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 explanation 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.
F
.a
7
&. :*
.,
*- . . I
.
-,.
'
:
'
h
.
.,,,..:
sb.:
E
..,. h..
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 abundant 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 chemically, 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 temperatures. 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 flowing into the fractures that developed. Country rock, inciden-
" B a W zone
/
'Chill
zone
Figure 1I .1
Igneous rock apparently intruded preexisting rock (country rock)
as a liauid.
Chapter 11
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
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 underCharacteristically, 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 composition). 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 positively identified by verifying that quartz is present. Rhyolite
Igneous Rocks, Intrusive Activity, and the Origin ofIprous Rocks
I
..
45% SO*
i>
I
I
SlLlClC
(FELSIC)
INTERMEDIATE
I
is usually cream-colored, tan, or pink. Its light color indicates that ferromagnesian minerals are not abundant. Diorite and andcsitc are composed of feldspars and significant
amounts of ferromagnesian minerals (30-50%). The minerals can be identified and their percentages estimated to indicate diorite. Andesite, being fine-grained, can usually be
identified by its medium-gray or medium-green color. Its
ULTRAMAFlC
MAFlC
I
I
. appearance
Classification chart for the mod
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
approximate composition of the,
rocks in which they are most
likely to be found.
is intermediate berween light-colored rhyolite
and dark basalt.
\
Use the chart in figure 11.4 along with table 11.1 to identify common igneous roch. You may also find it helpful to
turn to Appendix B,which includes a key for identifying common igneous rocks. (Photos of typical igneous rocks are shown
in figure 11.5.)
(porphyrltlc)
Igneous Rocks, IntrusiveAm'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 variation 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 minerals 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 sib, 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~llcateminerals 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.
dioc.wd&eho
original magma are left over as well. If no fioaun above
the pluton p m i w the fluids to e s q . Ih arc ecsltd in,
as in a p s u r e cooker. The watery rcsid
magma h a
phase of the plutorr Fluids,n&biy-%
3
low viscosiy which allows appropriate atoms to migrate '
easily toward growing er).rrplr. The crystals add more and
=more
atoms
. w
atice d e s arc g c n e d y quite small. Many q
mcturcp, locared either within the upper portion
te pluton or within the overlying country d
e concact with granite, the fluid body evidmdy h v ened into the country rock before sokdifging. Pegdikes are fairly common, especially within gnnite
utons, where they apparcndy filled cracks that dcyelopal
e already solid granite. Some pegmacites form am31
along contacts bcnvcen granite and wunny rock, tillcracks rhat developed as the cooling granite p l u m con#
Most oeematites contain only auara, Mdsoar ywt rrus mi;a."~inerals of conside;at;lc commer;ial valuiuc
in a few pegmatites. Large crystals of muscovite mica
'ned from pegmatite. Thcse crystals are died
" because the cleavage flakes (tens of cenrimemn
) look like pages. Because muscovite is an orcdienc
tor, the cleavage sheeo are used in electrical devices,
(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
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.
fll
with a chemical content between that offelsic and malic
sified as intermediate rocks. Ana'esite, which is usually
or medium gray, is the most common intermediate vol-
Roch
rock is composed entirely or almost entirely of
minerals. No feldspars are present and, of
Many rare clements are mined from pegmatires. These
dements were not absorbed by the minerals of the main
pluton and so were concentrated in the residual pegmatitic
magma, where they crystallized as constituents of unusual
minerals. Minerals containing the element lithium are
mined from pcgmatites. Lithium becomes part of a sheet
te structure to form a pink or purple
. varietv of mica I
(called Iepidolite). ~ranium'ores,similarly conce;luated in
the residual melt of magmas, are also exrracted from pegmatite~.
Some pegmatite are mined for gemstones. Emerald and
aquamarine, varieties of the mined beryl, occur in pegm a t e s that crystallized from a solution containing the element beryllium. A large number of the world's very rare
minerals are found only in pegmatites, many of these in
only one known pegmarire body. These rare minerals arc
mainly of interest to collectors and museums.
