Page 470 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

434 part III The Earth–Atmosphere Interface
(a) Chemical weathering processes act on the joints in granite to dissolve weaker minerals, leading to a rounding of the edges of the cracks in the Alabama Hills. Mount Whitney is visible on the crest of the Sierra Nevada in the background.
Hydrolysis is the decomposition of a chemical com- pound by reaction with water. In geomorphology, hydro- lysis is of interest as a process that breaks down silicate minerals in rocks. In contrast with hydration, in which water merely combines with minerals in the rock, hydro- lysis chemically breaks down a mineral, thereby produc- ing a different mineral through the chemical reaction.
For example, the weathering of feldspar minerals in granite can occur by reaction with the normal mild acids dissolved in precipitation:
feldspar (K, Al, Si, O) + carbonic acid and water S residual clays + dissolved minerals + silica
The products of chemical weathering of feldspar in granite include clay (such as kaolinite) and silica. The particles of quartz (silica, or SiO2) formed in this process are resistant to further chemical breakdown and may wash downstream, eventually becoming sand on some distant beach. Clay minerals become a major component in soil and in shale, a common sedimentary rock.
When minerals in rock are changed by hydrolysis, the interlocking crystal network consolidating the rock breaks down and granular disintegration takes place. Such disintegration in granite may make the rock appear corroded and even crumbly (Figure 14.12b).
In Table 12.2, Classification of Igneous Rocks, the sixth line compares the various rocks’ resistance to chemi- cal weathering. It shows that the “ultramafic” (low-silica) minerals pyroxene and olivine (on the far right side of the table) are most susceptible to chemical weathering. High- silica minerals such as feldspar and quartz are more resis- tant. The chemical properties of the minerals determine the resistance of the rock to weathering; for example, basalt (a mafic rock) weathers faster chemically than does gran- ite (a felsic rock).
oxidation Another type of chemical weathering occurs when certain metallic elements combine with oxygen to
(b) Rounded granite outcrop demonstrates spheroidal weathering and the disintegration of rock. The surface is actually crumbly.
form oxides. This process is known as oxidation. Perhaps the most familiar oxidation is the “rusting” of iron to pro- duce iron oxide (Fe2O3). You see the result of this oxida- tion after leaving a tool or nails outside only to find them, weeks later, coated with a crumbly reddish-brown sub- stance. Its rusty colour is visible on the surfaces of rock and in heavily oxidized soils such as those in the humid southeastern United States, the arid U.S. Southwest, and the tropics (Figure 14.13). Here is a simple oxidation re- action in iron:
iron (Fe) + oxygen (O2) S iron oxide (hematite; Fe2O3)
When oxidation reactions remove iron from the minerals in a rock, the disruption of the crystal structures makes the rock more susceptible to further chemical weathering and disintegration.
Dissolution of Carbonates Chemical weathering also occurs when a mineral dissolves into solution—for exam- ple, when sodium chloride (common table salt) dissolves in water. Remember, water is called the universal solvent because it is capable of dissolving at least 57 of the natu- ral elements and many of their compounds.
Water vapour readily dissolves carbon dioxide, thereby yielding precipitation containing carbonic acid (H2CO3). This acid is strong enough to dissolve many minerals, especially limestone, by carbonation, a type of chemical reaction. This type of chemical weathering breaks down minerals that contain calcium, magnesium, potassium, or sodium. When rainwater attacks forma- tions of limestone (mainly calcium carbonate, CaCO3), the constituent minerals dissolve and wash away with the mildly acidic rainwater:
calcium carbonate + carbonic acid and water S calcium bicarbonate (Ca22+ · CO2 · H2O)
The dissolution of marble, a metamorphic form of limestone, is apparent on tombstones in many
◀Figure 14.12 Chemical weathering and spheroidal weathering. [Bobbé Christopherson.]