Download 4- Igneous Rock (Intrusive)

INTROD
DUCTION
N
Rocks reesulting fro
om volcannic eruptio
ons are widespread,
w
but they probably
represent only a sm
mall portioon of the total rock
ks formed by the co
ooling and
crystallizaation of molten
m
rockk material called
c
mag
gma. Most magma co
ools below
the Earth''s surface and
a forms bbodies of rock
r
known
n as plutonss. The sam
me types off
magmas are
a involveed in both vvolcanism and the orrigin of pluutons, altho
ough some
magmas are
a more mobile
m
and m
more comm
monly reac
ch the surfaace. Plutons typically
underlie areas
a
of extensive vollcanism an
nd were the
e sources o f the overlying lavas
and fragm
mental maaterials ejeected from
m volcanoes during eexplosive eruptions.
Furthermore, like volcanism, most pluto
onism occu
urs at or neear plate margins.
m
in
pter we are concerneed primariily with th
he texturess, compossition, and
this chap
classification of ign
neous rockks and witth plutonic or intrusiive igneou
us activity.
Volcanism
m will be discussed
d
inn Chapter 5.
5
_ MAGM
MA AND LAVA
L
Magma iss molten ro
ock materiial below the
t Earth's surface, annd lava is magma at
the Earth
h's surface. Magma iss less densse than the solid rockk from wh
hich it was
derived, so
s it tends to move uupward tow
ward the su
urface. Som
me magma is erupted
onto the surface
s
as lava
l
Rows,, and somee is forcefu
ully ejectedd into the attmosphere
as particles called pyroclastic
p
materials (from the Greek
G
pyroo = fire an
nd klastos =
broken). Igneous ro
ocks (from
m the Latin ignis
i
= fire
e) form w
when magm
ma cools
and crystallizes or when pyroclasstic materrials such
h as volc
canic ash
become consolid
dated. Ma
agma extrruded onto
o the Earthh's surface as
a lava and
pyroclastic materials forms volcanic or extrusive igneous rocks, whereas magma that
crystallizes within the Earth's crust forms plutonic or intrusive igneous rocks.
Composition
Recall from Chapter 3 that the most abundant minerals in the Earth's crust are silicates,
composed of silicon, oxygen, and the other elements listed in Table 3-2. Accordingly,
when crustal rocks melt and form magma, the magma is typically silica rich and also
contains considerable aluminum, calcium, sodium, iron, magnesium, and potassium as
well as many other element' in lesser quantities. Not all magmas originate by melting of
crustal rocks, however; some are derived from upper mantle rocks that are composed
largely of ferromagnesian silicates. A magma from this source contains comparatively
less silica and more iron and magnesium.
Although silica is the primary constituent of nearly all magmas, silica content varies
and serves to distinguish felsic, intermediate, and mafic magmas (@ Table 4-1). A
felsic magma, for example, contains more than 65% silica and considerable sodium,
potassium, and aluminum, but little calcium, iron, and magnesium. In contrast to felsic
magmas, mafic magmas are silica poor, and contain proportionately more calcium, iron,
and magnesium. As one would expect, intermediate magmas have mineral
compositions intermediate between those of mafic and felsic magmas (Table 4-1).
Temperature
no direct measurements of temperatures of magma below the Earth's surface have
been made. Erupting lavas generally have temperatures in the range of 1,000° to
1,,200°C, although temperatures of 1 ,350°C have been recorded above Hawaiian lava
lakes where volcanic gases reacted with the atmosphere.
Most direct temperature measurements have been taken at volcanoes characterized by
little or no explosive activity where geologists can safely approach the lava. Therefore,
little is known of the temperatures of felsic lavas, because eruptions of such lavas are
rare, and when they do occur, they tend to be explosive. The temperatures of some lava
domes, most of which are bulbous masses of felsic magma, have been measured at a
distance by using an instrument called an optical pyrometer. The surfaces of these
domes have temperatures up to 900°C, but the exterior of a dome is probably much
cooler than its interior.
