CHAPTER 5 Weathering and Soils
146
plants—is a combination of mineral and organic matter, water,
and air. About half of the total volume of a good-quality soil is
a mixture of disintegrated and decomposed rock (mineral matter)
and humus (the decayed remains of animal and plant life); the
remaining half consists of pore spaces, where air and water circulate. The most important factors that control soil formation are
parent material, time, climate, plants and animals, and topography.
Soil-forming processes operate from the surface downward
and produce zones or layers in the soil called horizons. From the
surface downward, the soil horizons are respectively designated
as O (largely organic matter), A (largely mineral matter), E
(where the fine soil components and soluble materials have been
removed by eluviation and leaching), B (subsoil, often referred to
as the zone of accumulation), and C (partially altered parent material). Together the O and A horizons make up what is commonly
called the topsoil.
In the United States, soils are classified using a system known
as the Soil Taxonomy. It is based on physical and chemical
properties of the soil profile and includes six hierarchical
categories. The system is especially useful for agricultural and
related land-use purposes.
Soil erosion is a natural process. It is part of the constant
recycling of Earth materials that we call the rock cycle. Once in a
stream channel, soil particles, which can now be called sediment,
are transported downstream and eventually deposited. Rates of
soil erosion vary from one place to another and depend on the
soil’ s characteristics as well as on such factors as climate, slope,
and type of vegetation. Human activities have greatly accelerated
the rate of soil erosion in many areas.
Weathering creates ore deposits by concentrating minor
amounts of metals into economically valuable deposits. The
process, often called secondary enrichment, is accomplished by
either (1) removing undesirable materials and leaving the desired
elements enriched in the upper zones of the soil or (2) removing
and carrying the desirable elements to lower soil zones where
they are redeposited and become more concentrated. Bauxite,
the principal ore of aluminum, is one important ore created as a
result of enrichment by weathering processes. In addition,
many copper and silver deposits result when weathering
processes concentrate metals that were formerly dispersed
through low-grade primary ore.
Key Terms
chemical weathering (p. 125)
differential weathering (p. 132)
eluviation (p. 137)
erosion (p. 124)
exfoliation domes (p. 127)
external processes (p. 124)
frost wedging (p. 126)
horizons (p. 137)
internal processes (p. 124)
leaching (p. 137)
mass wasting (p. 124)
mechanical weathering (p. 125)
parent material (p. 133)
regolith (p. 133)
secondary enrichment (p. 144)
sheeting (p. 127)
soil (p. 133)
soil profile (p. 137)
Soil Taxonomy (p. 139)
solum (p. 138)
spheroidal weathering (p. 131)
weathering (p. 124)
146
plants—is a combination of mineral and organic matter, water,
and air. About half of the total volume of a good-quality soil is
a mixture of disintegrated and decomposed rock (mineral matter)
and humus (the decayed remains of animal and plant life); the
remaining half consists of pore spaces, where air and water circulate. The most important factors that control soil formation are
parent material, time, climate, plants and animals, and topography.
Soil-forming processes operate from the surface downward
and produce zones or layers in the soil called horizons. From the
surface downward, the soil horizons are respectively designated
as O (largely organic matter), A (largely mineral matter), E
(where the fine soil components and soluble materials have been
removed by eluviation and leaching), B (subsoil, often referred to
as the zone of accumulation), and C (partially altered parent material). Together the O and A horizons make up what is commonly
called the topsoil.
In the United States, soils are classified using a system known
as the Soil Taxonomy. It is based on physical and chemical
properties of the soil profile and includes six hierarchical
categories. The system is especially useful for agricultural and
related land-use purposes.
Soil erosion is a natural process. It is part of the constant
recycling of Earth materials that we call the rock cycle. Once in a
stream channel, soil particles, which can now be called sediment,
are transported downstream and eventually deposited. Rates of
soil erosion vary from one place to another and depend on the
soil’ s characteristics as well as on such factors as climate, slope,
and type of vegetation. Human activities have greatly accelerated
the rate of soil erosion in many areas.
Weathering creates ore deposits by concentrating minor
amounts of metals into economically valuable deposits. The
process, often called secondary enrichment, is accomplished by
either (1) removing undesirable materials and leaving the desired
elements enriched in the upper zones of the soil or (2) removing
and carrying the desirable elements to lower soil zones where
they are redeposited and become more concentrated. Bauxite,
the principal ore of aluminum, is one important ore created as a
result of enrichment by weathering processes. In addition,
many copper and silver deposits result when weathering
processes concentrate metals that were formerly dispersed
through low-grade primary ore.
Key Terms
chemical weathering (p. 125)
differential weathering (p. 132)
eluviation (p. 137)
erosion (p. 124)
exfoliation domes (p. 127)
external processes (p. 124)
frost wedging (p. 126)
horizons (p. 137)
internal processes (p. 124)
leaching (p. 137)
mass wasting (p. 124)
mechanical weathering (p. 125)
parent material (p. 133)
regolith (p. 133)
secondary enrichment (p. 144)
sheeting (p. 127)
soil (p. 133)
soil profile (p. 137)
Soil Taxonomy (p. 139)
solum (p. 138)
spheroidal weathering (p. 131)
weathering (p. 124)