium (or if calcium-rich plagioclase is removed from the melt), plagioclase will continue to crystallize through lower temperatures.

Any magma left after the crystallization is completed along the two branches is richer in silicon than the original magma and also contains abundant potassium and alu- minum. The potassium and aluminum combine with silicon to form potasrium fcldcpar. (If the water pressure is high, mwcouite may also form at this stage.) Excess Si02 crystal- lizes as quartz.

Normally a newly erupted cooling basalt lava progresses only a short distance down the reaction series before all the magma is consumed by growing crystals. Olivine dwelops, but only part of it reacts with the melt to form pyroxene before all the magma is solidified. Simultaneously, calcium-rich plagio- clase grows and becomes increasingly sodic; but its growth ceases when all liquid is consumed. The rock becomes a com- pletely solid aggregate of calcium-rich plagioclase, pyroxene, and ol ivinein other words, what one expects to find in a basalt.

However, Bowen used this experimentally determined reaction series to support the hypothesis that all magmas (ma&, intermediate, and silicic) derive from a single parent (mafic) magma by differentiation. The early-dweloping min- erals are separated from the remaining magma. These minerals collectively result in a rock that is more mafic than the original magma. The remaining magma is deficient in iron, magne- sium, and calcium; therefore, upon cooling, it solidifies into a silicic or intermediate rock.

Crystal Settling - Only if the original basaltic magma cools slowly, and the earliest-formed minerals physically separate from the magma, can the minerals on the lower part of the reaction series crys- tallize. Crystal settling is the downward movement of min- erals that are denser (heavier) than the magma from which they crystallized. What is pictured happening is that as the olivine crystallizes from the magma, the crystals settle to the

. . . : . . .r': ~~

. . ". . . ~ . ~- . ...

Inirusiuc Actiuitqr and the Origin of &mow Rock$

Cross section showing the process of differentiation by crystal settling in a sill and dike. ( A ) Recently intruded magma is completely liquld. (6) uponslow cooiing, mineralskuch as olivine crystallize first. (C) The heavier crystals that formed early sink, leaving the remaining magma depleted in mafic constituents.

bottom of the magma chamber (figure 11.17). Calcium-rich plagioclase also separates as it forms. The remaining magma is, therefore, depleted of calcium, iron, and magnesium. Because these minerals were economical in using the rela- tively abundant silica, the remaining magma becomes richer in silica as well as in sodium and potassium. If enough mafic ingredients are removed in this manner, the remaining residue of magma eventually solidifies into a granite.

Undoubtedly this method of differentiation does take place in nature, though probably not to the extent that Bowen envisioned. The lowermost portions of some large sills are composed predominantly of olivine, whereas upper levels are considerably less mafic. Even in large sills, however, differentia- tion has rarely progressed fir enough to produce any granite within the sill.

If we assume that the maiic minerals settle ever deeper in large magma bodies, there is still a problem in trying to explain the origin of granite by Bowen's theory. Calculations show that to produce a given volume of granite, about ten times as much maiic rock first has to form and settle out. If this is true, m would expect to find far more mafic plutonic rock than granite in the continental crust.

This is not to say that Bowen's work is discredited. Quite the opposite. His work has led to other theories on the behav- ior of magmas. Moreover, differentiation does occur and can explain relatively minor compositional variations within intru- sive bodies, even if it does not satisfactorily explain the origiq of large granite bodies.

Ore Deposits Due to Crystal Settling Crystal settling accounts for important ore deposits that are mined for chromium and platinum. Most of the world's chromium and platinum come from a huge sill in South Africa. The sill, the famous Bushveldt Complex, is 8 kilome- ters thick and 500 kilometers long. Layers of chromite (a chromium-bearing mineral) up to 2 meters thick are found, and mined, at the base of the sill. Layers containing platinum overlie the chromite-rich layers.

Partial Melting A granitic magma could be created by partial melting ok rock-visualize a progression upward through part of Bowen's reaction series (going from cool to hot). As might be expected, the first portion of a rock to melt as tempera- tures rise forms a liquid with the chemical composition of quartz and potassium feldspar. The oxides of silicon plus potassium and aluminum "sweated out" of the solid rock could accumulate into a pocket of felsic magma. If higher temperatures prevailed, more mafic magmas would be crc- ated. Small pockets of magma could merge and form a large enough mass to rise as a diapir. The lower part of the conti- nental crust is a very plausible source for felsic magma gen- erated by partial melting.