When Mount St. Helens erupted in 1980, it ejected felsic magma as particulate matter
in pyroclastic flows. Two weeks later, these flows still had temperatures between 300°
and 420°C.
Viscosity
Magma is also characterized by its viscosity, or resistance to flow. The viscosity of
some liquids, such as water, is very low; they are highly fluid and flow readily. The
viscosity of some other liquids is so high that they flow much more slowly. Motor oil
and syrup flow readily when they are hot, but become stiff and flow very slowly when
they are cold. Thus, one might expect that temperature controls the viscosity of magma,
and such an inference is partly correct. We can generalize and say that hot lava flows
more readily than cooler lava.
Magma viscosity is also strongly controlled by silica content. In a felsic lava,
numerous networks of silica tetrahedra retard flow, because the strong bonds of the
networks must be ruptured for flow to occur. Mafic lavas, on the other hand, contain
fewer silicca tetrahedrra networks and conseq
quently flow
w more readdily. Felsic lavas form
thick, slow
w moving flows,
f
wherreas mafic lavas tend to form thiinner flows that move
rather rapiidly over grreat distancces. One succh flow in Iceland in 11783 flowed about 80
km, and soome ancien
nt flows in thhe state of Washington
W
n can be tracced for morre than 500
km.
IGNEOU
US ROCK
KS
All intrusiive and maany extrusivve igneous rocks form
m when minnerals crystaallize from
magma. The
T process of crystallizzation invollves the form
mation and subsequentt growth off
crystal nuuclei. The attoms in maagma are in
n constant motion,
m
but when cooliing begins,
some atom
ms bond to
o form sm
mall groups, or nucleii, whose ar
arrangementt of atoms
correspondds to the arrangemennt in minerral crystals. As otherr atoms in the liquid
chemicallyy bond to th
hese nucleii, they do so
o in an ordeered geomeetric arrangeement, and
the nucleii grow into crystalline mineral graains, the ind
dividual parrticles that comprise
c
a
rock. Duriing rapid co
ooling, the rrate of nuclei formation
n exceeds thhe rate of growth,
g
and
an aggregate of many
y small graiins results (~
( Figure 4-2a). With slow coolin
ng, the rate
of growth exceeds thee rate of nuccleation, so relatively large
l
grainss form (Figu
ure 4-2b).
texture
hanitic textu
ure, individuual minerall grains are
grained teexture termeed aphaniticc. In an aph
too small to be obseerved withoout magnificcation (~ Figure 4-3a)). In contrast, igneous
rocks withh a coarse-grained or phaneritic texture hav
ve mineral grains thatt are easily
visible wiithout magn
nification (F
Figure 4-3b
b). Such larrge minerall grains ind
dicate slow
cooling annd generallly an intruusive origin
n; a phanerritic texturee can deveelop in the
interiors of
o some thicck lava flow
ws as well.
Rocks with
w
porphy
yritic texturres have a somewhat more com
mplex coolin
ng history.
Such rockks have a com
mbination oof mineral grains
g
of maarkedly diffferent sizes. The larger
grains are phenocrystss. and the sm
maller ones are referred
d to as grouundmass (Figure 4-3c).
Suppose that
t
a magm
ma begins 'cooling sllowly as an
n intrusive body, and that some
mineral crrystal nuclei form and begin to grrow" Suppose further th
that before the
t magma
has complletely crystaallized, the remaining liquid phase and solid mineral graains within
it are extrruded onto the Earth'ss surface where
w
it coo
ols rapidly, forming an
n aphanitic
texture. The
T resultin
ng igneous rock woulld have large mineral grains (ph
henocrysts)
suspendedd in a finely
y crystallinee groundmaass, and thee rock woulld be charaacterized as
porphyry
A lava may cool so
s rapidly tthat its constituent atoms do not have time to become
i the orderred. three diimensional framework
ks typical off minerals. As a result
arranged in
of such raapid cooling
g. a naturaal glass succh as obsidiian forms ((~ Figure 4-4a).
4
Even
though obbsidian is not
n composeed of mineerals. it is still
s consideered to be an
a igneous
rock.
some magmas contain large amounts of water vapor and other gases. These
gases may be trapped in cooling lava where they form numerous small holes or
cavities called vesicles: rocks possessing numerous vesicles are termed vesicular;
as in vesicular basalt (Figure 4-4b).
