Earth Materials, Geologic Time, and Geologic Processes - Investigations in Environmental Geology - page 8

8 I. Introduction to Geology

COMPOSITION Generally, the composition of igneous
rocks can be divided into two major groups: (1) those in
which the light-colored minerals quartz and orthoclase
feldspar predominate (acidic igneous rocks); and (2)
those in which the dark-colored minerals olivine,
augite (pyroxene), and hornblende predominate (basic
igneous rocks). Plagioclase feldspar is usually present
in the darker igneous rocks.

QUESTIONS 1, PART B1
1. Your instructor will provide specimens of igneous rocks
for you to identify. Determine the texture, composition, and
any other important characteristics of each specimen and
record your observations in Table 1 .4. Using the information
you have compiled and Table 1 .3, determine the name of
each rock.

2. After you have made a record of the names and charac-
teristics of the igneous rocks, note information regarding
the geologic, environmental, and economic significance of
these rocks in the proper column in Table 1 .4.

SIGNIFICANCE Igneous rocks are economically impor-
tant as most metallic minerals are found in or asso-
ciated with them. Many other economic products,
including building stone, aggregate, gemstones, and
ceramic and glass materials are derived from igneous
rocks. Geothermal energy, a source of which is heat
from igneous rocks, is a potentially important,
although limited, future energy source. Since igneous
rocks are relatively nonporous and impermeable, the
groundwater obtainable from them is generally limited
in quantity, although usually of good quality. However,
some igneous rocks may be jointed or fractured, and
some volcanic rocks may have lava tunnels so that
large voids exist that provide a reservoir of abundant
water. When these rocks are used for waste disposal,
groundwater may readily become polluted.
Important engineering properties of igneous
rocks include their strength and stability; that is, they
provide a sound foundation for construction projects.
Commonly, soils that have developed on igneous
rocks are thin and on slopes such soils may slide when
saturated with water. Although igneous rocks usually
provide a firm foundation, the benefits may be over-
come by adverse factors such as excavation difficulty,
thin soils, and lack of groundwater.

Sedimentary Rocks
Sedimentary rocks are formed from the products of
the weathering (breakdown) of preexisting rocks.
When these products (sediments) are compacted by
pressure of overlying material or cemented together by
precipitation of mineral matter between the particles,
new rock is formed. Sedimentary rocks may also be
formed from the accumulation of organic materials
and from chemical precipitation of mineral matter
from water. The common sedimentary rocks and their
characteristics are listed in Table 1 .5.

SIGNIFICANCE Since sedimentary rocks cover appro-
ximately 75 percent of the Earth's surface, human
interaction with earth materials is more likely to be
with these rocks than with any other type. When we
seek a water supply, dispose of waste materials,
engage in construction projects, or extract mineral
resources, the rock encountered is likely to be
sedimentary.

TABLE 1.3 Classification of Igneous Rocks

Composition Quartz, Orthoclase,

Biotite

Na-rich plagioclase.
Hornblende, Biotite Texture -—^^^ Ca-rich plagioclase, Augite, Olivine

Phaneritic Granite Diorite Gabbro

Phaneritic (with phenocrysts) Porphyritic granite

Aphanitic

Rhyolite (Felsite) Andesite Basalt

Aphanitic (with phenocrysts) Porphyritic rhyolite Porphyritic andesite Porphyritic basalt

Glassy

Obsidian

Vesicular Pumice Scoria

Pyroclastic

Tuff and Volcanic breccia