Geology Reference
In-Depth Information
Volcanoes have a cylindrical conduit known as a
volcanic pipe
that connects to an underlying magma chamber. Magma rises
through this structure; however, when a volcano ceases to erupt,
its slopes are attacked by weathering and erosion, but the magma
in the pipe is commonly more resistant to erosion and is left as a
remnant called a
volcanic neck
. Several volcanic necks are found
in the southwestern United States, especially in Arizona and New
Mexico, and others are recognized elsewhere (see Geo-inSight
on pages 100 and 101 and Geo-Focus on pages 102 and 103).
granite porphyry. Granitic rocks also are the primary source
of gold, which forms from mineral-rich solutions moving
through cracks and fractures of the igneous body.
INTO EARTH'S CRUST?
Geologists realized long ago that the origin of batholiths
posed a space problem. What happened to the rock that was
once in the space now occupied by a batholith?
One solution to the space problem is that these large ig-
neous bodies melted their way into the crust. In other words,
they simply assimilated the country rock as they moved
upward (Figure 4.7). The presence of inclusions, especially
near the tops of some plutons, indicates that assimilation
does occur. Nevertheless, as we noted, assimilation is a lim-
ited process because magma cools as country rock is assimi-
lated. Calculations indicate that far too little heat is available
in magma to assimilate the huge quantities of country rock
necessary to make room for a batholith.
Geologists now generally agree that batholiths were em-
placed by
forceful injection
as magma moved upward. Recall
that granite is derived from viscous felsic magma and there-
fore rises slowly. It appears that the magma deforms and
shoulders aside the country rock, and as it rises farther, some
of the country rock fi lls the space beneath the magma.
Some batholiths do indeed show evidence of having been
emplaced forcefully by shouldering aside and deforming the
country rock. This mechanism probably occurs in the deeper
parts of the crust where temperature and pressure are high
and the country rocks are easily deformed in the manner de-
scribed. At shallower depths, the crust is more rigid and tends
to deform by fracturing. In this environment, batholiths may
move upward by
stoping
, a process in which rising magma
detaches and engulfs pieces of country rock (
By definition, a
batholith
, the largest of all plutons, must
have at least 100 km
2
of surface area, and most are far larger.
A
stock
, in contrast, is similar but smaller. Some stocks are
simply parts of large plutons that once exposed by erosion
are batholiths (see Geo-inSight on pages 100 and 101). Both
batholiths and stocks are mostly discordant, although locally
they may be concordant, and batholiths, especially, consist of
multiple intrusions. In other words, a batholith is a large com-
posite body produced by repeated, voluminous intrusions of
magma in the same region. The coastal batholith of Peru, for
instance, was emplaced during a period of 60 to 70 million
years and is made up of as many as 800 individual plutons.
The igneous rocks that make up batholiths are mostly
granitic, although diorite may also be present. Batholiths and
stocks are emplaced mostly near convergent plate boundar-
ies during episodes of mountain building. One example is
the Sierra Nevada batholith of California (see the chapter
opening photograph), which formed over millions of years.
Other large batholiths in North America include the Idaho
batholith, the Boulder batholith in Montana, and the Coast
Range batholith in British Columbia, Canada.
Mineral resources are found in rocks of batholiths and
stocks, and in the adjacent country rocks. The copper depos-
its at Butte, Montana, are in rocks near the margins of the
granitic rocks of the Boulder batholith. Near Salt Lake City,
Utah, copper is mined from the mineralized rocks of the
Bingham stock, a composite pluton composed of granite and
Figure 4.18).
According to this concept, magma moves up along fractures
and the planes separating layers of country rock. Eventually,
pieces of country rock detach and settle into the magma. No
new room is created during stoping; the magma simply fi lls
the space formerly occupied by country rock (Figure 4.18).
◗
◗
Figure 4.18
Emplacement of a Batholith by Forceful Injection and Stoping
Blocks of country rock are detached and engulfed in the magma,
thereby making room for the magma to rise farther. Some of the
engulfed blocks might be assimilated, and some may remain as
inclusions (Figure 4.7b).
Magma rises and forces its way into fractures and planes
between layers in the country rock.
a
b
Search WWH ::
Custom Search