A pyroclastic or fragmental texture characterizes igneous rocks formed by explosive
volcanic activity. Ash may be discharged high into the atmosphere and eventually settle
to the surface where it accumulates: if it is turned into solid rock. it is considered to be a
pyroclastic igneous rock.
Composition
Magmas are characterized as mafic (45-52% silica), intermediate (53-65% silica), or
felsic (>65% silica) (see Table 4-1). The parent magma plays a significant role in
determining the mineral composition of igneous rocks, yet the same magma can yield
different igneous rocks because its composition can change as a consequence of the
sequence in which minerals crystallize. crystal settling. assimilation, and magma
mixing.
Bowen's Reaction Series. During the early part of this century, N. L. Bowen
hypothesized that mafic, intermediate, and felsic magmas could all derive from a parent
mafic magma. He knew that minerals do not all crystallize simultaneously from a
cooling magma, but rather crystallize in a predictable sequence. Based on his
observations and laboratory experiments, Bowen proposed a mechanism, now called
Bowen's reaction series, to account for the derivation of intermediate and felsic
magmas from a basaltic (mafic) magma (~ Figure 4-5). Bowen's reaction series consists
of two branches: a discontinuous branch and a continuous branch. Crystallization of
minerals occurs along both branches simultaneously, but for convenience we will
discuss them separately.
In the discontinuous branch, which contains only ferromagnesian minerals, one
mineral changes to another over specific temperature ranges (Figure 4-5). As the
temperature decreases, it reaches a range where a given mineral begins to crystallize.
Once a mineral forms, it reacts with the remaining liquid magma (the melt) such that it
forms the next mineral in the sequence. For example, olivine I(Mg, Fe),SiO2,] is the
first ferromagnesian mineral to crystallize. As the magma continues to cool, it reaches
the temperature range at which pyroxene is stable; a reaction occurs between the olivine
and the remaining melt, and pyroxene forms.
A similar reaction takes place between pyroxene and the melt as further cooling
occurs, and the pyroxene structure is rearranged to form amphibole. Further cooling
causes a reaction between the amphibole and the melt, and its structure is rearranged so
that the sheet structure typical of biotite mica forms. Although the reactions just
described tend to convert one mineral to the next in the series, the reactions are not
always complete. Olivine might have a rim of pyroxene, indicating an incomplete
reaction. If a magma cools rapidly enough, the early-formed minerals do not have time
to react with the melt, and all the ferrornagnesian minerals in the discontinuous branch
can be in one rock. In any case, by the time biotite has crystallized, essentially all
magnesium and iron present in the original magma have been used up.
Plagioclase feldspars are the only minerals in the continuous branch of Bowen's
reaction series (Figure 4-5). Calcium-rich plagioclase crystallizes first. As cooling of
the magma proceeds, calcium-rich plagioclase reacts with the melt, and plagioclase
containing proportionately more sodium crystallizes until all of the calcium and sodium
are used up. In many cases, cooling is too rapid for a complete transformation from
calcium-rich to sodium-rich plagioclase 10 occur. Plagioclase forming under these
conditions is zoned, meaning that it has a calcium-rich core surrounded by zones
progressively richer in sodium.
Magnesium and iron on the one hand and calcium and sodium on the other are used
up as crystallization occurs along the two branches in Bowen's reaction series. Accordingly, any magma left over is enriched in potassium, aluminum, and silicon. These
elements combine [0 form potassium feldspar (KAISi3,O8), and if the water pressure is
high, t he sheet silicate muscovite mica will form. Any remaining magma is
predominantly silicon and oxygen (silica) and forms the mineral quartz (Si02). The
crystallization of potassium feldspar and quartz is not a true reaction series because
they form independently rather than from a reaction of the orthoclase with the
remaining melt.