Nydrorhermal veins (described in chapter IS) are closely
A & d to pegmatites. Veins of quartz ar; common in coun- 1
try rock I&&granite. Many of these arc believed to be
awed by water that emolpes from the magma. S
solved in du wry hot warcr cokes on the walls of cr
the water cools while traveling +vard.
So
valuable me& such as p k l s k r , lad. dne, .aB
arc depodd with jhe q u ~ in
m veins.
.
. .
course, no quartz. Most ultrarnafics are composed of coarsegrained pyroxene and/or olivine. Chemically, these ro& contain less than 45% silica.
Note that the chart (figure 11.4) does not include a finegrained 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 temperatures. 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.
Intrusive Bodies
Intrusions, or intrusive structures, are bodies of intrusive
rock whose names are based on their size and shape, as well as
their relationship to surrounding rocks. They are important
aspects of the architecture, or strplcturc, of the earth's crust. The
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 stmccan also be seen at ShipAock. The low, wallridge 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 counDikes 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-coloredrmks) 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 i k 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 (figure 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 predominantlycomposed of granite. Detailed studies of batholiths indicate that they are formed
Ignaw M,
inirusimActivirjl and thr Origin ofIpeovr Rmkr
of numerous. coalesced olutons. A~oarentlv.
I rounding rock that is shouldered aside as the
I
I
I
=
-
.--.- -1.9
'2
magma rises. Batholiths occupy large portions of
North America, particularly in the west. Over
half of California's Sierra Nevada mountains (figure 11.12 and map, figure 1.5) is a batholiFh
whose individual plutons were emplaced during a
period of over 100 million years. An even larger
batholith extends almost the entire lenmh of the
mountain ranges of Canada's west coast and
southeastern Alaska-a distance of 1,800 kilometers. Smaller batholiths are also found in the
Appalachian Mountains in eastern North America. (The extent and location of North American
batholiths are shown on the geologic map on the
inside cover.)
0
Sill (dark rock) in sedimentary layered rock, Grand Canyon, Arizona.
-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.
Chapter 11
mmer
Figure 11.ll
Diap~rsof 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 intermediate 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 mountain ranges have, over time, eroded and become the stable interior 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
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.
1
I
-
If a rock is heated sufficiently, it begins melting 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~erraNevaaa batnol~tn.Al. Ifghtcolored rock shown new (inclxl ng ma1 ~naerthe
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 II .I3
Geothermal gradients at two parts of the earth's crust.
over 1,000'C are required to create basaltic magma. However, there arc several factors that control the melting temperature 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 geothermal gradient, described below, is a manifestation of heat transfer in the mantle.
Geothermal Gradient
A miner descending a mine shaft notices a rise in temperature. 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 average, to be about 3°C for each 100 meters (3O0C/km) of
depth in the upper part of the crust. The geothermal gradient is not the same everywhere. Figure 11.13 shows geothermal gradients for two regions. The curve for the volcanic
Chapter I I
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 convection 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 emptions. 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
The melting point of a mineral generally incwaes with increasing pressure. Pressure increases with depth in the earth's crust,
just as temperature does. So a rock that melts at a given temperature at the surface of the earth requires a higher temperature 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 s o 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.)
1
E*periments have ahown that, uadq moderately high p m r
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. (Pat depth k usually erp"ed in kilobats; one kilobar is
equal m l,Q00bars.) votsr
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. Ifno 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~llmelt 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
-
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.
.
1300'
Differentiation and Bowen's
Reaction Theory
1 looo
/Melting begins above
this temperature
Quartz
Potassium
feldspar
75%
75%
58%
25%
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 minerals melt at a lower temperature than either mineral alone.
Figure 11.15 shows the melting temperatures for quartz and
potassium feldspar mixed in various proportions. If the mixture is 42% quartz (58% potassium feldspar), melting takes place at just above
1,00O0C.On the other hand, 1,50O0Cis 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.
How Magmas of Different
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
I
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,
a considerable range ofcdmpositions, mostly varieties of granite, but many also will contain minor
,.
Chapter I I
L
Fieum II. I S
Bowen's reaction series.
