Geoscience Reference
In-Depth Information
Edges
Water can act on
two surfaces
Hydrolysis
Rounded rocks
Water can act on
only one surface
Corners
Water can act on
three surfaces
(a)
(b)
Figure 14.10 Spheroidal weathering. (a) Corners and edges of rocks offer more surfaces on which water can act, so they are worn away
faster than the rest of the rock. (b) Rounded rocks such as these in Joshua Tree National Park in California usually result from spheroidal
weathering. Note the small trees for scale.
rocks, for example, hydrolysis causes feldspars to weather into
clays, such as kaolinite and silica, which may be subsequently
washed or blown away and deposited somewhere in a soil or
stream system. If a sufficient amount of clay is removed in this
fashion, more resistant quartz grains are exposed and, with fur-
ther weathering, liberated from the host rock, to be transported
by a stream or perhaps blown into sand dunes.
You can see the effects of hydrolysis in several different
ways. The most prominent impact of hydrolysis on the landscape
is that the sharp edges and corners of rocks become rounded
in a process called spheroidal weathering (FigureĀ  14.10).
This process has the greatest impact where numerous joints
occur in rocks, providing more corners and edges on which
decomposition can occur.
dissolve and wash away in solution. This form of weathering
is called carbonation (because carbon is reacting with miner-
als) and occurs because carbon dioxide is easily dissolved by
atmospheric water vapor, forming carbonic acid. As a result,
Oxidation Another form of chemical weathering is called
oxidation . In contrast to hydrolysis, which requires water, oxi-
dation occurs when oxygen is added to chemical compounds
and causes electrons within the compounds to be lost. In ev-
eryday life, we are most familiar with oxidation when tools
and household equipment rust when left outside for a few days.
Rusting occurs because oxygen reacts with iron to produce iron
oxides, which are reddish-colored and flake off with time. The
iron underneath is then exposed to the air and forms rust in its
turn. The same process happens in rocks that contain abundant
iron, such as those in the southwestern United States. These
rocks have been heavily oxidized, resulting in their orange
appearance (Figure 14.11).
Figure 14.11 The effects of oxidation. Oxidized sandstones
in the southwestern United States. Scenes like this at the Fiery
Furnace in Arches National Park are very common in this part of
the country because iron-bearing rocks crop out in many places
due to the arid climate and lack of vegetation.
Carbonation A third kind of chemical weathering occurs
when water containing carbon dioxide causes minerals to
Oxidation A form of chemical weathering in which oxygen
chemically combines with metallic iron to form iron oxides,
resulting in the loss of electrons.
Carbonation A type of chemical weathering caused by rain-
water that has absorbed atmospheric carbon dioxide and
formed a weak carbonic acid that slowly dissolves rock.
 
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