PART A - WEATHERING
There are two broad categories of weathering-mechanical, or physical, weathering (disintegration) and chemical
weathering (decomposition).
1. Mechanical Weathering. Mechanical weathering is the physical disintegration or fracturing of rock material
with no regard for any chemical changes that may occur simultaneously. This type of weathering tends to
predominate in arid environments and generally results in the production of relatively large, angular rock
fragments (gravel-sized particles or larger) that are composed of the same material as the parent rock. There are
several common natural processes that may be responsible for mechanical weathering.
a. Freeze/Thaw Water Cycle. When temperatures drop below 32 degrees Fahrenheit, any water that may
be present in the pores or fractures of a rock is subject to freeing. Initial crystallization of water molecules takes
place on the surface therefore, liquid water may still exist below a cap of frozen ice. If temperatures remain
depressed, the confined water underlying the ice layer will eventually freeze, and as it freezes, its volume will
expand by nine percent. This will exert a great amount of pressure on the surrounding rock. After several cycles
of freezing followed by thawing and then by freezing once again, the rock may eventually break into angular
fragments.
b. Temperature Changes. Daily and seasonal temperature changes are accompanied by the alternate
heating and cooling of rock material. In general, rocks expand when heated and contract when cooled. However,
if a rock is made up of several different minerals, it may experience differential expansion and contraction when
exposed to cycles of alternate heating and cooling. This could result in a weakening of the rock to the point
where it is finally fractured.
c. Exfoliation. Many of the rocks that are now exposed at the surface of the earth were, at one time, buried
at great depths. The release of confining pressures brought about by the wearing away of overlying rock material
may cause expansion of the exposed surface. The expansion is normally accompanied by concentric fracturing
parallel to the land surface. The broken rock fragments eventually spall off, or exfoliate from, the main rock
body, which then takes on the appearance of an onion.
d. Abrasion. The physical grinding of rock material may result in the formation of scratches or grooves on
the rock surfaces. Crustal movements, like faulting and folding, as well as mass movements, such as rock slides
and avalanches, impart this type of mechanical destruction in a very short time. Running water in the form of
streams and rivers and ice in the form of glaciers can also act as agents of abrasion.
e. Organic Destruction. Significant amounts of mechanical weathering are caused by the actions of
organisms. For example, the root systems of plants have the ability to penetrate and enlarge small rock fractures,
and burrowing animals, such as earthworms and ants, are capable of extensive sediment disturbance. In addition,
human activities also contribute to the rapid disintegration of rock material. Mining, quarrying, and excavating
construction sites are only a few manifestations of the influence of man.
EN5341
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