Page 39 - Geotechnical Engineering Soil and Foundation Principles and Practice

Igneous Rocks, Ultimate Sources for Soils
34 Geotechnical Engineering

are sources for occasional earthquakes. Granitic mountains in the U.S. include
the Colorado Front Range, Sangre de Cristo Mountains, Bighorns, Beartooth,
Belt, Idaho Batholith, Sierra Nevada, and the Coastal Range of southeast
Alaska and British Columbia.

Granite mountains often are flanked by once-horizontal layers of sedimen-
tary rocks that are tilted to lap up on the sides of the mountain range. The
more resistant layers of sedimentary rock become high ridges called hogbacks.
The Black Hills are a relatively small granite body that is surrounded by
hogbacks.

2.6.3 Composition of Granite
Granite and closely related rocks such as granodiorite generally are light gray
or pink in color, with a scattering of dark minerals such as biotite, or black
mica, creating a ‘‘salt and pepper’’ effect. The main components in granite are
feldspars, which are light-colored gray or pink minerals that show cleavage,
meaning that the grains break along flat fracture surfaces. The existence of
cleavage in minerals was an early clue that there might be an orderly internal
arrangement of atoms comprising a crystalline structure.

Feldspars are readily weathered, forming clay minerals that incorporate
OH ions from water into their crystalline structure. The resulting increase in
volume of feldspar crystals causes granite to fall apart into sand. Granitic
rocks therefore are the ultimate sources for most natural sands that occur in
dunes and river bars.

The clear, colorless mineral that may be seen in a broken face of granite is
quartz. Quartz is readily recognized from its clear, glass-like appearance and the
lack of any cleavage. Quartz is the last mineral to crystallize from a granitic
magma, and therefore fills voids between other crystals. Quartz grains therefore
initially are somewhat rounded, and become more rounded as they are repeatedly
transported and deposited, then eroded, transported, and redeposited as sand,
an action that can be observed with every wave washing up on a beach. The
large quartz crystals that are found for sale in rock shops and are cut into
slices for controlling frequencies in electronic devices are not igneous in origin, but
have formed in caverns through slow accretion of ions from cold solutions.

2.6.4 Magmatic Differentiation

Whereas basalt is a heavy, black rock composed of high-temperature minerals,
granite is a light-colored rock mainly composed of low-temperature minerals.
According to a theory called magmatic differentiation, granite can form from
a basalt magma if the heavy, high-temperature minerals crystallize first and
sink. Field evidence confirming this has been discovered in thick horizontal
ledges of intruded rocks called sills.

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