the number of freeze-thaw cycles, the greater the amount of frost shattering. A
unique characteristic of water is that it increases its volume by 9 percent as it
freezes. This volume increase causes the water to exert considerable pressure on
the walls of the confined openings in rocks. These pressures can exceed 20 million
Pascals in winter and high-altitude situations. This is great enough to overcome
the resistance of rocks to breakage.
Temperaturechanges also weather rocks. The diurnal temperature cycle heats
and cools rocks and the various minerals within the rocks expand and contract at
different rates. Millions of diurnal cycles are needed to mechanically reduce rock,
although the intense temperatures from fire or lava can quickly do the same work.
In general, temperature changes are accountable for limited weathering.
Salt wedging is a third form of mechanical weathering that occurs when salts
form as water evaporates. The salt crystals can grow large enough to push against
the rock and weaken its structure; this form is not nearly as potent as frost shattering.
Mechanical weathering alone is incapable of reducing rock to its smallest
weathered remnants known as clays; chemical weathering takes care of that. The
agents of chemical weathering are varied and work in combination Water is key
in weathering because it is brought by thehydrologic cycleto all places on the
continental surfaces.
Hydrolysis occurs when the hydrogen in the water reacts with minerals and
becomes part of the crystalline structure of the rock. A common example is the
alteration of the mineral feldspar, a major component of granite. The feldspar is
changed into a type of clay known as kaolinite and the original rock is easily
crumbled and removed.
As mentioned elsewhere,Karstsolution dissolves all rock types, albeit at
widely varying rates. This is because water readily combines with other common
substances. The solution flows away from the originallocationof the rock that
was dissolved. Oxidation is a profound expression of the action of water. Just as
automobiles rust, so too do rocks. Oxidation chemically combines the oxygen
atoms of water with metallic atoms in the rocks with the oxidation of iron-
bearing silicate minerals being a widespread example. The resulting iron oxide
and other oxides invariably take up more volume and are easier to remove than
unaltered rocks.
Carbonation results from carbon dioxide dissolved in water such that the water
becomes carbonic acid. Although the carbonic acid is not strong, it inexorably
works on calcium-rich rocks to produce calcium bicarbonate that is much weaker
and more readily removed than the original rock.
Life is capable of weathering crustal materials. Both animal and plant kingdoms
are involved in this type of weathering. Although it can be locally important it can-
not have the impacts of inorganic chemical and mechanical weathering, because362 Weathering and Mass Wasting