Page 41 - Geotechnical Engineering Soil and Foundation Principles and Practice

Igneous Rocks, Ultimate Sources for Soils
36 Geotechnical Engineering

The complex nature of rocks exposed in a shield relates to its origin as the core of
a former mountain range. In North America the Canadian Shield constitutes most
of eastern Canada and extends southward into New York as the Adirondack
Mountains, and into northern Minnesota and Wisconsin as the Superior Upland.
This shield also has been eroded and stripped of much of its residual soil by
continental glaciation.

2.6.7 The Basement Complex
The elevated positions of continents relative to oceans suggests that they are
underlain by rocks that are less dense than basalt. Deep drilling has confirmed
the existence of granitic rocks underlying the sedimentary layers. Another
contributing factor may be a higher temperature from innate radioactivity, which
would result in a lower density. Because of its unknown features it sometimes
is referred to as the basement complex.

2.6.8 Geomorphology of Granite Areas

Exposures of granite, whether in a mountain range or shield area, are marked by
a distinctive criss-cross network of fractures that are readily seen from the air
and on airphotos, Fig. 2.8. Because weathering and erosion proceed faster along
the fractures, they also influence the locations of streams, and the resulting
drainage pattern. The tendency for streams to bend at nearly right angles when
they follow fractures gives a rectangular drainage pattern. Rectangular drainage
also can be apparent on topographic maps.

Other important clues can be seen from a car window. The tendency for weathering
to proceed along intersecting fractures creates round boulders embedded in a
matrix of sand, so as the sand washes away the boulders become exposed as
rounded knobs. Boulders that eventually are freed to roll down the hill are
sometimes mistakenly assumed to have been rounded by rolling along in streams.

As the surface of a granite mass weathers and expands, it can create enough
shearing stress that surface layers will become loose and scale off in a process
called ‘‘exfoliation,’’ leaving a rounded exposure. An example is Stone Mountain,
Georgia.

Problems

2.1. What is meant by the ‘‘Ring of Fire’’?
2.2. Describe three classes of volcanoes. Would you expect their mineralogical
compositions to be the same or different?
2.3. Look up Alfred Wegener in an encyclopedia or other article or book. Does
the article mention the shabby treatment that he received from the scientific
hierarchy?

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