Earth An Introduction to Physical Geology 10 e

Earth: An Introduction to Physical Geology, 10 e Tarbuck & Lutgens © 2011 Pearson Education, Inc.

Weathering and Soil Earth, 10 e - Chapter 6 Stan Hatfield Southwestern Illinois College © 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 © 2011 Pearson Education, Inc.