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

Weathering and SoilsWeathering and SoilsGEOLOGY TODAY - Chapter 7

Barbara W. MurckBrian J. Skinner

N. Lindsley-Griffin, 1999

Acid Rain DamageSoil Profile

The physical and chemical breakdown of

rocks at or near Earth’s surface

Rock Cycle and

Weathering

Rock Cycle and

Weathering

J.R. Griffin, 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 WeatheringMechanical Weathering


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)



Heat spalling - flakes of rock loosened by expansion from extreme heat of forest fires.

Yellowstone National Park, WY(Fig. 7.5, p. 194)



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)


© Lutgens & Tarbuck, 1999; N. Lindsley-Griffin, 1999


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


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)



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 WeatheringChemical Weathering

Acid Rain Damage

© Houghton Mifflin 1998; N. Lindsley-Griffin, 1999

Great Smoky Mountains, Tennessee

10 to 1000 timesmore acidic thannatural rain water

Cause: burningsulfur-rich coal,industrial emissions,automobile exhausts

Damages lakes,forests, buildings

Acid RainAcid Rain


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)



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.

1810 marble tombstone,

New England cemetery

(Fig. 7.9, p. 197)



Oxidation adds oxygen to iron in minerals to produce hematite and stain rocks and soil red.

Oxidized soil, Hawaii

(Fig. 7.10, p. 197)



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 WeatheringClimate and Weathering


Differential WeatheringDifferential 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 WeatheringDifferential Weathering


SoilsSoils


Soil-forming processes need water, solar heat

- both are climate dependent

Weathering of

sedimentary rock,

South Africa

Fig. 7.2, p. 192

Soil ProfileSoil Profile


Typical sequence of soil horizons developed in a cool moist climate.

(Fig. 7.13, p. 201)


Different combinations of climate and vegetation produce different soil horizons. (Fig. 7.14, p. 201)

Soils and ClimateSoils and Climate



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



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)



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)

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