Geologists generally regard basaltic magma (Hawaiian lava, for example) as the product of partial melting of ultra- mafic rock in the mantle, at temperatures hotter than those in the crust. The solid residue left behind in rhe mantle when the basaltic magma is removed is an even more silica-deficient ultramaiic rock.

Chapter I I

ed~ate in composition absorbed country rock.

ocks of country rock (the iiths of country rock with magma melt. (C) The original magma, leaving

of the country rock and i d into the magma (ftgun ce cubes into a cup of hot cools w it brim diiund.

a, perhaps merated fmm the ntinentd crust, the mqma

Ignmu Roc&, ,

Sfi~cs magma movlnp slowly upward

Mafic mapma moving rapialy

C

Figure 11.19 Mixing of magmas. (A ) Two bodies of magma moving suffaceward. (6) The mafic magma catches up with the silicic magma. (C) The twu magmas combine and become an Intermediate magma.

simultaneously becomes richer in silica and cooler. Possibly intermediate magmas such as are associated with circum- Pacific andesite volcanoes may &rive from assimilation of some crustal rocla by a basaltic magma.

Mixing of M a p a s The idea that some of our igneous rocks may be ' ' ccdds" of different magmas is cur rend^ receiving more attention by geolo- gists than it did in the past. The concept is quite simple. If two magmas meet and merge within the crust, the combined magma will be compositiondy intermediate (figure 11.19). If you had appmximatdy equal amounts of a granitic magma mixing with a basaltic magma, the resulting magma should crysdizc under- ground as diorite or erupt on the s& to solidify as andesite.

InmrrivtActivir)r and the Origin ofIpoous Rocks

Explaining Igneous Activity by Plate Tectonics a One of the appealing aspects of &%theory of plate tectonics is that it accounts reasonably well For the variety of igneous rocks and their distribution patterns. Divergent boundaries are associated with creation of basalt and gabbro of the oceanic crust. Andesite and granite am associated with conver- gent boundaries.

Igneous Processes at Divergent Boundaries Geologists agree that basaltic magma produced at divergent boundaries is due to partial melting of the asthenosphere. The asthmosphcre, as described in chapter 1, is the plastic zone of the mantle beneath the rigid lithsphm (the upper mantle and crust that make up a plate). Along divergent boundaries, the asthenosphere is relatively close (5 to 10 kilometers) to the sur- face (fimrc 1 1.20). . -. . \..- -. . . . - .

The probable reason the asthenosphere is plastic or "soft" is that temperatures there are only slightly lower than the tem- peratures required for partial melting of mantle rock.

If extra heat is added, or pressure is reduced, partial melt- ing should take place. The asthenosphere beneath divergent boundaries probably represents mantle material that has welled upward from deeper levels of the mantle. As the hot astheno- sphere gets dose to the surface, pressure is reduced sufficiently for partial melting. The resulting magma is mafic and will solidify as basalt or gabbro. The portion that did not melt remains behind as a silica-depleted, iron and magnesium enriched ultramafic rock.

Some of the basaltic magma erupts along a submarine ridge to form pillow basalts (described in chapter lo), while some fills near-surface fissures to create dikes. Deeper down, magma solidifies more slowly into gabbro. The newly solidi- fied rock is pulled apart by spreading plates; more magma fills the new fracture and erupts on the sea floor. The process is repeated, resulting in a continuous production of mafic crust.

The basalt magma that builds the oceanic crust is removed from the underlying mantle, depleting the mantle beneath the ridge of much of its calcium, aluminum, and silicon oxides. The unmelted residue (olivine and pyroxene) becomes depleted mantle, but it is still a variety of ultramafic rock. The rigid ultramafic rock, the overlying gabbro and basalt, and any sediment that may have deposited on the basalt wllectively are the lithosphere of an oceanic plate, which moves away from a spreading center over the asthenosphere. (The nature of the oceanic crust is described in more detail in chapter 3.)

Intraplate Igneous Activity - - Hawaiian volcanism is unusual because Hawaii is not at a plate boundary. Most geologists think that basaltic magma there is created because the Pacific plate is overriding a hot mantle plume in a process described and illustrated in chapter 4.

