• External processes include 1) weathering—the disintegration and decomposition of rock at or near the surface, 2) mass wasting—the transfer of rock material downslope under the influence of gravity, and 3) erosion—the 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 interior.

• 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 size—small pieces generally weather faster than large pieces; 2) mineral makeup—calcite 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 jointing.

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• Soil is a combination of mineral and organic matter, water, and air—that 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, animals, and slope.

• Soil-forming process operate from the surface downward and produce zones or layers in the soil that soil scientists call horizons. From the surface downward the horizons are designated as O, A, E, B, and C respectively. Although there are hundreds of soil types and subtypes worldwide, the three very generic types are 1) pedalfer—characterized by an accumulation of iron oxides and aluminum-rich clays in the B horizon, 2) pedocal—characterized by an accumulation of calcium carbonate, and 3) laterite—deep soils that develop in the hot, wet tropics that are poor for growing because they are highly leached and brick-like.

• Soil erosion by wind and water is the ultimate fate of practically all soils. 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.

• Weathering creates mineral deposits by consolidating metals into economical concentrations. 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 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 effect 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 the 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.

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