The following statements summarize and describe many of the key terms and concepts presented
in the chapter.
- External processes include (1) weatheringthe disintegration and
decomposition of rock at or near the surface, (2) mass wastingthe
transfer of rock material downslope under the influence of gravity, and (3)
erosionthe incorporation and transportation of material by a mobile
agent, usually water, wind, or ice. They are called external processes because they occur at
or near Earth's surface and are powered by energy from the Sun. By contrast, internal
processes, such as volcanism and mountain building, derive their energy from Earth's
- Mechanical weathering is the physical breaking up of rock into smaller
pieces. Chemical weathering alters a rock's chemistry, changing it into
different substances. Rocks can be broken into smaller fragments by frost wedging,
unloading, and biological activity. Water is by far the most important
agent of chemical weathering. Oxygen in water can oxidize some materials, while carbon
dioxide (CO2) dissolved in water forms carbonic acid. The chemical
weathering of silicate minerals frequently produces (1) soluble products containing sodium,
calcium, potassium, and magnesium, (2) insoluble iron oxides, and (3) clay minerals.
- The rate at which rock weathers depends on such factors as (1) particle
sizesmall pieces generally weather faster than large pieces; (2) mineral
makeupcalcite readily dissolves in mildly acidic solutions, and silicate
minerals that form first from magma are least resistant to chemical weathering; and (3)
climatic factors, particularly temperature and moisture. Frequently, rocks
exposed at Earth's surface do not weather at the same rate. This differential
weathering of rocks is influenced by such factors as mineral makeup and degree of
- Soil is a combination of mineral and organic matter, water, and
airthat portion of the regolith (the layer of rock and mineral fragments
produced by weathering) that supports the growth of plants. Soil texture refers to the
proportions of different particle sizes (clay, silt, and sand) found in soil. 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 horizons are
designated as O, A, E, B, and C, respectively.
- 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. Rates of soil erosion vary from one
place to another and depend on the soil's characteristics as well as such factors as
climate, slope, and type of vegetation. Human activities have greatly accelerated the rate
of soil erosion in many areas.
- Weathering creates mineral deposits by concentrating metals into economically
valuable deposits. The process, 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 thus become more concentrated. Bauxite, the
principal ore of aluminum, is one important ore created by secondary enrichment.
- In the evolution of most landforms, mass wasting is the step that follows
weathering. The combined effects of mass wasting and erosion by running water produce stream
valleys. Gravity is the controlling force of mass wasting. Other factors that
influence or trigger downslope movements are saturation of the material with water,
oversteepening of slopes beyond the angle of repose, removal of anchoring
vegetation, and ground vibrations from earthquakes.
- The various processes included under the name of mass wasting are classified and
described on the basis of (1) the type of material involved (debris, mud, earth, or rock),
(2) the kind of motion (fall, slide, or flow), and (3) the rate of movement (fast, slow).
The various kinds of mass wasting include the more rapid forms called slump,
rockslide, debris flow, and earthflow, as well as the slow movements
referred to as creep and solifluction.