Wherever rocks are exposed on the earth’s surface, they are attacked by
the agents of _weathering_. They are dissolved by rainwater, pried apart
by frost and ice, and blasted by windblown sand. Some of the changes
produced by weathering are purely mechanical, that is, the rock is
simply reduced to smaller fragments without being broken down chemically
or undergoing any change in its mineral composition. This _mechanical
weathering_, or _disintegration_, takes place in a number of ways.
Changes are especially noticeable in rocks that are subjected to large
daily temperature variations. If a crack in these rocks becomes filled
with water and the temperature drops below freezing, ice forms. When
water freezes it expands by about 10 percent of its volume—this is the
reason why water pipes often split open during the winter. Just as in a
water pipe, the pressure of the expanding ice is commonly great enough
to widen and deepen the crack in the rock. This process, called _frost
wedging_, may ultimately cause the rock to split and fall apart. The
cumulative effects of frost wedging have probably played a significant
role in prying off large blocks of rocks from the walls and rim of the
canyon.
Animals and plants may also hasten rock disintegration. Plant roots
commonly grow in rock crevices and as the roots become larger they wedge
the rock apart. Burrowing animals such as rabbits, gophers, and ground
squirrels also promote rock disintegration. Although they do not attack
the rocks directly, their digging exposes new rock surfaces to
weathering processes. The holes these creatures make also permit water
and air to enter the earth more easily, thereby hastening rock
destruction.
Man, of course, promotes more rock disintegration than all other animals
combined. Thus, as one explores the canyon’s trails and climbs its
walls, he will not only see evidence of the various types of mechanical
weathering, he will also be contributing to the further wearing away of
the rocks.
_Decomposition_, or _chemical weathering_, works hand in hand with
mechanical weathering. But unlike disintegration, decomposition produces
rock materials that are basically different from the original
unweathered rock. These changes are brought about as the result of
chemical reactions between minerals in the rocks and water, carbon
dioxide, and oxygen. Although the arid climate and severe winters of the
Panhandle generally facilitate mechanical weathering, some of the red
shales and gypsum deposits show the effect of oxidation, hydration, and
other forms of chemical weathering (fig. 10).