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Chapter 6: Weathering and Soils . Introduction: Weathering—The Breakdown of Rock. At the Earth’s surface, rocks are exposed to the effects of weathering: the chemical alteration and mechanical breakdown of rock, when exposed to air, moisture, and organic matter.
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Introduction: Weathering—The Breakdown of Rock • At the Earth’s surface, rocks are exposed to the effects of weathering: the chemical alteration and mechanical breakdown of rock, when exposed to air, moisture, and organic matter. • Weathering is an integral part of the rock cycle. • Weathering converts rock to regolith.
Physical Weathering • Rocks break at weak spots when they are twisted, squeezed, or stretched by tectonic forces. • Such forces form joints. • Rocks adjust to removal of overlying rock by expanding upward. • Removal of the weight of overlying rocks releases stress on the buried rock and causes joints to open slightly, thereby allowing water, air, and microscopic life to enter.
Joints • Joints occur as a widespread set or sets of parallel fractures. • When dikes, sills, lava flows, and welded tuffs cool they contract and form columnar joints (joints that split igneous rocks into long prisms or columns).
Crystal Growth • Water moving slowly through fractured rocks contains ions, which may precipitate out of solution to form salts. • The force exerted by salt crystals growing can be very large and can result in the rupture or disaggregation of rocks.
Frost Wedging • Wherever temperatures fluctuate about the freezing point, water in the ground periodically freezes and thaws. • As water freezes to form ice, its volume increases by about 9 percent. • This process leads to a very effective type of physical weathering known as frost wedging. • Frost wedging probably the most effective at temperatures of -5o to -15oC.
Daily Heating and Cooling • Surface temperatures as high as 80oC have been measured on exposed desert rocks. • Daily temperature variations of more than 40o have been recorded on rock surfaces, • Despite a number of careful tests, no one has yet demonstrated that daily heating and cooling cycles have noticeable physical effects on rocks.
Spalling and Wedging by Roots • Fire can be very effective in disrupting rocks. • Because rock is a relatively poor conductor of heat, only a thin outer shell expands and breaks away as a spall. • When plants grow they extend their roots into the cracks in rock, where their growth can force the rock apart.
Chemical Weathering Pathways • In chemical weathering, chemical reactions transform rocks and minerals into new chemical combinations. • There are four different chemical pathways by which chemical weathering proceeds: • Dissolution. • Hydrolysis. • Leaching. • Oxidation.
Dissolution • The easiest reaction pathway to comprehend is dissolution; this means that chemicals in rocks are dissolved in water. • Halite (NaCI) is a mineral that can be removed completely from a rock by dissolution.
Hydrolysis • Any reaction involving water that leads to the decomposition of a compound is a hydrolysis reaction. • Potassium feldspar, for instance, decomposes in the clay mineral kaolinite. • Hydrolysis is one of the chief processes involved in the chemical breakdown of common rocks.
Leaching • Leaching is the continued removal, by water solution, of soluble matter from bedrock or regolith. • Soluble substances leached from rocks during weathering are present in all surface and ground water. • Sometimes their concentrations are high enough to give the water an distinctive taste.
Oxidation • Oxidation is a process by which an ion loses an electron. • The oxidation state of the ion is said to increase.
Chemical Weathering of Iron (1) • A common example is the oxidation of iron. • An iron atom that has given up two electrons forms a ferrous ion (Fe2+). • When a ferrous ion is oxidized further by giving up a third electron, the result is a ferric ion (Fe3+). • The incorporation of water in a mineral structure is called hydration. • The ferric hydroxide will soon dehydrate, meaning it will lose some water, in which case it will form goethite (FeO.OH).
Chemical Weathering of Iron (2) • Goethite may dehydrate still further to form hematite (Fe2O3). • The intensity of the colors of ferric hydroxide, goethite, and hematite, ranging from yellowish through brownish red to brick red, can provide clues to how much time has elapsed since weathering began, and the degree or intensity of weathering.
Combined Reactions (1) • Almost all instances of chemical weathering involve more than one reaction pathway. • Dissolution plays a part in virtually all chemical weathering processes and is usually accompanied by hydrolysis and leaching.
Combined Reactions (2) • Calcite, if carbonic acid is present, dissolves rapidly in rainwater. • The effects of these processes are widely seen in the distinctive landscapes—including caves, caverns, and sink-holes—underlain by carbonate rocks.
Effects of Chemical Weathering on Common Minerals and Rocks (1) • When a granite decomposes, it does so by the combined effects of dissolution, hydrolysis, and oxidation. • Feldspar, mica, and ferromagnesian minerals weather to clay minerals and soluble Na1+, K1+, and Mg2+ ions. • The quartz grains, being relatively inactive chemically, remain essentially unaltered.
Effects of Chemical Weathering on Common Minerals and Rocks (2) • When basalt weathers, the plagioclase feldspar and ferromagnesian minerals it contains form clay minerals and soluble ions (Na1+, Ca2+, and Mg2+). • The iron from ferromagnesian minerals, together with iron from magnetite, forms goethite.
Effects of Chemical Weathering on Common Minerals and Rocks (3) • When limestone, the most common sedimentary rock that contains calcium carbonate, is attacked by dissolution and hydrolysis, it is readily dissolved, leaving behind only the nearly insoluble impurities (chiefly clay and quartz) that are always present in small amounts in the rock. • Minerals such as gold, platinum, and diamond persist during weathering.