Chapter I I

Fiuure through tlw wlrt

Hot asthenosp rock moves up' /

A

Magma soi~d~fies to basalt (gabbro at depth)

/

Flgum 1 1.20 Schematic representation of how basaltic oceanic crust and tht underlying ultramafic mantle rock form at a dlverging boundary. The process is more continuous than the two-step diagram implies. (A ) Partial melting of asthenosphere takes place benes a mid-oceanic ridge. (6) The magma squeezes into the fissure system. Solid maflc mlnerals are left behind as ultramafic rock.

The huge volume of mafic magma that erupted to fc the Columbia plateau basalts (described in chapter 4) is art] uted to a past hot mantle plume, according to a recent hypc esis (figure 11.21). In this case, the large volume of basal due to the arrival beneath the lithosphere of a mantle plu with a large head on it (sort of a mega-diapir).

The very explosive rhyolitic eruptions at yellow st^ National Park are also credited to a hot mantle plume t caused extensive partial melting of the conrincntal crust. ' resulting silicic magma bodies reached the surface (unlike N granitic-magmas &at solidify as pluths).

h i t p : / / m . mhhc. mm/c~rtks~dgcologY/plum~

- Continental lithosphere

bulged upward and thinned Basalt floods

\ /

nure 11.21 lot mantle plume with a large head rises from the lower mantle. ien it reaches the base of the lithosphere it uplifts and stretches ,overlying lithosphere. The reduced pressure results in partial ~Iting, producing basaltic magma. Large volumes of magma vel through fissures and flood the earth's surface.

peous Processes at Convergent oundaries termediate and silicic magmas are clearly related to the con- rgence of two plates and subduction. However, exactly what res place is debated by geologists. Compared to divergent

boundaries, there is much less agreement about how magmas are generated at converging boundaries. The scenarios that fol- low are currently believed by geologists to be the best explana- tions of the data.

The Origin ofAndesite Magma for most of our andesitic composite volcanoes (such as are found along the west coast of the Americas) seems to origi- nate from a depth of about 100 kilometers. This coincides with the depth at which we would expect the subducted oceanic plate to slide under the asthenosphere (figure 11.22). Partial melting of the asthenosphere takes place, resulting in a malic magma. A plausible reason for the melting to occur is that the subducted oceanic crust releases water into the asthenosphere. The water would have collected in the oceanic crust when it was beneath the ocean and is driven out because of heating of the descending plate. The water lowers the melt- ing temperature of the ultramafic rocks in this part of the man- tle. Partial melting produces a mafic magma. On its slow journey through tLeAcrust, the mafic magma evolves into an intermediate magma by differentiation, assimilation of silicic crustal rocks, and by magma mixing (see box 11.3).

The Origin of Granite To explain the great volumes of granitic plutonic rocks, most geologists think that partial melting of the lower continental crust must take place. The continental crust contains the high amount of silica needed for a silicic magma. As the silicic rocks of the continental crust have relatively low melting tempera- tures (especially if water is present), partial meIting of the lower continental crust is likely. Currently, geologists think that magmatic un&rplating by andesitic or basaltic magma

me processes that may contribute to magma generation at a convergent boundary.

Ignmw Rocks, Inhrrive Actiui9 and the Origin oflgneow Rocks

A B

Flgum I I .23 How mafic magma could add heat to the lower crust and result in partial melting to form a granitic magma. (A) Mafic magma from the asthenosphere rises through closely spaced fissures in the lower crust (wldths are highly exaggerated in diagram). (8) Magmatic underplating of the continental crust.

Chapter 11

plays an important role providing the extra heat source needed ro generate granitic magmas in the lower continental crust. Initially, some of the mafic magma coming from the astheno- sphere works its way through fissures systems in the lower crust (figure 11.23A). As the lower crust gets hotter, the rock becomes more plastic and melting begins. Fissures are sealed. The denser, mafic magma then pools under (underplates) the lighter, partially molten, lowest crust (figure 11.238). Heat from the cooling and crystallizing mafic magma is conducted upward to create larger volumes of silicic magma by partially. melting more of the continental crust. The silicic magma, in

turn, separates from its solid residue and works its way upward in diapirs to a higher level of the crust where it slowly solidifies to a pluton, usually as part of a batholith.

