GEO-113: ENVIRONMENTAL GEOLOGY

WEATHERING AND SOILS

LECTURE NOTES

Soil is the raw material required for almost all agriculture activity, particularly in the third world. In many ways it is the world's most valuable natural resource.

Soil is mixture of loose unconsolidated rocks or minerals, called regolith, plus organic matter, known as humus. Different types of soils vary in the compositions and relative amounts of the regolith and humus. Soil best suited for agriculture is known as loam. They have a good balance between humus and regolith of the right grain size. Soil production is controlled by a combination of the local climate, organic activity, topographic relief, and parent material and time. Soil = f(Cl, O, R, P, T)

1) Weathering Processes and Soil

Soil produced by a combination of three weathering types, chemical, mechanical, and biological.

1) Chemical Weathering: Most intense in warm, humid climate. Very little in cold, dry climates. Many minerals are not stable at earth surface conditions. They react with surface waters, atmospheric gases, and dissolved compounds (acids) and form a new set of minerals. Process also tends to remove, or leach, elements and compounds from the weathered rock and add them to the surface and ground water.

Carbonate minerals dissolve, silicate minerals undergo hydrolysis (take up water) and form clay minerals, and iron-bearing compounds oxidize (rust). Some minerals, like quartz are resistant to chemical weathering.

2) Mechanical Weathering: Physical breaking of minerals or rocks. Make "little rocks out of big rocks." Can be caused by a number of different processes.

1) Freeze-thaw: Water gets into cracks in rock. When the water freezes the ice expands pushing the sides of the crack apart. Eventually causes the rock to crumble. Potholes in roads are often due to freeze-thaw cycles during the winter.

2) Contraction/expansion: Rocks expand when the temperature goes up during the day and contract when it falls at night. This constant working of the rock eventually cracks it and causes it to crumble. Most effective in environments where there is a big temperature difference between day and night, as in the desert.

3) Salt precipitation: When water evaporates in a crack, salts crystallize from solution. The growth of the crystals can slowly pry open the crack.

All three processes (and others) may be involved. Regardless of the process, mechanical weathering dramatically increases the available surface area by decreasing the size of the particles. This allows chemical weathering to proceed more rapidly. The greater the chemical weathering the weaker the rock and the easier it is to mechanically weather. Both processes help the other (a synergistic relationship).

3) Biological Weathering: A combination of the first two. Organisms break down minerals by both chemical (they secrete acids from their roots) and mechanical (roots splits rocks or burrowing animals mix soil) means.

II. Soil Profiles and Horizons

Soils are not homogenous with depth. They usually are layered chemically and physically. Exactly how varies, depending on local conditions (Cl, O, R, P, T).

1) O horizon: Purely organic material or humus. Sometime referred to as the leaf litter.

2) A horizon: Top layer of the soil. It is very rich in organic material or humus. Dictates the fertility of the soil. Also known as the topsoil. Water percolating down reacts with minerals and leaches material from this horizon.

3) B horizon: Layer where leached material from above is deposited. Also known as the zone of accumulation. Coarser grained and with less organic material than A horizon. This horizon may contain minerals deposited from the water percolating through (caliche). This may make the soil very hard to plow.

4) C horizon: Very coarse and broken up rock. Mostly just regolith with little or no organic material. Very little chemical weathering occurs so minerals are only slightly altered.

5) D horizon: Unaltered bedrock.

Each horizon may be subdivided further than this; not all soils contain all horizons.

III. Soil development and types

Soil thickness is the result of the balance between weathering processes (generate soil) and erosional process (remove soil). Soils can be either generated in place (residual soils) or brought in and deposited by wind, water, glaciers, and volcanoes (transported soils).

Two broad types of soils:

1) Pedalfer: Found in warm, humid regions. Usually rich in iron and aluminum. Rest of the material is largely is leached away by high rainfall. This is the typical soil found in the eastern U.S.

2) Pedocal: Found in drier and cooler regions. Less chemical weathering and, therefore, less leaching of compounds. Usually rich in calcium.

Modern classification schemes may have thousands of sub-types ≠ U.S. has ten major categories (orders) with a total of 14,000 different series!

Laterite: The ultimate residual soil. A pedalfer type, but with extreme leaching. Often not much left other than Al and Fe. Laterites are the source of bauxite, the ore for aluminum.