Exfoliation and Spheroidal Weathering • During weathering, concentric shells of rock may spall from the outside of an outcrop or a boulder, a process known as exfoliation. • Exfoliation is caused by differential stresses within a rock that result mainly from chemical weathering. • Spheroidal weathering produces, by such progressive decomposition, rounded boulders.
Surface Area • The effectiveness of chemical weathering increases as the surface area exposed to weathering increases. • Surface area increases simply from the subdivision of large blocks into smaller blocks. • Chemical weathering therefore leads to a dramatic increase in the surface area.
Factors Influencing Weathering (1) • Mineralogy. • The resistance of a silicate mineral to weathering is a function of three principal things: • The chemical composition of the mineral. • The extend to which the silicate tetrahedra in the mineral are polymerized. • The acidity of the waters with which the mineral reacts.
Factors Influencing Weathering (2) • Most stable chemical compositions: • Ferric oxides and hydroxides. • Aluminum oxides and hydroxides. • Quartz. • Clay minerals. • Muscovite. • Potassium feldspar. • Biotite. • Sodium feldspar (albite-rich plagioclase). • Amphibole.
Factors Influencing Weathering (3) • Least stable chemical compositions: • Pyroxene. • Calcium feldspar (anorthite-rich plagioclase). • Olivine. • Calcite. • Rock type and structure. • Differences in the composition and structure of adjacent rock units can lead to contrasting rates of weathering and to landscapes that reflect such differential weathering.
Factors Influencing Weathering (4) • Slope angle. • On a steep slope, solid products of weathering move quickly away, continually exposing fresh bedrock to renewed attack. • On gentle slopes, weathering products are not easily washed away and in places may accumulate to depths of 50 m or more.
Factors Influencing Weathering (5) • Climate. • Moisture and heat promote chemical reactions. • Therefore, weathering is more intense and generally extends to greater depths in a warm, moist climate than in a cold, dry one. • In moist tropical lands, like Central America and Southeast Asia, obvious effects of chemical weathering can be seen at depths of 100 m or more.
Factors Influencing Weathering (6) • Rocks such as limestone and marble are highly susceptible to chemical weathering in a moist climate and commonly form low, gentle landscapes. • In a dry climate, however, the same rocks form bold cliffs because, with scant rainfall and only patchy vegetation, little carbonic acid is present to dissolve carbonate minerals.
Factors Influencing Weathering (7) • Burrowing animals. • Large and small burrowing animals bring partly decayed rock particles to the land surface. • Although burrowing animals do not break down rock directly, the amount of disaggregated rock they move over many millions of years must be enormous.
Factors Influencing Weathering (8) • Time. • Hundred to thousands of years are required for a hard igneous or plutonic rock to decompose. • Weathering processes are speeded up by increasing temperature and available water, and by decreasing particle size. • The rate of weathering tends to decrease with time as the weathering profile, or a weathering rind, thickens.
Soil: Origin And Classification Soils are one of the most important natural resources. Soils support the plants that are the basic source of our nourishment and provide food for domesticated animals. Soils store organic matter, thereby influencing how much carbon is cycled in the atmosphere as carbon dioxide, and they also trap pollutants.
Origin of Soils • Soils are produced by: • The physical and chemical breakdown of solid rock by weathering processes. • The organic matter derived from the decay of dead plants and animals.
Soil Profiles (1) • Soil profiles: • As a soil develops from the surface downward, an identifiable succession of approximately horizontal weathered zones, called soil horizons, forms. • The soil horizons constitute a soil profile. • The uppermost horizon may be a surface accumulation of organic matter (O horizon). • An A horizon may either underlie an O horizon or lie directly beneath the surface. • The A horizon is dark because of the presence of humus (decomposed residue of plant and animal tissues, which is mixed with mineral matter).
Soil Profiles (2) • The B horizon is enriched in clay and/or iron and aluminum hydroxides produced by the weathering of minerals within the horizon. • The C horizon is the deepest horizon and consists of rock in various stages of weathering.
Soil Types • Different soils result from the influence of six formative factors: • Climate. • Vegetation cover. • Soil organisms. • Composition of the parent material. • Topography. • Time.
Polar soils • Polar soils generally are dry and lack well-developed horizons. • They are classified as entisols. • In wetter high-latitude environments, mat-like tundra vegetation overlies perennially frozen ground: they form water-logged soils that are rich in organic matter, called histosols. • On well-drained sites soils develop recognizable A and B horizons called inceptisols.
Temperate Latitude Soils • Temperate-latitude soils: • Alfisols, characteristic of deciduous woodlands, have a clay-rich B horizon beneath. • Acidic spodosols develop in cool, moist evergreen forests. • Grasslands and prairies typically develop mollisols having thick, dark-colored, organic-rich A horizons. • Soils formed in moist subtropical climates, commonly displaying a strongly weathered B horizon, are called ultisols.
Desert Soils • In dry climates, where lack of moisture reduces leaching, carbonates accumulate in the profile during the development of aridosols. • Over extensive arid regions of the southwestern United States, carbonates have in this way built up a solid, almost impervious layer of whitish calcium carbonate known as caliche.
