Geology Reference
In-Depth Information
10
I. Introduction to Geology
The fact that shale is the most abundant sedi-
mentary rock is somewhat unfortunate, as it has sev-
eral undesirable properties. These properties are
sometimes ignored in construction and engineering
projects. Some shales absorb large quantities of water
and expand greatly; others are plastic and flow easily.
Shales typically weather rapidly, are weak and unable
to support great weight, and, when lubricated by
water, slide readily. The impermeability of shales
makes them a poor source of groundwater.
The coarser clastic (made of fragments) rocks,
conglomerate and sandstone, are generally porous and
permeable and as a result are the source of abundant
groundwater, usually of high quality. These rocks gen-
erally possess good foundation-bearing qualities or
support strength.
Limestones may be highly soluble and hence
unsuitable sites for many construction projects. Solu-
tion channels, caves, and sinkholes which develop in
limestone may result in subsidence problems and
make unsuitable sites for surface water reservoirs and
for waste disposal.
Sedimentary rocks are of great economic signif-
icance. Building materials, aggregate, glass, ceramic
products, and the fossil fuels, as well as some metal-
lic mineral deposits are the products of sedimentary
rocks.
QUESTIONS 1, PART B2
1.
Your instructor will provide specimens of sedimentary
rocks for you to identify. Using the information presented in
Table 1.5, determine the composition and texture of each spec-
imen. On this basis draw your conclusions about the origin of
the sediments, the process of lithification, and the name of the
rock. Record this information in Table 1.6.
2.
After you have identified the rocks, determine some of
the important uses and geologic and environmental signifi-
cance of each rock and record this in the last column in
Table 1.6.
Metamorphic Rocks
Metamorphic rocks are formed from preexisting rocks
that have undergone substantial alteration by pres-
sure, heat, and chemically active fluids. Metamorphic
changes include (1) recrystallization of minerals, nor-
mally into larger grains; (2) chemical recombination
and growth of new minerals; and (3) deformation and
reorientation of constituent minerals. Classification of
metamorphic rocks is based primarily on texture and
composition. Two general categories result: foliated
and nonfoliated. Foliated texture is characterized by
an alignment of platy minerals such as mica, giving
the rock a banded or layered appearance. Nonfoliated
