Geoscience Reference
In-Depth Information
other things) that they have (1) high silica content; (2) rela-
tively low melting temperatures; and (3) lighter colors. The
remaining four of the seven silicate minerals are
mafic
,
which essentially have opposite characteristics to felsic
minerals.
Figure 12.7 indicates how silicate minerals can combine
in different ways to form various kinds of igneous rocks. A
good example on the mafic side of the diagram is the intrusive
rock
gabbro
, which forms when plagioclase feldspar mixes
with minerals from the pyroxene group and olivine. If the
same minerals combine in an extrusive environment, the mafic
rock is called
basalt
. On the felsic side of the diagram, a good
example is the intrusive rock
granite
, which forms when pot-
ash feldspar combines with plagioclase feldspar, biotite mica,
and quartz. In an extrusive setting, these minerals combine to
form
rhyolite
. Note the other kinds of combinations that can
occur.
You may wonder how you can tell the difference between
extrusive and intrusive igneous rocks if they are made from
the same materials? The answer is that extrusive and intrusive
rocks differ fundamentally in their appearance. Intrusive igne-
ous rocks have coarse crystalline grains that you can see with
your unaided eye. These coarse textures evolve because magma
cools relatively slowly, perhaps over several thousand years,
when it is trapped within the Earth's crust, such as in a sill, dike,
or laccolith (see Figure 12.6a). A good example of a coarse-
grained igneous rock is
granite
(Figure 12.7). Granite is a felsic
rock that has distinctive dark grains consisting of the minerals
amphibole
and
biotite
, pinkish-tan grains of
feldspar
, and clear
grains of
quartz.
In contrast to intrusive igneous rocks, extrusive igneous
rocks reach the surface and cool relatively quickly, sometimes
in a matter of days. A good example of an extrusive igneous
setting can be seen in Figure 12.6b, which shows a lava flow
in Hawaii. Given that the lava cools relatively quickly in this
setting, silicate minerals do not have time to congregate with
one another, as they do in intrusive igneous rocks. The re-
sult is a fine-grained rock. A common kind of extrusive igne-
ous rock is
basalt
, which is a dark mafic rock (Figure 12.7).
Perhaps the most distinctive extrusive igneous rock is
obsid-
ian
(Figure 12.8). Obsidian is also called
volcanic glass
and
forms when nongaseous lava cools so fast that crystals cannot
form.
Figure 12.8 Obsidian.
Obsidian is an extrusive igneous
rock that develops when nongaseous lava cools very quickly,
before crystals can develop. Prehistoric Native Americans
prized this rock for its rich, dark color and smooth glassy
surface.
setting, such as a river floodplain, shallow seafloor,
interior valley, lake, or marsh (Figure 12.9). These minerals
can arise from any of the three major rock groups—igneous,
sedimentary, or metamorphic (to be discussed later)—but their
ultimate source is usually some form of igneous rock that was
weathered in a way that liberated countless individual parti-
cles such as clay (from feldspar) or sand (from quartz). These
particles could then be transported as
sediment
by wind,
water, or glaciers to a new place, where they subsequently
accumulate as sedimentary deposits. Most sedimentary rocks
are the remnants of sediments that accumulated in an oceanic
environment. Many of these sediments were eroded from the
continents in various ways and then carried to the ocean where
they settled to the bottom.
After the sediment accumulates, all the grains gradu-
ally become cemented to one another in a gradual process
called
lithification
to form rock. This process occurs be-
cause water is squeezed out of the lower layers of sediments
when they are compacted by the cumulative weight of the
deposits above them. As a result of compaction and associ-
ated water loss, minerals such as calcium and silica recrys-
tallize in the sediments and essentially glue them together
to form rock.
Within the overall category of sedimentary rocks, three
major classes of sediments occur: clastic, chemical, and
organic. Table 12.1 lists the basic characteristics of these
rocks.
Sedimentary Rocks
Now let's turn to a second major rock class, which is
sedimentary rocks
. Although some forms of sedimentary rock
have organic origins, most consist of vast quantities of formerly
loose minerals that collected in some kind of depositional
Sedimentary rocks
Rocks that form through the deposition
and lithification of small fragments or dissolved substances
from other rocks or, in some cases, marine animals.
Sediment
Solid fragments of rocks that are transported to
some location and deposited by wind, water, or ice.
Lithification
The process whereby sediments are cemented
through compaction to form rock.
