Weathering and Soils GEOLOGY TODAY - Chapter 7 Barbara W. Murck Brian J. Skinner N. Lindsley-Griffin, 1999 Acid Rain Damage Soil Profile

The physical and chemical breakdown of rocks at or near Earth’s surface Rock Cycle and Weathering J.R. Griffin, N. Lindsley-Griffin, 1999

N. Lindsley-Griffin, 1999 Joints and bedding planes cause rocks to break into blocks. Joints and fractures aid weathering. Red sandstone, Wales (Fig. 7.3, p. 194) Mechanical Weathering

N. Lindsley-Griffin, 1999 Frost Wedging - rocks split by repeated freezing and thawing. Temperature must cycle back and forth across freezing point. Important in temperate and cold climates Granite boulder, Sierra Nevada, CA (Fig. 7.4, p. 194) Mechanical Weathering

N. Lindsley-Griffin, 1999 Heat spalling - flakes of rock loosened by expansion from extreme heat of forest fires. Yellowstone National Park, WY (Fig. 7.5, p. 194) Mechanical Weathering

N. Lindsley-Griffin, 1999 Root Wedging - tree roots pry rock apart by growing into cracks. As roots enlarge, flakes of rock break off. Ponderosa pine on granite (Fig. 7.6, p. 195) Mechanical Weathering

© Lutgens & Tarbuck, 1999; N. Lindsley-Griffin, 1999 Mechanical Weathering Exfoliation - thin sheets flake off. When rocks formed at depth are uplifted to the surface, confining pressure decreases. They flake into concentric layers. Common in sandstone, granite, and other even- grained rocks. Granite dome, Yosemite National Park, CA

N. Lindsley-Griffin, 1999 Exfoliation - Corners weather faster because they are attacked on all sides, causing boulders to become rounder. The Devil’s Marbles, Australia (Fig. 7.7, p. 195) Mechanical Weathering

N. Lindsley-Griffin, 1999 Normal chemical weathering: Slightly acidic rainwater in which atmospheric carbon dioxide is dissolved, or more acidic ground water from decaying organic matter. Acid Rain - human-generated sulfur and nitrogen compounds in the atmosphere mix with rainwater to form strong acids. Marble statue, Italy Chemical Weathering Acid Rain Damage

© Houghton Mifflin 1998; N. Lindsley-Griffin, 1999 Great Smoky Mountains, Tennessee 10 to 1000 times more acidic than natural rain water Cause: burning sulfur-rich coal, industrial emissions, automobile exhausts Damages lakes, forests, buildings Acid Rain

N. Lindsley-Griffin, 1999 Hydrolysis - any chemical reaction that involves water Ion Exchange - hydrogen ions in natural acid solutions displace cations to form new minerals (Fig. 7.8 A, B; p. 195) Chemical Weathering

N. Lindsley-Griffin, 1999 Solution can dissolve some minerals completely. Calcite and dolomite are especially vulnerable (limestone and marble). Sometimes solution of silicates leaves a residue of clay behind marble tombstone, New England cemetery (Fig. 7.9, p. 197) Chemical Weathering

N. Lindsley-Griffin, 1999 Oxidation adds oxygen to iron in minerals to produce hematite and stain rocks and soil red. Oxidized soil, Hawaii (Fig. 7.10, p. 197) Chemical Weathering

N. Lindsley-Griffin, 1999 Climate - combination of temperature and rainfall. Controls type and rate of weathering. Strength and type of mineral bonds also important. (Fig. 7.12, p. 199) Climate and Weathering

N. Lindsley-Griffin, 1999 Differential Weathering Limestone weathers quickly in humid climates by hydrolysis and solution. In arid climates sandstone weathers more quickly than limestone. Limestone and marble in New England weather faster than granite and basalt dikes Folded marble cut by granite and basalt dikes, central Maine

In arid climate of the Grand Canyon: Limestone and sandstone form steep cliffs, resistant ledges. Shale is easily eroded, forms gentle slopes. Differential Weathering N. Lindsley-Griffin, 1999

Soils N. Lindsley-Griffin, 1999 Soil-forming processes need water, solar heat - both are climate dependent Weathering of sedimentary rock, South Africa Fig. 7.2, p. 192

Soil Profile N. Lindsley-Griffin, 1999 Typical sequence of soil horizons developed in a cool moist climate. (Fig. 7.13, p. 201)

N. Lindsley-Griffin, 1999 Different combinations of climate and vegetation produce different soil horizons. (Fig. 7.14, p. 201) Soils and Climate

N. Lindsley-Griffin, 1999 Pedalfer soils are rich in clays, with aluminum and iron oxides. Very fertile - moderate rainfall, temperate climates. Houghton-Mifflin, 1998 Fig. 7.15, p. 202

Soils and Climate N. Lindsley-Griffin, 1999 Pedocal soils are rich in calcium carbonate, gypsum, and other soluble minerals. Fertile with enough water. Warm, dry climates. White caliche horizon, central New Mexico (Fig. 7.16, p. 203)

Soils and Climate N. Lindsley-Griffin, 1999 Laterite soils are made of insoluble iron and aluminum oxide residues left by leaching - most original materials removed by solution. Low fertility. Warm, wet, tropical forests. (See Fig. 7.17, p. 203)