1. Weathering

  2. Weathering Processes

  3. Parent Material

    1. Residual

    2. Transported--Alluvial, Marine, Glacial, Eolian, Colluvium, Man, Organic

  4. Physiographic regions of N.C.

  1. Weathering of Rocks and Minerals

    Weathering is important because it:

    1. Produces unconsolidated material (parent material) from which soil is formed.

    2. Provides plant nutrients

    3. Results in the formation of secondary minerals, the most important group being the clay minerals.

    Rocks are the starting point for the weathering processes:

    • big rocks are weathered to smaller rock
    • smaller rocks are weathered to the minerals that make up the rocks.
    • secondary minerals are formed from primary minerals

  2. Weathering Processes

    1. Types of Weathering

      1. Physical (mechanical)

      2. Chemical - the combination of breakdown and synthesis yields weathering products

    1. Physical Processes

      1. Temperature changes

        Differential Expansion (exfoliation) - changes in temperature between day and night may cause the breakdown of rock. This is because of the different coefficients of expansion for different minerals.

        • Exfoliation: Difference in temperature between the outer layer of rock and the inner layer. This may cause the surface layers to peel away. (Click to enlarge)


        • Freezing of water (click to enlarge)


          Ice may exert a force of 150 tons/ft2. This is called frost wedging.

      2. Abrasion processes caused by water, ice and wind: (Click to enlarge)


        • river sediments: Rivers that carry sediments have great cutting power. The Grand Canyon is an extreme example of this.
        • glaciers: Ice is a very important weathering agent when it is in the form of a glacier.
        • dust storms: Sand causes erosion when it is wind-borne. The rounded rocks of the west are examples of wind erosion.

      3. Plant rooting: (click to enlarge)

        plant rooting

        Plant roots may exert a prying force on rocks and widen existing cracks.

      4. Human Impacts:

        Erosion and removal of material for building, road construction, coal mining, etc.

    2. Chemical Processes Water and solutions are the most important types of weathering of minerals. It is involved in the following reactions:

      Acid solution - Water + carbon dioxide produces a weak acid called carbonic acid. Acid may come from:
      rain (acid rain) CO2 produced by plants and other pollution sources

      All of the chemical weathering processes are interdependent. When one process occurs it may lead to the occurrence of another process. It is important to remember that weathering is continuous. It is a process in nature that never stops. By understanding this process, it is easy to see how many of the earth's features and soils were formed.

  3. Parent Material

    • Residual
    • Transported

    Soils Formed in Residual Parent Material --material that has weathered in place. The weathered material that formed the soil was like or very similar to the underlying rock. The Piedmont has some of the best examples of residual parent material in the US. It is a very old surface of metamorphic and igneous rocks.

    1. Sedimentary

      Rock Mineral Soil Characteristics
      Limestone Calcite, Dolomite, Clay minerals clayey / fertile
      Sandstone quartz sandy and low fertility
      Shale clay minerals clayey / fertile

    2. Igneous

      • Granite
        coarse texture, low fertility, acid soils.

      • Basalt and Gabbro

        higher fertility, finer texture, more weathered, deeper soils

        (remember the relationship between texture and color to fertility)

      • Metamorphic

        Original Rock Soil Characteristics
        Marble Limestone clayey / fertile
        Gneiss Granite sandy with some clay
        Slate Shale clayey / fertile

  4. Soil forming factors that act on parent material

    climate, topography, biotic, time

    1. Climate

      Temperature and precipitation are the main climatic factors that affect soil properties

      Weathering, organic matter production, and organic matter decomposition are affected by climate

      1. Temperature

        1. Temperature changes can cause enough expansion and contraction to crack hard rock.

        2. Temperature has a direct effect on the amount of organic matter that is produced. Organic matter production increases as temperature increases, provided there is rainfall for good plant growth.

        3. Increasing the temperature also increases the rate of decomposition. The higher the temperature the greater the rate of decomposition.

        4. A balance is reached between these two factors resulting in a percentage of soil organic matter that remains relatively constant.

      2. Percipitation:

        Rainfall affects weathering and the amount of organic matter production and decomposition.

        1. As rainfall increase the rate of erosion increases.

        2. When rainfall increases, organic matter production increases provided the temperature is high enough for good plant growth.

        3. If rainfall is high enough to waterlog the soil, then organic matter decomposition will not occur because of less oxidation.

          These two factors are closely related. Compare net organic matter accumulation in a tropic area that has good rainfall to a cooler area that also has good rainfall.


        Mountains-- 50-60 in/yr

        Piedmont--40-50 in/yr

        Coastal plain 45-55 in/yr

        The most organic matter will accumulate in wetter, cooler soils.

    2. Topography

      Components of topography:

      • Slope - angle of the land surface
      • Height - how high above the river or stream
      • Direction that a slope faces - aspect

      1. Slope

        % Slope = Vertical distance x 100 / Horizontal distance
        ex) 5 ft X 100 / 100 ft= 5% slope

        Effects of steep slopes, > 15% are important because

        1. runoff

        2. erosion increases as slopes get steeper

        3. generally these are shallow soils

        4. soils have thin A and B horizons

        5. if the soils are on very steep slopes there will be no B horizon

        Level slopes, 0-5%, have:

        1. less water runoff, more infiltration

        2. less erosion

        3. generally thicker soils

        4. think A and B horizons, more leaching occurs

        5. waterlogging may occur in some soils

      2. Elevation:

        Elevation, the height above a water source, will help control the soil drainage. The elevation will affect the soil wetness.