Laterites are typically found in hot, moist tropical climates. This type of soils has a very low fertility because of the extreme chemical weathering. The often lush, tropical vegetation found where lateritic soils occur get their nutrients from recently dead or decaying material found in the O horizon. A very efficient type of recycling system. When vegetation is removed (usually to make way for development or agriculture, the soil's fertility is quickly lost (O horizon destroyed) and the soil may harden to a brick-like consistency. This makes continued farming almost impossible and very expensive due to the high cost of man-made fertilizers. Farms are abandoned and new tropical forest is cleared for new ones. Cycle simply repeats itself.

Tundra soils: Thin soils found in cold, dry climates. Typically have very low organic content. Very low levels of chemical weathering.

IV. Soil erosion

Two major agents of soil erosion:

1) Water ≠ surface runoff carries loose soil particles into local stream systems. Can occur by sheet erosion (no defined channels), rill erosion (shallow channels), and gully erosion (greater than 25 cm deep).

2) Wind ≠ usually less effective at removing soil particles than water-related processes unless there is drought or no vegetation holding soil in place.

Both processes are aided by a lack of vegetation and root systems.

U.S. averages 4.5 tons/acre/year of soil loss. This averages to 0. 04 cm/year of removal. This is largely due to agriculture. Rates of removal are usually higher in the third world.

Weathering processes replace soil at .006 cm/year or less. Takes thousands of years to produce a mature soil. The problem is we are losing soil at a rate that may be 10X the rate at which it is made. Eventually strip away the soil we depend upon for agriculture.

Human activities only make the problem worse:

1) Urbanization: Vegetation removed during development. Usually for a relatively short duration (covered eventually).

2) Agriculture: More long-term effects. Fields uncovered for long periods of time. If enough of the topsoil is lost crop yields are reduced, making food more expensive. New land is used for agriculture and the problem repeats. May lead to desertification.

3) Mining: surface mining in particular destroys vegetation, leaving soils unprotected.

4) Lumbering: Same. Leaves soils bare.

5) Recreational vehicles: Often used in environmentally sensitive areas where vegetation is sparse and easily destroyed.

Washing in of eroded soils may silt up rivers and increase food hazard. Also pollutes lakes and streams because of chemicals, pesticides, and fertilizers found in many soils.

V. Erosion Controls

The idea is to slow down the velocity of the wind and/or water and to keep the soil from being disturbed. A variety of strategies can be employed.

1) Wind breaks: Line of trees and large bushes surround fields to slow wind.

2) Wind fences: Slatted fences slow the wind and trap soil particles.

3) Contour plowing: Plow furrows across slope rather than up and down the slope. Furrows act as small steps that slow water velocity. May lower crop yields.

4) Terracing of slopes: Create flat areas on steep slopes. Has been used for thousands of years.

5) Minimum tillage or no-till agriculture: Reduce the number of time a field is plowed during the growing season. Everything is done at the same time (plowing, planting, fertilizers, pesticides). This reduces the number of times the soil is disturbed and loosened.

6) Reforestation: Replant trees after they've been cut down for lumber or cleared for agriculture, mining, or development.

7) Crop rotation: Helps preserve soil fertility.

8) Recreational use restrictions: Restrict recreational use of lands with soils prone to erosion. Protect the vegetation that helps hold the soil in place.

Many of these remedies require financial assistance to the individuals trying them. U.S. Soil Conservation Service provides these funds. $25 billion in last 50 years.

Failure to do these things can lead to complete loss of topsoil and desertification. Dust Bowl conditions can be created. Last seen in the 1930s in the U.S. Occurred in Texas, New Mexico, Oklahoma, Kansas, and Colorado. 200 million tons of soil removed during a single dust storm. Locally turned day into night. Dust in the air caused respiratory illnesses and muddy rains and snows in the eastern U.S. Desertification is a problem today in the southern Sahara, eastern Africa, and central Asia.

VI. Expansive Soils and Permafrost

Clay rich soils expand when wet and contract when dried. Causes $6 billion a year in property damage, mostly in the Rocky Mountains, southwest, Texas, and Gulf Coast.

Problem is hard to control. Can use chemicals, keep the soils wet, remove the soils, or build specially designed structures. Best method is recognition and avoidance.

Permafrost: permanently frozen soil as much as 5000 feet thick. Found in 20 percent of the earth's surface. Problems occur when human activity causes thawing. Soil flows, resulting in creep, landslides, and subsidence. Try to build without disturbing thermal balance.