Crystal Settling. A magma's composition may change by crystal settling, which
involves the physical separation of minerals by crystallization and gravitational settling
(~ Figure4-6). Olivine. the first ferromagnesian mineral to form in the discontinuous
branch of Bowen's reaction series, has a specific gravity greater than that of the
remaining magma and tends [0 sink downward in the melt. Accordingly. the remaining
melt becomes relatively rich in silica, sodium. and potassium, because much of the iron
and magnesium were removed when minerals containing these dements crystallized.
Although crystal settling does occur, it does not do so on the scale envisioned by
Bowen. In some thick, tabular, intrusive igneous bodies called sills, the first fanned
minerals ill the reaction series are indeed concentrated. The lower parts of these bodies
contain more olivine and pyroxene than the upper parts, which are less mafic. But, even
in these bodies, crystal settling has yielded very little felsic magma from an original
mafic magma.
If felsic magma could be derived on a large scale from mafic magma as Bowen
thought, there should be far more mafic magma than felsic magma. For crystal settling
to yield a particular volume of granite (a felsic igneous rock), about 10 times as much
mafic magma would have to be present initially. If this were so, then mafic intrusive
igneous rocks should be much more common than felsic ones. Just the opposite is the
case, however, so it appears that mechanisms other than crystal settling must account
for the large volume of felsic magma. Partial melting of mafic oceanic crust and silicarich sediments of continental margins during subduction yields magma richer in silica
than the source rock. Furthermore, magma rising through the continental crust can
absorb some felsic materials by assimilation and become more enriched in silica.
Assimilation. The composition of a magma can be changed by assimilation, a process
whereby a magma reacts with preexisting rock. called country rock, with which it comes
in contact (~ Figure 4-7). The walls of a volcanic conduit or magma chamber are, of
course, heated by the adjacent magma. which may reach temperatures of 1 ,300°c.
Some of these rocks can be partly or completely melted, provided their melting
temperature is less than that of the magma. Because the assimilated rocks seldom have
the same composition as the magma, the composition of the magma is changed.
The fact that assimilation occurs can be demonstrated by inclusions, incompletely
melted pieces of rock that are fairly common within igneous rocks. Many inclusions
were simply wedged loose from the country rock as the magma forced its way into
preexisting fractures (Figures 4-7 and ~ 4-8).
no one doubts that assimilation occurs, but its effect on the bulk composition of
most magmas must be slight. The reason is that the heat for melting must come from
the magma .• itself. and this would have the effect of cooling the magma.
Consequently, only a limited amount of rock can be assimilated by a magma, and that
amount is usually insufficient to bring about a major compositional change.
Neither crystal settling nor assimilation can produce a significant amount of felsic
magma from a mafic one. But the two processes operating concurrently can change the
composition of a mafic magma much more than either process acting alone. Some
geologists think that this is one way that many intermediate magmas form where
oceanic lithosphere is subducted beneath continental lithosphere.
Magma Mixing. The fact that a single volcano can erupt lavas of different
composition indicates that magmas of differing composition must be present. It seems
likely that some of these magmas would come into contact and mix with one
another. If this is the case, we would expect that t he composition of the magma
resulting from magma mixing would be a modified version of the parent
magmas. Suppose that a rising mafic magma mixes with a felsic magma of
about the same volume (~ Figure 4-9). The resulting "new" magma would have
a more intermediate composition.
Classification of Igneous Rocks
Most igneous rocks are classified on the basis of their textures .md composition (~
Figure 4-10). Notice in Figure -I-Ill th.ir all of the rocks, except peridotite, constitute
pairs; the members of a pair have the same composition but different textures. Basalt
and gabbro. andesite and diorite, and rhyolite and granite are compositional
(mineralogical) equivalents, but basalt, andesite, and rhyolite are aphanitic and most
commonly extrusive, whereas gabbro, diorite, and granite have phaneritic textures that
generally indicate an intrusive origin.
The igneous rocks shown in Figure 4-10 are also differentiated by composition.
Reading across the chart from rhyolite to andesite to basalt, for example, the relative
proportions of nonferromagnesian and ferromagnesian minerals differ. The differences
in composition are gradual, however, so that a compositional continuum exists. In other
words, there are rocks whose compositions are intermediate between rhyolite and
andesite, and so on.