        Discuss the role of iron color, changes in iron forms and how this information is useful.

      3. Aspect

        This deals with the direction of the slope is facing. South facing will generally be drier, warmer, and less moist than north facing slopes because they get more direct sunlight. This slope idea can be related to mountainsides, hillsides, sides of roads or row furrows.

    3. Biotic

      Two parts: Soil organisms, Vegetation

      1. Soil Organisms

        Organisms play a role in: Organic matter accumulation, profile mixing, nutrient cycling, soil stability

        Examples of soil organisms:

        • ants, termites, crayfish, earthworms, etc.

        • microbes; Important in the biochemical activities that occur in the soil.

        The biggest influence of plants may be seen in the differences in soils that formed under grass vegetation and soils that formed under tree vegetation.

      2. Vegetation

        We generally look at vegetation in two different classes.

        Soils that formed under trees and soils that formed under grasses.

        1. Soils that formed under (grasses usually have thick dark A horizons. These A horizons have good organic matter accumulations. These would be the soils of the prairie. This occurs because 50% of the organic matter produced by grasses is in the roots, Also, many of these roots the each year and become organic matter. The grasses are also important because grasses use a nutrient, die, and cycle that nutrient back into the soil. This helps keep the nutrients from being leached out of the soil.

        2. Soils that formed under tree vegetation have of their organic matter concentrated at the surface. Tree roots do not the each year and add o c matter to the soil. Because of this trees do not cycle nutrients as fast as grasses do, but they do cycle nutrients from a greater depth. Because nutrients are cycled slower, these soils are more highly leached and may develop E horizons. E horizons are not generally seen under grasslands.

        3. Humans play a role also. We can affect the type of soil that forms by removing existing vegetation, moving large amounts of soil material, changing the chemical nature of the soil, etc..

    4. Time
      1. age (chronological)

      2. degree of rofile development SOIL DEVELOPMENT = "SOIL AGE"


        1. little profile development

        2. contains easily weathered minerals

        3. properties of parent material


        1. strong profile development

        2. leached, highly weathered

        3. few weatherable minearls

        Time is an important soil forming factor because it determines the degree to which the other factors are expressed.

        Soils that form from granite take much longer to form horizons because the material weathers slowly, whereas soil form from shale will develop horizons much more quickly because the material weathers fairly quick.

      Effects of time on soil formation:

      1. Older soils have deeper soil profiles.

      2. Older soils are usually more highly weathered.

      3. Older soils have usually lost their plant nutrients due to leaching.


      soil = f (pm, cl, to, or, ti) Any change in any one of the factors will cause a different type of soil to be formed. Even if 4 of the factors are the same, changes in one factor will produce a soil with different features

  5. Physiographic Regions of North Carolina

    North Carolina extends through 3 physiographic provinces that are generally present in the eastern and southeastern part of the U. S.

    Coastal Plain, Piedmont, Mountains

    1. The Coastal Plain

      This regions extends from the Atlantic ocean to the higher land of the piedmont. The break between the two provinces is called the fall line and occurs between the sediment, and the harder metamorphic and igneous rocks.

      1. 45% of the state is coastal plain.

      2. it varies in width from 100 to 140 miles

      3. goes from sea levle to 500 ft above sea level in the sand hill region.

      4. except near the edge of the piedmont the relief relatively flat.

      5. slow flowing rivers

      6. many soils have poor drainage

      7. oldest soils are highly weathered. Remember that the parent material for these soils was weathered before the soils started forming after the sediments were deposited.

      The coastal plain was formed at a time when sea levels were higher. The sediments and soils are marine in origin. periods are

      Coastal Plain Soil properties:

      1. related to sediment texture and landscape position

      2. marine sediments

      3. alluvial sediments

    2. Piedmont

      This province lies between the Coastal plain and the Mountains, it is separated from the mountains by the Blue Ridge Scarp

      1. 40% of the state is piedmont.

      2. Elevations range from 500 to 1500 ft above sea level.

      3. The piedmont is an ancient erosion surface that has rolling hills.

      The piedmont is underlain mostly by metamorphic rocks..

      These original igneous rocks were both granite and gabbro. There are some sedimentary rocks in the region known as the Triassic basin. The rocks in the piedmont are covered by a mantle of highly weathered residual material called saprolite. The soils in this region formed from this saprolite material.

      Piedmont soil characteristics

      1. acid, clayey soils on metamorphic gneiss and schist.

      2. heavy clay soils in Triassic Basin sediments

      3. more sandy soils weathered from granite

      4. silty soils in Slate Belt

      5. residual material

    3. Mountains

      The mountains of North Carolina are about 200 miles long and vary from 15 to 50 miles wide.

      1. They cover 10% of the state.

      2. They range from 1500 to 6000 ft above sea level.

      The rocks of the mountains are mainly metamorphic.

      Mountain soil characteristics:

      1. thinner, rocky, less clay

      2. colluvial material

      3. residual material

      All regions of the state have alluvial deposits, aeolian, and man influenced deposits