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 ferromagnesian. 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 exceptionally high proportion (2:l) of iron and magnesium to
chains of tetrahene, with a formula of
ount of silicon rela1 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 fersian 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 doudrons. More of the silicon
um, calcium, and minor
ted into the newly develphibole has formed, on further coolthe melt to produce biotite (which is
he last of the ferromagnesian minera remaining after biotite has finry little iron or magnesium.
o n t i n u o w ~rancib
feldspars, combine with calcium and sodium to form plagioclase. 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 plagioclase 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-tosodium 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 crystallizes 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 aluminum. 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 crystallizes 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 plagioclase grows and becomes increasingly sodic; but its growth
ceases when all liquid is consumed. The rock becomes a completely solid aggregate of calcium-rich plagioclase, pyroxene,
and olivinein 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 minerals 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, magnesium, and calcium; therefore, upon cooling, it solidifies into a
silicic or intermediate rock.
CrystalSettlingOnly 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 crystallize. Crystal settling is the downward movement of minerals 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$
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, differentiation 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 behavior of magmas. Moreover, differentiation does occur and can
explain relatively minor compositional variations within intrusive 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 kilometers 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
Cross section showing the process of differentiationby 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 relatively 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.
Chapter I I
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 temperatures 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 crcated. Small pockets of magma could merge and form a large
enough mass to rise as a diapir. The lower part of the continental crust is a very plausible source for felsic magma generated by partial melting.
Geologists generally regard basaltic magma (Hawaiian
lava, for example) as the product of partial melting of ultramafic 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.
Sfi~cs
magma
movlnp slowly
upward
Maficmapma
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.
ed~atein composition
absorbed country rock.
ocks of country rock (the
iiths of country rock with
magma melt. ( C )The
original magma, leaving
simultaneously becomes richer in silica and cooler. Possibly
intermediate magmas such as are associated with circumPacific andesite volcanoes may &rive from assimilation of
some crustal rocla by a basaltic magma.
Mixing of M a p a s
of the country rock and
i d into the magma (ftgun
ce cubes into a cup of hot
cools w it brimdiiund.
a, perhaps merated fmm the
ntinentd crust, the m q m a
The idea that some of our igneous rocks may be ' ' c c d d s " of
different magmas is cur rend^ receiving more attention by geologists 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 underground as diorite or erupt on the&s
to solidify as andesite.
Ignmu Roc&, InmrrivtActivir)r and the Origin ofIpoous Rocks
,
Explaining Igneous Activity
by Plate Tectonics
Fiuure through tlw wlrt
a
One of the appealing aspects of &%theoryof 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 convergent 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 surface
.
-.
.-(fimrc
.
\
-.
.1
.1.20).
.- .
The probable reason the asthenosphere is plastic or "soft"
is that temperatures there are only slightly lower than the temperatures required for partial melting of mantle rock.
If extra heat is added, or pressure is reduced, partial melting 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 asthenosphere 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 solidified 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
/
Hot asthenosp
rock moves up'
A
Magma soi~d~fies
to basalt
(gabbro at depth)
/
Flgum 11.20
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~
Basalt floods
Continental lithosphere
bulged upward and thinned
\
/
boundaries, there is much less agreement about how magmas
are generated at converging boundaries. The scenarios that follow are currently believed by geologists to be the best explanations of the data.
The Origin ofAndesite
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 conrgence of two plates and subduction. However, exactly what
res place is debated by geologists. Compared to divergent
Magma for most of our andesitic composite volcanoes (such as
are found along the west coast of the Americas) seems to originate 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 melting temperature of the ultramafic rocks in this part of the mantle. 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 temperatures (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 II.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 asthenosphere 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
Ignrour rocks form from solidification of
mapa. If the rock forms at the earth's surface it is extrusive. Intrusive rocks are igneous
rocks that formed underground. Some intrusive rocks have solidified near the surface as a
direct result of volcanic activity. Volcanic
necks solidified within volcanoes. Finegrained 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
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 available data. There are a number of variations of this picture
based on different interpretations of the data (box 11.3).
Plate tectonics has not ~ r o v i d e dfinal answers to the roblems raised by analyzing igneous phenomena. But it has provided a broad framework in which to work toward solutions
to the problems.
chemical composition of the magmas from
which they formed, and on grain sizes.