Ultramafic Rocks. Ultramafic rocks «45% silica) are composed largely of
ferrornagnesian silicate minerals. The ultramafic rock peridotite contains mostly olivine,
lesser amounts of pyroxene, and generally a little plagioclase feldspar (Figure 4-10).
Another ultramafic rock (pyroxenite) is composed predominantly of pyroxene. Because
these minerals are dark colored, the rocks are generally black or dark green. Peridotite
is thought to be the rock type composing the upper mantle, but ultramafic rocks are rare
at the Earth's surface. Ultramafic rocks are generally thought to have originated by
concentration of the early-formed ferromagnesian minerals that separated from mafic
magmas.
Basalt-Gabbro. Basalt and gabbro (45-52% silica) are the fine-grained and coarsegrained rocks, respectively, that crystallize from mafic magmas (~ Figure 4-11). Both
have the same composition-mostly calcium-rich plagioclase and pyroxene, with smaller
amounts of olivine and amphibole (Figure 4-10). Because they contain a large
proportion of ferromagnesian minerals, basalt and gabbro are dark colored;
those that are porphyritic typically contain calcium plagioclase or olivine phenocrysts.
Basalt is generally considered to be the most common extrusive igneous rock.
Extensive basalt lava flows were erupted in vast areas in Washington, Oregon, Idaho,
and northern California (sec Chapter 5). Oceanic islands such as Iceland, the
Galapagos, the Azores, and the Hawaiian Islands are composed mostly of basalt.
Furthermore, the upper part of the oceanic crust is composed almost entirely of basalt.
Gabbro is much less common than basalt, at least in the continental crust or where it
can be easily observed. Small intrusive bodies of gabbro do occur in the continental
crust, but intermediate to felsic intrusive rocks such as diorite and granite arc much
more common. The lower pan of the oceanic crust is composed of gabbro, however.
Andesite-Diorite. Magmas intermediate in composition (53-65% silica) crystallize to
form andesite and diorite, which are compositionally equivalent fine- and coarse-grained
igneous rocks. Andesite and diorite are composed predominantly of plagioclase
feldspar, with the typical ferromagnesian component being amphibole or biotite (Figure
4-10).
Andesite is a common extrusive igneous rock formed from lavas erupted in volcanic
island arcs at 'convergent plate margins. The volcanoes of the Andes Mountains of
South America and the Cascade Range in western North America ate composed in part
of andesite. Intrusive bodies composed of diorite are fairly Common in the continental
Crusts.
Rhyolite-Granite. Rhyolite and granite (>65% silica) crystallize from felsic magmas and
are therefore silica-rich rocks (~ Figure 4-12). They consist largely of potassium
feldspar, sodium-rich plagioclase, and quartz, with perhaps some biotite and rarely
amphibole (Figure 4-10). because nonferromagnesian minerals predominate, these
rocks are generally light colored. Rhyolite is fine-grained, although most often it
contains phenocrysts of potassium feldspar or quartz, and granite is coarse-grained.
Granite porphyry is also fairly Common.
Rhyolite lava flows are much less common than andesite and basalt flows. Recall that
the greatest control of viscosity in a magma is the silica content. Thus, if a felsic
magma rises to the surface, it begins to cool, the pressure on it decreases, and gases are
released explosively, usually yielding rhyolitic pyroclastic materials. The rhyolitic lava
flows that do occur are thick and highly viscous and move only short distances.
Granitic rocks ate by far the most common intrusive igneous rocks, although they are
restricted to the continents. Most granitic rocks were intruded at or near convergent
plate margins during episodes of mountain building. When these mountainous regions
are uplifted and eroded, the vast bodies of granitic rocks forming their cores are
exposed. The granitic rocks of the Sierra Nevada of California form a composite body
measuring about 640 km long and 110 km wide, and the granitic rocks of the Coast
Ranges of British Columbia. Canada, are much more voluminous.
