Geology Reference
In-Depth Information
+
0
-
O
O
O
Fe - OH
2
Fe - OH
Fe - OH
O
O
O
Fe - OH
2
Fe - OH
2
Fe - OH
Iron oxide surface below pzc
Overall positive charge
pzc = point of zero charge
pH at which net surface charge is zero
Iron oxide surface above pzc
Overall negative charge
Figure 7 Variable, pH-dependent charge on an iron oxide surface
Roots tend to grow into existing fissures and channels and can
exert significant pressure.
Crystallization of some salts can also cause swelling, which can
exert pressure.
The key factor about physical weathering is the increase in surface area
of rock that is then available for attack by various chemical-weathering
processes. Chemical weathering is the reaction with natural waters, and
dissolved components, resulting in partial or complete dissolution of the
rock and formation of a new mineral phase. In most cases, it is the
interaction of water with the minerals in the rock that is the crucial issue.
5.2.4.1 Dissolution. Some minerals, such as gypsum (CaSO
4
.2H
2
O),
are readily soluble in water and so can persist only in soils of arid and
semi-arid regions, where water is limiting. In temperate and tropical
regions sucient water will pass through the soil to cause dissolution
and loss of gypsum:
CaSO
4
.2H
2
O
Ca
21
þ
SO
2
4
þ
2H
2
O
!
(5.1)
Quartz is a much less soluble mineral than gypsum, but even this is
slightly soluble:
SiO
2
þ
2H
2
O
"
H
4
SiO
4
(5.2)
This reaction is driven by the loss of the soluble silicic acid from the soil.
In temperate regions insucient water passes through the soil, and
weathering has proceeded over a relatively short time since the last ice
