Geology Reference
In-Depth Information
The groundmass can be either aphanitic or phaneritic; the only
requirement for a porphyritic texture is that the phenocrysts be
considerably larger than the minerals in the groundmass. Igne-
ous rocks with porphyritic textures are designated
porphyry
, as
in basalt porphyry. These rocks have more complex cooling his-
tories than those with aphanitic or phaneritic textures and might
involve, for example, magma partly cooling beneath the surface
followed by its eruption and rapid cooling at the surface.
Lava may cool so rapidly that its constituent atoms do
not have time to become arranged in the ordered, three-
dimensional frameworks of minerals. As a consequence,
nat-
ural glass
, such as
obsidian
, forms (Figure 4.9d). Even though
obsidian with its glassy texture is not composed of minerals,
geologists nevertheless classify it as an 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 known as
vesicles
; rocks with many vesicles are termed
vesicular
, as in
vesicular basalt (Figure 4.9e).
A
pyroclastic
or
fragmental texture
characterizes igne-
ous rocks formed by explosive volcanic activity (Figure 4.9f).
For example, ash discharged high into the atmosphere even-
tually settles to the surface where it accumulates; if consoli-
dated, it forms pyroclastic igneous rock.
same magma to yield a variety of igneous rocks because its
composition can change as a result of the sequence in which
minerals crystallize, or by crystal settling, assimilation, and
magma mixing (Figures 4.4, 4.7, and 4.8).
Geologists use texture and composition to classify most
igneous rocks. Notice in
Figure 4.10 that all rocks except
peridotite are in pairs; the members of a pair have the same
composition but different textures. Basalt and gabbro,
andesite and diorite, and rhyolite and granite are compo-
sitional (mineralogical) pairs, but basalt, andesite, and
rhyolite are aphanitic and most commonly extrusive (vol-
canic), whereas gabbro, diorite, and granite are phaneritic
and mostly intrusive (plutonic). The extrusive and intrusive
members of each pair can usually be distinguished by
texture, but remember that rocks in some shallow plutons
may be aphanitic and rocks that formed in thick lava fl ows
may be phaneritic. In other words, all of these rocks exist in
a textural continuum.
The igneous rocks in Figure 4.10 are also differentiated
by composition—that is, by their mineral content. Reading
across the chart from rhyolite to andesite to basalt, for exam-
ple, we see that the proportions of nonferromagnesian and
ferromagnesian silicates change. The differences in composi-
tion, however, are gradual along a compositional continuum.
In other words, there are rocks with compositions that cor-
respond to the lines between granite and diorite, basalt and
andesite, and so on. For example, Lassen Peak in California,
which erupted from 1914 through 1917, is made up mostly
of
dacite
, a rock with a composition between andesite and
rhyolite.
◗
Most igneous rocks, like the magma from which they
originate, are mafi c (45-52% silica), intermediate (53-65%
silica), or felsic (>65% silica). A few are called
ultramafic
(<45% silica); however, these are probably derived from
mafi c magma by a process to be discussed later. The parent
magma plays an important role in determining the min-
eral composition of igneous rocks, yet it is possible for the
Ultramafic Rocks
Ultramafic rocks
(<45% silica) are composed mostly of
ferro magnesian silicates.
Peridotite
contains
mostly olivine, lesser amounts of pyroxene,
and usually a little plagioclase feldspar
(Figure 4.10 and
Aphanitic:
Rhyolite
Andesite
Basalt
—
Texture
Granite
Diorite
Gabbro
Peridotite
Phaneritic:
4.11), and pyroxe-
nite is composed predominately of pyroxene.
Because these minerals are dark, the rocks
are black or green. Peridotite is a likely can-
didate for the rock that makes up the upper
mantle (see Chapter 8). Ultramafi c rocks in
Earth's crust probably originate by concen-
tration of the early-formed ferromagnesian
minerals that separated from mafi c magmas.
Ultramafi c lava fl ows, called
komatiites,
are known in rocks older than 2.5 billion
years, but are rare or absent in younger
ones. The reason is that to erupt, ultramafi c
lava must have a near-surface temperature
of approximately 1600°C; the surface tem-
peratures of present-day mafic lava flows
are rarely more than 1200°C. During early
Earth history, though, more radioactive de-
cay heated the mantle to as much as 300°C
◗
Figure
100
Quartz
80
Plagioclase
feldspars
Potassium
feldspars
60
40
Pyroxene
20
Olivine
Biotite
Hornblende
0
Type of
Magma
Felsic
Intermediate
Mafic
Ultramafic
Darkness and specific gravity increase
Silica increases
◗
Figure 4.10
Classifi cation of Igneous Rocks This diagram shows the percentages of
minerals, as well as the textures of common igneous rocks. For example, an aphanitic
(fi ne-grained) rock of mostly calcium-rich plagioclase and pyroxene is basalt.
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