Pegmatite. is a very coarsely crystalline igneous rock. It contains minerals measuring at
least I cm across, and many crystals are much larger (~ Figure 4-13). The name
pegmatite refers to texture rather than a specific composition. but most pegmatite’s are
composed largely of quartz, potassium feldspar, and sodium-rich plagioclase-s-a
composition similar to granite. Many pegmatite’s are associated with granite batholiths
and appear to represent the minerals that formed from the fluid and vapor phases that
remained after most of the granite crystallized.
The water-rich vapor phase that exists after most of a magma has crystallized as
granite has properties that differ from the magma from which it separated. It has a
lower density and viscosity and commonly invades the country rock where it
crystallizes. The water-rich vapor phase ordinarily contains a number of elements that
rarely enter into the common minerals that form granite.
The formation and growth of mineral crystal nuclei in pegmatite’s are similar to those
processes in magma, but with one critical difference: the vapor phase from which
pegmatite’s crystallize inhibits the formation of nuclei. Some nuclei do form, however,
and because the appropriate ions in the liquid can move easily and attach themselves to
a growing crystal. individual mineral grains have the opportunity to grow to very large
sizes, several meters long in some cases.
Other igneous Rocks. Some igneous rocks, including tuff, volcanic breccia, obsidian,
and pumice, are identified solely by their textures. Much of the fragmental material
erupted by volcanoes is ash, a designation for pyroclastic materials less than 2.0 mm in
diameter: much ash consists of broken pieces or shards of volcanic glass. The
consolidattion of ash forms the ppyroclastic rock tuff. Some ash Roows are so hot that as
they comee to rest, th
he ash partiicles fuse to
ogether and
d form a weelded tuff Co
onsolidated
deposits of
o larger pyrroclasts, succh as cinderrs. blocks, an
nd bombs, aare volcanic breccia.
b
Both obsiddian and pum
mice are varieeties of volccanic glass.
Obsidian may be blaack, dark ggray, red, or
o brown, with
w the coolor depend
ding on the
presence of
o tiny parrticles of iroon minerals (~ Figuree 4-14a). A
Analyses off numerous
samples inndicate thatt most obsiidian has a high silicaa content annd is comp
positionally
similar to rhyolite.
Pumice is a variety
y of volcannic glass co
ontaining numerous
n
buubble-shapeed vesicles
va and form
ms a froth ((Figure 4-1
14b). Some
that devellop when gas escapes through lav
pumice foorms as crusts on lavva Rows, and
a
some forms
f
as paarticles eru
upted from
explosive volcanoes. If pumice ffalls into waater, it can be
b carried ggreat distancces because
ght that it flooats.
it is so porrous and lig
INTRUS
SIVE IGN
NEOUS B
BODIES: PLUTON
P
S
magma cools
Intrusive igneous bodies ccalled plu
utons form
m when m
c
and
crystallizzes within the Earth's crust (~ Figure 4-15). Geolo
ogists face
e a special
challengee in studyiing the oriigins of pllutons for,, unlike ex
xtrusive orr volcanic
activity which
w
can be observ
ved, intrusiive igneou
us activity can be stu
udied only
indirectly
y. Althoug
gh plutons can be ob
bserved aft
fter erosion
n has expo
osed them
at the su
urface, we cannot d
duplicate th
he conditiions that eexisted de
eep in the
crust wheen they forrmed, exceept in sma
all-scale lab
boratory eexperimentts.
Several types of pllutons arc rrecognized, all of whicch are definned by theirr geometry
(three-dim
mensional sh
hape) and thheir relation
nship to the country rocck (Figure 4-15).
4
Geometricallyy. plutons may
m be charaacterized ass massive orr irregular. ttabular. cylindrical. or
mushroom
m shaped. Plutons arre also deescribed ass concordan
ant or disccordant. A
concordannt pluton, su
uch as a silll. has bound
daries that are parallell to the layeering in the
intruded rock
r
or whaat is commoonly called the countryy rock A diiscordant plluton, such
as a dike, has boundaaries that rutt across the layer in!( of
o the countrry rock (Fig
gure 4-15).
