© 2011 Pearson Education, Inc. Weathering and Soil Earth, 10e - Chapter 6

© 2011 Pearson Education, Inc. Earth’s External Processes Weathering—the physical breakdown and chemical decomposition of rock Mass wasting—the transfer of rock and soil downslope under the influence of gravity Erosion—the physical removal of material by water, wind, ice, or gravity

© 2011 Pearson Education, Inc. Weathering Two types: 1.Mechanical weathering — breaking of rocks into smaller pieces 2.Types of mechanical weathering – Frost wedging – Unloading – Thermal expansion – Biological activity

© 2011 Pearson Education, Inc. Sheeting and Exfoliation Due to Unloading

© 2011 Pearson Education, Inc. Weathering Chemical weathering Breaks down rock components and internal structures of minerals The most important agent is water. – Responsible for transport of ions and molecules involved in chemical processes

© 2011 Pearson Education, Inc. Weathering Major processes of chemical weathering Dissolution – Aided by small amounts of acid in the water Oxidation – Any reaction when electrons are lost from one element Hydrolysis – The reaction of any substance with water. – A hydrogen ion attacks and replaces other ions.

© 2011 Pearson Education, Inc. Weathering Alterations caused by chemical weathering Decomposition of unstable minerals Formation or retention of stable materials Physical changes such as the rounding of corners or edges

© 2011 Pearson Education, Inc. Products of Weathering

© 2011 Pearson Education, Inc. Rates of Weathering Factors affecting weathering Surface area Rock characteristics – Rocks containing calcite (marble and limestone) readily dissolve in weakly acidic solutions. – Silicate minerals weather in the same order as their order of crystallization.

© 2011 Pearson Education, Inc. Surface Area and Chemical Weathering

© 2011 Pearson Education, Inc. Rates of Weathering Climate – Temperature and moisture characteristics – Chemical weathering is most effective in areas of warm, moist climates. Differential weathering – Rocks do not weather uniformly due to regional and local factors. – Results in many unusual and spectacular rock formations and landforms

© 2011 Pearson Education, Inc. Spheroidal Weathering of Extensively Jointed Rock

© 2011 Pearson Education, Inc. Differential Weathering in Bryce Canyon National Park, Utah

© 2011 Pearson Education, Inc. Soil Soil = mineral and organic matter + water + air Rock and mineral fragments produced by weathering (regolith) that support the growth of plants Humus (decayed animal and plant remains) is a small, but essential component.

© 2011 Pearson Education, Inc. Composition of a “Good” Soil

© 2011 Pearson Education, Inc. Soil Factors controlling soil formation Parent material – Residual versus transported soil Time – Amount of time for soil formation varies depending on geologic and climatic conditions.

© 2011 Pearson Education, Inc. Soil Climate – Most influential control of soil formation – Key factors are temperature and precipitation. Plants and animals – Influence the soil’s physical and chemical properties Topography – Steep slopes often have poorly developed soils. – The optimum terrain is a flat-to-undulating upland surface.

© 2011 Pearson Education, Inc. Variations in Soil Development

© 2011 Pearson Education, Inc. Soil The soil profile Soil-forming processes operate from the surface downward. Vertical differences are called horizons, which are zones or layers of soil. – O horizon—organic matter – A horizon—organic and mineral matter » High biological activity » O and A horizons make up the topsoil

© 2011 Pearson Education, Inc. Soil Soil profile – E horizon—little organic matter » Zone of eluviation and leaching – B horizon—zone of accumulation – C horizon—partially altered parent material Collectively, the O, A, E, and B horizons make up the solum, or “true soil.”

© 2011 Pearson Education, Inc. Idealized Soil Profile

© 2011 Pearson Education, Inc. Soil Profile Showing Different Horizons

© 2011 Pearson Education, Inc. Soil Classifying soils Variations in soil formation over time and distances has led to a great variety of recognized soil types. Groups have been established using common characteristics. In the United States, a system was devised, and called the Soil Taxonomy.

© 2011 Pearson Education, Inc. Soil Classifying soils Soil Taxonomy – Emphasis is placed on physical and chemical properties. – Six hierarchical categories exist. – Descriptive names are derived from Latin and Greek. – 12 basic soil orders are recognized.

© 2011 Pearson Education, Inc. Basic Soil Orders

© 2011 Pearson Education, Inc. Soil Soil erosion Part of the natural recycling of all Earth materials Natural rates of soil erosion depend on: – Soil characteristics – Climate – Slope – Type of vegetation

© 2011 Pearson Education, Inc. Soil Erosion Where Vegetation Is Lacking

© 2011 Pearson Education, Inc. Soil In many regions, soil erosion is much greater than the rate of soil formation. Sedimentation and chemical pollution – Related to excessive soil erosion – Occasionally, soil particles are contaminated with pesticides.

© 2011 Pearson Education, Inc. End of Chapter 6