2. Chemical Weathering. Chemical weathering of geologic material involves the alteration of its atomic
structure. There are several methods by which this alteration may take place; however, all require optimal
temperatures and the presence of water. Consequently, chemical weathering predominates in warm, humid
environments. Weathered products generally consist of very fine-grained (clay and silt) particles. Some of the
more common means of chemical alteration are listed below.
a. Oxidation. Oxidation is the chemical union of a compound with oxygen. This type of reaction often
occurs when iron-bearing compounds are, under moist conditions, exposed to atmospheric oxygen. The process,
commonly known as rusting, transforms original green or gray metallic substances to fine-grained red, yellow, or
b. Hydration. Hydration is the chemical addition of water to the structure of a mineral. When the minerals
that comprise a rock are hydrated, there is a corresponding increase in the volume of the rock. The volumetric
expansion causes a decrease in both the density and cohesiveness of the rock. Some minerals are much more
likely than others to accept water into their structures; therefore, they are more susceptible to chemical
c. Hydrolysis. Hydrolysis, like hydration, involves a chemical reaction between water and the minerals
composing a rock. However, in this case, both the minerals and the water decompose and react to form new, less
resistant compounds. Many competent silicate minerals are hydrolyzed to incompetent clays.
d. Carbonation. Carbonation refers to chemical processes in which carbon dioxide, contained either in the
atmosphere or in carbonated waters, reacts with rocks composed of magnesium, sodium, or potassium ions to
produce carbonates and bicarbonates. These types of rocks, in turn, are extremely soluble in water.
e. Solution. All surface water contains carbon dioxide derived from the atmosphere. A portion of the
dissolved carbon dioxide reacts with the water to form a weak acidic solution called carbonic acid. Carbonic acid,
in turn, readily dissolves carbonate rocks, such as limestones and dolomites. The dissolved ions are carried away
in the acidic solution, sometimes leaving huge void spaces called caves. Solutioning is the type of weathering
responsible for the development of karst topography discussed in Lesson 1.B.2.b(1)(a), page 1-21.
PART B - EROSION AND DEPOSITION
The unconsolidated materials formed from mechanical or chemical weathering may be transported, or eroded,
from their place of origin and later deposited in another location, thereby forming the basis for new sedimentary
rocks. Each transportation agent (water, wind, ice, and gravity) is capable of forming its own unique erosional as
well as depositional features. These agents and their associated features are discussed below.
1. Water. There are numerous types of erosional and depositional features created by the action of water. For
example, streams, freshwater lakes, and waves are all masses of water, and each one is associated with its own
types of features.