Dikes and Sills Both dikes and sills arc tabular or sheet like plutons, but dikes aft'
discordant whereas sills arc concordant (Figure 4-15). Dikes are common intrusive
features (Figure 4-15). May are small bodies measuring I or 2 m across, but they ranges
from a few centimeters to more than 100 m thick. Dikes are emplaced within
preexisting zones of weakness where fractures already exist or where the fluid pressure
is great enough for them to form their own fractures during emplacement.
Erosion of the Hawaiian volcanoes exposes dikes in rift zones. the large fractures that
cut across these volcanoes. The Columbia River basalts in Washington issued from
long fissures. and the magma that cooled in the fissures formed dike's. Some of the
large historic fissure eruptions are underlain by dikes; for example. dikes underlie both
the Laki fissure eruption of 1783 in Iceland and the Eldgja fissure, also in Iceland.
where eruptions occurred in A.D. 950 from a fissure 300 km long.
Sills are concordant plutons, many of which are a meter or less thick, although some
are much thicker (Figure 4-15). A well-known sill in the United States is the Palisades
sill that tonus the Palisades along the west side of the Hudson River ill New York and
New Jersey. It is exposed for 60 km along the river and is up to 300 m thick.
Most sills have been intruded into sedimentary rocks, but eroded volcanoes also
reveal that sills are commonly injected into piles of volcanic rocks. In fact, some of the
inflation of volcanoes preceding eruptions may be caused by the injection of sills.
In contrast to dikes, which follow zones of weakness, sills are emplaced when the
fluid pressure is so great that the intruding magma actually lifts the overlying rocks.
Because emplacement requires fluid pressure exceeding the force exerted by the weight
of the overlying rocks, sills are typically shallow intrusive bodies.
Laccoliths
Laccoliths .1rt' similar to sill. in that they are concordant. but instead of being
tabular, they have a mushroom like geometry (Figure 4-15). They tend
to have a flat floor and are domed up in their central part. Like sills,
laccoliths are rather shallow intrusive bodies that actually lift up the
overlying strata. 111 this cast'. however, [he overlying rock layers are arched upward
over the pluton (Figure 4-15). Most laccoliths are rather small bodies. The best-known
laccoliths in the United States are in the Henry Mountains of southeastern Utah.
Volcanic Pipes and Necks
The conduit connecting the crater of a volcano with an underlying magma chamber is a
volcanic pipe (Figure 4-15). In other words, it is the structure through which magma
rises to the surface. When a volcano ceases to erupt, it is eroded as it is attacked by
water, gases, and acids. The volcanic mountain eventually erodes away, but the magma
that solidified in the pipe is commonly more resistant to weathering and erosion and is
often left as an erosional remnant, a volcanic neck (Figure 4-15). A number of volcanic
necks are present in the southwestern United States, especially in Arizona and New
Mexico (see Perspective 4-1), and others are recognized elsewhere.
Batholiths and Stocks
Batholiths arc the largest intrusive bodies. 13y definition they must have at least 100
km2 of surface area, and most are much larger than this (Figure 4-15). Stocks have the
same general features as batholiths but are smaller, although some stocks arc simply the
exposed parts of much larger intrusions. Batholiths ate generally discordant, and most
consist off multiple in
ntrusions. IIn other wo
ords, a bath
holith is a llarge comp
posite body
produced by repeated
d, voluminoous intrusio
ons of magm
ma in the saame area. The
T coastal
batholith of
o Peru, forr instance, was emplacced over a period of 660 to 70 miillion yeats
and consissts of perhap
ps as many as 800 indiividual pluto
ons.
The ignneous rocks composingg batholithss are mostly granitic, although diorite
d
may
also occur. Most baatholiths aree emplaced
d along con
nvergent pllate margin
ns. A good
example is
i the Coastt Range battholith of 13ritish Colo
ombia, Cannada. It wass emplaced
over a perriod of milliions of yearrs. Later upllift and erossion exposeed this huge composite
pluton at the
t Earth's surface.
s
Othher large baatholiths in North
N
Ameerica includee the Idaho
batholith and
a the Sierrra Nevada bbatholith in
n California (~ Figure 44-16).