Granite, diorite, and gabbw are the coarsegained 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 predominant 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 convergent 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 hypotheses 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 mixing 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 incorporates 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 ferromagnesian portion that stays solid remains in the
mantle as ultramalic rock. Granite and
andesite are associated with subduction. Differentiation, assimilation, partial melting,
and mixing of magmas may each play a part
in creating the appropriate rocks.
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
'Terms to R e m e m b e r
~dcsite272
d1271
uholith 277
mven's reaction series 282
ill zone 270
wsc-grained rock 271
urcordant 277
bnract 270
tuntry rock 270
Ignrour Rocks, Intrusive Activi8 and the Origin ofIpcow Rocks
manrlc plum 280
plumn 277
plutonic rock 271
volcanic ncck 276
xenolith 270
silicic (felsic) 279
. ,
sill 277
... ,
mdr 277
I
'resting Your Knowledge
... .
below to prepare for q b a s e d dn this chaptc,
Use the
. . questiohh
1. Why dodom&cmagmas tend to nichdreslir..e mudh mote
o f m than silicic magmas?
2. %t r& d&s t & ~ t h e n ~ ~ h ~ f e p s$ay
s i bin1 genendng
~
.
m&aat (a) (a) anverging boundasy: (b) a div6rgirig'bo~ndary?
3. How do batholiths form?
4. How.would you distinguish, on the basis
...,
of mincrds present,
amonggianite, gabbro, and diorirc?
5. Howwould
. ...
you distinguish andesire from a diorite?
6. What rock would probably f o m if magma that was feeding
composite volcanoes solidified at cbnside'nbledepth?
7. Why is ahigher tcmpulture i t q u i d to form magma.at the
9. What is the difference between a dike and
a sill?
, ,
10. ~escribethe differences be&etl:th c~ntin$o%and the
discontinuousbranches of Bowen's reaction series.
11. A surfaceseparatingdifferent rock types is called a (a) xenolith
(b) contact (c)chill zone (d) none ofthe above
12, The major difference between inirusive igneous roch and
extrusive ignequs rocks is (a) w h e t they solidify (b) chemical
composition (c) type of minerals id) all of the above
13. Which is not an intrusive igneous rock! (a) gabbro (b) diorite
(c) granke (d) andesite
. .
are sequences of rocks that geologists
interprct as slices of ancient oceanic
lithosphere. Assuming that such a
sequence formed at a divergent
boundary and was moved toward a
Chaptrr I I
16. The georlrtrmal gradient is, on the average, about (a) 1'CI
(b)1o"Cllua (c) 30nC/km (d) 5CPClkm
17. Thc continuous branch of Bowen's reaction series contains
(b) plagioclase (c) amphibole (d) biotite
mineral (a)
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'
(c) mineralogy (d) the kind oJmagma that each crystallii
feldspar found in granite?
1. In parts of major mountain belts there
.
14, By definition, stocks differ from batholiths in (a) size (b) sha
(c) chemical omp position (d) all of the above
15. Which is not a source of heat for melting rock?(a) geother
gradient (b) tho hotter mantle ( c ) man& plumcc (dl water
under pressure
2 1. The difference in texture between intrusive and extrusive r
magma
texture (d) chemistry
23. A +ge
in:magma composition due to melting of surroun
cnuntty rock is
(a) magmamixing (b) assimilation
(c) q f a i setting(d) pa& melting
convergent boundary by plate motion,
what rock types would you cxpect to
make up this sequulce, going from the
top downward?
2. What would happen, according to
Bowen's reaction series, under the
dw
following circumstances: olivine crystals
form and only the surface of each
crystal reacts with the melt to form a
mating of pyroxene that prevents the
interior of olivine from reacting with
the melt?
i
Barker, D. S. 1983. Igneouc rock
Englewood Cliffs, N.J.: PrenticeHall.
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
'
(
IRobi
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
~hotosof various gianites and links to other
sites that have more images.
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://www.geolab.unc.edu/Perunil/
IgMerAtlasimainmenu.html
http://seis.natsci.csulb.edu/basicgeo/
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
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