WEATHERING AND SOILS

I. Weathering

II. Effects of Weathering on Rock

III. Soil

IV. Types of Soil

V. Possible Essay Questions

VI. Practice Questions

I. Weathering -

• Weathering is the disintegration and decomposition of rock into small pieces of sediment and dissolved minerals, respectively.

• Mechanical weathering processes (disintegration) -
  • Frost wedging - pushing rock apart by alternate freezing and thawing of water in cracks.
    VIEW RESULTS OF FROST WEDGING

  • Unloading - cracking of rocks as they expand due to removal of overburden pressure near the surface.
    VIEW RESULTS OF UNLOADING

  • Thermal expansion - cracking of rocks from alternate expansion and contraction due to daily changes in temperature.

  • Organic activity - expansion of cracks by growth of roots and the breakup of rock by burrowing activities.

• Chemical weathering processes (decomposition) -
  • Oxidation - oxygen anions combine with mineral cations to break down and soften the original mineral. Example - iron turns into hematite

  • Hydration - hydroxide (OH) anions combine with mineral cations to break down and soften the original mineral. Example - feldspar turns into clay

  • Dissolution - mineral anions and cations are dissolved by reaction with acids in the environment.
    • Natural acids - carbonic acid in water, humic acid in soil

    • Man-made acids - sulfuric acid in acid rain

• Rates of Weathering
  • Chemical processes are most active (work most rapidly) in warm and/or moist climates where water is abundant.

  • Mechanical processes are most active (work most rapidly) in cold and/or arid climates where water is scarce.

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II. Effect of weathering on rock -

• Angular rock fragments that are originally created in a rock due to fracturing of the rock, are slowly rounded at the corners and reduced in size.

• Spheroidal weathering - production of rounded forms by weathering of homogenous rock.
  VIEW OF SPHEROIDAL WEATHERING

• Differential weathering - production of alternating resistant and nonresistant layers of rock due to the varying effect of weathering on different types of rock.

• Exfoliation - production of loose surface slabs of rock that fall off the rock like the skin of an onion.

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III. Soil -

• A layer of broken, unconsolidated rock fragments produced over hard, bedrock surfaces by weathering processes.

• The process of erosion converts soil into sediment.

• Three soil horizons are recognized -
  • "A" horizon - the uppermost horizon, a zone of leaching, infiltrating rain water dissolves minerals and carries the ions to lower horizons.

  • "B" horizon - the middle horizon, a zone of accumulation, ions brought here by infiltrating rain water are recombined to precipitate new minerals.

  • "C" horizon - the lowermost horizon, a zone of partially weathered rock.
  DIAGRAM OF A SOIL PROFILE

• Three factors determine the type of soil that will form -
  • Climate - Is it dry or wet?

  • Slope - Is it gentle or steep?

  • Time - How long has weathering gone on undisturbed?

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IV. Basic types of soil -

• Pedocal - dry climate; little organic matter; little leaching; most nutrient ions are still present, in places water evaporates and calcite precipitates in the "B" horizon to form a hard layer called caliche or hardpan.

• Pedalfer - moist climate; moderate organic matter; moderate leaching; many soluble nutrients are leached out, only clay, aluminum, and iron are left.

• Laterite - very wet climate; abundant organic matter; extensive leaching; all soluble nutrients are leached out, iron and aluminum oxide forms.

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V. Possible essay questions -

• Describe four types of mechanical weathering.

• Describe three types of chemical weathering.

• Describe how weathering is related to climate.

• Name and describe the three horizons found in a layer of soil.

• What three factors determine the type of soil that will form in an area?

• Describe the three basic types of soil found in the world.

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VI. Practice Questions

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