A numbber of mineeral resourcces occur in
n rocks of batholiths
b
aand stocks and in the
country roocks adjacen
nt to them. For examplle, silica-ricch igneous rrocks, such as granite,
are the prrimary sou
urce of goldd, which forms
f
from mineral riich solution
ns moving
through cracks
c
and fractures oof the igneeous body. The coppper depositss at Butte,
Montana, are in rock
ks near the m
margins off the graniticc rocks of tthe Boulderr batholith.
d from the mineralized
m
d rocks adjaacent to the
Near Salt Lake City, Utah, coppper is mined
s
a com
mposite plutton compossed of granitte and graniite porphyry
y.
Bingham stock,
MECHAN
NICS OF BATHOLIT
B
TH EMPLA
ACEMENT
T
Geologistss realized lo
ong ago that
at the emplacement of batholiths
b
poosed a spacce problem;
that is, whhat happened to
phenomennon so it is essentially
e
aan extreme type of mettamorphism
m (see Chaptter 8).
Most geeologists think that onlyy small quaantities of grranite are foormed by grranitization
and that itt cannot acccount for thee huge gran
nite batholiths of the w
world. Thesee geologists
think an igneous
i
origin for graanite is cleaar, but then
n they mustt deal with
h the space
problem. One
O solution is that theese large ign
neous bodiees melted thheir way into the crust.
In other words.
w
they simply
s
assim
milated the country rocck as they m
moved upwaard (Figure
4-7). The presence of
o inclusionns, especiallly near thee tops of ssuch intrusiive bodies,
indicates that assim
milation doocs occur. Neverthelless, as w
we noted previously,
p
mited proccess becausse magma is cooledd as countrry rock is
assimilatioon is a lim
assimilated; calculatiions indicatte that far too little heat
h
is avaiilable in a magma to
assimilate the huge qu
uantities off country rocck necessary
y to make ro
room for a batholith.
b
Most geeologists no
ow agree thhat batholitths were em
mplaced byy forceful in
njection as
magma moved
m
upwaard toward tthe surface,, Recall that granite is derived fro
om viscous
felsic maggma and th
herefore risses slowly.. It appears that the magma deeforms and
shoulders aside the country
c
rockk, and as it rises furtheer, some off the country
y rock fills
the space beneath th
he magma. A somewh
hat analogo
ous situatioon occurs when
w
large
masses off sedimentary rock callled rock saalt rise through the ovverlying roccks to form
salt domess.
Salt dom
mes are reco
ognized in sseveral areaas of the wo
orld, includding the Gu
ulf Coast off
the Unitedd States. Laayers of rocck salt exisst at some depth,
d
but ssalt is less dense than
most otheer types of rock materrials. When under presssure, it risees toward the
t surface
even though it remain
ns solid, annd as it mov
ves upward, it pushes aaside and deforms
d
the
Natural exam
mples of rocck salt flow
wage are kno
own, and it
country roock (~ Figure 4-17). N
can easilyy be demon
nstrated expperimentally
y. In the arid
d Middle E
East, for exaample, salt
moving uppward in thee manner deescribed acttually flowss out at the ssurface.
Some batholiths do
o indeed shhow eviden
nce of havin
ng been em
mplaced forrcefully by
d deformingg the countrry rock. This mechanissm probably
y occurs in
shoulderinng aside and
the deeperr parts of th
he crust whhere temperature and pressure
p
aree high and the
t country
rocks are easily deformed in thee manner deescribed. At
A shallowerr depths, ho
owever, the
crust is more
m
rigid an
nd tends to deform by
y fracturing.. I n this ennvironment,, batholiths
may be em
mplaced by stopping, a process in
n which magma detachhes and eng
gulfs pieces
of countryy rock (~ Figure
F
4-188). Accordiing to this concept. m
magma mov
ves upward
along fracctures and th
he planes sseparating laayers of cou
untry rock. Eventually
y, pieces off
country roock are detaached and ssettle into the
t magma. No new rooom is creaated during
stopping, the magmaa simply fillls the spacee formerly occupied
o
byy country ro
ock (Figure
4-18).
Ansswers
Additiona
al readings