Geology Reference
In-Depth Information
metamorphism of the rocks below is easy, less obvious is
whether an igneous body is intrusive or extrusive in a rock
outcrop where sedimentary rocks occur above and below
the igneous body. Recognizing which sedimentary rock
units have been metamorphosed enables geologists to de-
termine whether the igneous body is intrusive (such as
a sill or dike) or extrusive (lava fl ow). Such a determina-
tion is critical in reconstructing the geologic history of an
area (see Chapter 17) and may have important economic
implications as well.
Fluids also play an important role in contact metamor-
phism. Magma is usually wet and contains hot, chemically
active fluids that may emanate into the surrounding rock.
These fl uids can react with the rock and aid in the formation
of new minerals. In addition, the country rock may contain
pore fluids that, when heated by magma, also increase
reaction rates.
The formation of new minerals by contact metamor-
phism depends not only on proximity to the intrusion,
but also on the composition of the country rock. Shales,
mudstones, impure limestones, and impure dolostones are
particularly susceptible to the formation of new minerals by
contact metamorphism, whereas pure sandstones or pure
limestones typically are not.
Because heat and fluids are the primary agents of
contact metamorphism, two types of contact metamorphic
rocks are generally recognized: those resulting from baking
of country rock and those altered by hot solutions. Many of
the rocks that result from contact metamorphism have the
texture of porcelain; that is, they are hard and fi ne grained.
This is particularly true for rocks with a high clay content,
such as shale. Such texture results because the clay minerals
in the rock are baked, just as a clay pot is baked when fi red
in a kiln.
During the final stages of cooling when an intruding
magma begins to crystallize, large amounts of hot, watery
solutions are often released. These solutions may react with
the country rock and produce new metamorphic minerals.
in all public buildings, including all
public and private schools. However,
important questions have been raised
concerning the threat posed by asbes-
tos and the additional potential haz-
ards that may arise from its improper
removal.
Current EPA policy mandates that
all forms of asbestos are to be treated
as identical hazards. Yet studies indicate
that only the amphibole forms consti-
tute a known health hazard. Chrysotile,
whose fi bers tend to be curly, does not
become lodged in the lungs. Further-
more, its fi bers are generally soluble
and disappear in tissue. In contrast,
crocidolite has long, straight, thin fi bers
that penetrate the lungs and stay there.
These fi bers irritate the lung tissue and
over a long period of time can lead to
lung cancer (
material contain-
ing the asbestos is
disturbed, asbes-
tos does not shed
fi bers and thus
does not contrib-
ute to airborne
asbestos that
can be inhaled.
Furthermore,
improper removal
of asbestos can
lead to contami-
nation. In most
cases of improper
removal, the
concentration of
airborne asbestos
fi bers is far higher
than if the asbes-
tos had been left
in place.
The problem
of asbestos
contamination is
a good example
of how geol-
ogy affects our
lives and why we
should have a basic knowledge of
science before making decisions that
◗
Figure 2). Thus crocido-
lite, and not chrysotile, is overwhelm-
ingly responsible for asbestos-related
lung cancer. Because approximately
95% of the asbestos in place in the
United States is chrysotile, many people
question whether the dangers from
asbestos are exaggerated.
Removing asbestos from buildings
where it has been installed could cost
as much as $100 billion. Unless the
◗
Figure 2
Lung cancer. Colored tomography (CT) scan of an axial
section through the chest of a patient with a mesothelioma cancer
(light red). It is surrounding and constricting the lung at right (pink).
The other lung (dark blue) has a healthy pleura (dark red). The
spine (lower center, light blue), the descending aorta (green), and
the heart (dark green between lungs) are also seen. Mesothelioma
is a malignancy of the pleura, the membrane lining the chest cavity
and lungs. It is usually caused by asbestos exposure. It often
reaches a large size, as here, before diagnosis, and the prognosis
is then poor.
could have broad economic and soci-
etal impacts.
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