We should emphasize that the picture we have presented is not an observation but a reasoned interpretation of avail- able data. There are a number of variations of this picture based on different interpretations of the data (box 11.3). Plate tectonics has not ~rovided final answers to the rob- lems raised by analyzing igneous phenomena. But it has pro- vided a broad framework in which to work toward solutions to the problems.

Ignrour rocks form from solidification of mapa. If the rock forms at the earth's sur- face it is extrusive. Intrusive rocks are igneous rocks that formed underground. Some intru- sive rocks have solidified near the surface as a direct result of volcanic activity. Volcanic necks solidified within volcanoes. Fine- grained dikes and sills may also have formed in cracks during local extrusive activity. A sill is concordan+parallel to the planes within the country rock. A dike is discordant. Both are tabular bodies. Coarser grains in either a dike or a sill indicate that it probably formed at considerable depth.

Most intrusive rock is plutonic-that is, muse-grained rock that solidified slowly at wnsiderable depth. Most plutonic rock -sed at the earth's surface is in batholitht lvge plutonic bodies with no particular shape. A smaller, irregular body is called a stuck.

Silicic (or felsic) rocks are rich in silica, whereas malic rocks are silica deficient. Most igneous rocks are named on the basis of their .&era1 content, which in turn reflects the t

chemical composition of the magmas from which they formed, and on grain sizes. Granite, diorite, and gabbw are the coarse- gained equivalents of rbolite, andesitc, and barak respectively. Ultramajic rocks are made entirely of ferromagnesian minerals and are mostly associated with the mantle.

Basalt and gabbro are strongly predomi- nant in the oceanic crust. Granite strongly predominates in the continental crust. Younger granite batholiths occur mostly within younger mountain belts. Andesite is largely restricted to narrow zones along con- vergent plate boundaries.

The geothermal gradient is the increase in temperature with increase in depth. Hot mantle plumes, from the lower mantle, and magma at shallow depths in volcanic regions locally raise the geothermal gradient.

No single process can satisfactorily account for all igneous rocks. Several hypothe- ses adequately explain some igneous rocks. In the process of d~@nmtiation, based on Botumi. wanion s&, a residual magma more silicic

than the original m&c magma is created when the early-forming minerals separate out of the magma. In mimilation, a hot, original magma is contaminated by picking up and absorbing rock of a different composition. Magma mix- ing produces a magma whose composition is intermediate, between that of the two types of magma that were mixed.

Partial melting of the mantle usually produces basaltic magma whereas granitic magma is most likely produced by partial melting of the lower crust.

The theory of plate tectonics incorpo- rates various parts of previous theories. Basalt is generated where hot mantle rock partially melts, most notably along divergent boundaries. The fluid magma rises easily through fissures, if present. The ferromagne- sian portion that stays solid remains in the mantle as ultramalic rock. Granite and andesite are associated with subduction. Dif- ferentiation, assimilation, partial melting, and mixing of magmas may each play a part in creating the appropriate rocks.

'Terms to Remembe r

~dcsite 272 d 1 2 7 1 uholith 277 mven's reaction series 282 ill zone 270 wsc-grained rock 271 urcordant 277 bnract 270 tuntry rock 270

crystal reding 283 diapir 278 differentiation 282 dike 277 diorirc 272 discordant 277 extrusive rock 270 fin~-~rained rock 271 gabbm 271

geothermal gradient 280 granite 270 igneous rock 270 intermediate 275 intrusion (intrusive structure) 275 intrusive rock 270 lava 270 mafic 274 magma 270

Ignrour Rocks, Intrusive Activi8 and the Origin ofIpcow Rocks

volcanic ncck 276 xenolith 270

manrlc plum 280 silicic (felsic) 279 . ,

plumn 277 sill 277 . . . , plutonic rock 271 mdr 277

'resting Your Knowledge I Use the . . questiohh ... . below to prepare for q b a s e d dn this chaptc, 14, By definition, stocks differ from batholiths in (a) size (b) sha

1. Why dodom&c magmas tend to nichdreslir..e mudh mote (c) chemical omp position (d) all of the above

o f m than silicic magmas? 15. Which is not a source of heat for melting rock? (a) geother

2. %t r& d&s t & ~ t h e n ~ ~ h ~ f e p s s i b 1 ~ $ay in genendng . . gradient (b) tho hotter mantle ( c ) man& plumcc (dl water under pressure m&aat (a) (a) anverging boundasy: (b) a div6rgirig'bo~ndary?

