Page 40 - Geotechnical Engineering Soil and Foundation Principles and Practice

Igneous Rocks, Ultimate Sources for Soils
Igneous Rocks, Ultimate Sources for Soils 35

On the other hand, a theory that works qualitatively can suffer irreparable
damage when numbers are put to it, a precaution that must be remembered. There
is too much granite in the world for it all to have been derived from magmatic
differentiation. Another hypothesis called ‘‘granitization’’ suggests that the high
temperatures and solutions associated with intrusion of magma can cause it to
react with and incorporate existing siliceous rocks and limestone. Evidences for
this include the limestone layered structure that is incorporated into granite.
Granite that originates by this means is not strictly an igneous rock but is a
metamorphic rock, discussed later.

2.6.5 Older Mountains and Continents

Granite and related rocks also outcrop in geologically older and less topo-
graphically rugged mountains such as those in the Appalachian system, Piedmont
Plateau, and the Blue Ridge Mountains. Granite and associated metamorphic
rocks are prominent in Stone Mountain, Georgia, the White Mountains in
New England, and a small area designated the St. Francis Mountains in southeast
Missouri.

2.6.6 The Really Old Shield Areas
As erosion proceeds through hundreds of millions of years, the exposed base of
a granitic mountain range broadens, so more and more granite and associated
metamorphic rocks are exposed. This results in broad continental shield areas.

Figure 2.8
Aerial photo of a
granitic shield area
scoured clean by
glacial erosion. A
subtle criss-cross
pattern of streams
and more heavily
vegetated areas
defines vertical
fractures. (USDA
photo, Northern
Minnesota.)

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