3. How do batholiths form? 16. The georlrtrmal gradient is, on the average, about (a) 1'CI (b) 1o"Cllua (c) 30nC/km (d) 5CPClkm

4. How. would you distinguish, on the basis ..., of mincrds present, amonggianite, gabbro, and diorirc? 17. Thc continuous branch of Bowen's reaction series contains

mineral (a) (b) plagioclase (c) amphibole (d) biotite 5. Howwould . ... you distinguish andesire from a diorite?

6. What rock would probably f o m if magma that was feeding composite volcanoes solidified at cbnside'nble depth?

7. Why is ahigher tcmpulture i t q u i d to form magma.at the 19. The most common igneous rock of the continents is (a) basal (b) granite (c) rhyolite (d) ultramafic

20. Granite and rhyolite are different in (a) texmre (b) chem' feldspar found in granite? (c) mineralogy (d) the kind oJmagma that each crystallii

9. What is the difference between a dike and a sill? , ,

10. ~escribe the differences be&etl:th c~ntin$o% and the 2 1. The difference in texture between intrusive and extrusive r discontinuous branches of Bowen's reaction series.

11. A surfaceseparating different rock types is called a (a) xenolith (b) contact (c) chill zone (d) none ofthe above magma

12, The major difference between inirusive igneous roch and extrusive ignequs rocks is (a) whet they solidify (b) chemical composition (c) type of minerals id) all of the above texture (d) chemistry

13. Which is not an intrusive igneous rock! (a) gabbro (b) diorite 23. A +ge in:magma composition due to melting of surroun

(c) granke (d) andesite cnuntty rock is dw (a) magmamixing (b) assimilation . . (c) q f a i setting(d) pa& melting

following circumstances: olivine crystals i 1. In parts of major mountain belts there convergent boundary by plate motion,

are sequences of rocks that geologists what rock types would you cxpect to form and only the surface of each interprct as slices of ancient oceanic make up this sequulce, going from the crystal reacts with the melt to form a lithosphere. Assuming that such a top downward? mating of pyroxene that prevents the sequence formed at a divergent 2. What would happen, according to interior of olivine from reacting with boundary and was moved toward a Bowen's reaction series, under the the melt?

Chaptrr I I

Barker, D. S. 1983. Igneouc rock Englewood Cliffs, N.J.: Prentice-

Hall. Blatt, H., and R C. Tracy 1996. Petmlogy: Igneous, Sedrmmtar3: andMetamorphic. 2d ed. New York: W. H. Freeman. Hibbard, M. J. 1995. Petlographj to perrogenesu. Englewood Cliffs, N.J.: Prentice-Hall. MacKenzie, W. S., C. H. Donaldson, and C. Guilford. 1982. A t h of igneous rocks and then textures. New York: Halsted Press.

& hrrp:llnts.~c.~..~.;.edw/-ml '(I Robi Granite Pagc.

This site has a lot of information on granite and related igneous activity. The site is useful for people new to geology as well as for professionals. There are numerous images of granite. Click on "Did you know that granite is like ice cream?" for an interesting comparison. The page also has ~hotos of various gianites and links to other sites that have more images. http://www.geolab.unc.edu/Perunil/ IgMerAtlasimainmenu.html

A t h of rocks, minerah, and textures (from University of North Carolina). This site contains some photomicrographs of plutonic and volcanic rocks. The images are

thin sections (slices of rock so thin that most minerals are transparent) seen in a polarizing microscope. Most images are taken from cross-polarized light, which causes many minerals to appear in distinctive, bright colors. For some of the rocks (gabbro, for instance), you can also see what it looks like under plain polarized light by clicking the cirde with the horizontal, gray lines. http://seis.natsci.csulb.edu/basicgeo/

IGNEOUS-TOUR.htm1 Igneouc Rocks Tour. This site has some hand specimen images of common igneous rocks and should provide a useful review for rock identification.

Igneous Racks, Innnurive Activig and the Or@ ofIgnrow Rocks