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Book: Whole Regolith Pedology
Published by: Soil Science Society of America



  1.  p. 21-40
    SSSA Special Publication 34.
    Whole Regolith Pedology

    David L. Cremeens, Randall B. Brown and J. Herbert Huddleston (ed.)

    ISBN: 978-0-89118-929-9


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The Pedologic Nature of Weathered Rock

  1. R. C. Graham,
  2. K. R. Tice and
  3. William R. Guertal
  1. Foothill Engineering, Inc., Mercury, Nevada


Weathered rock, a common regolith in many areas unaffected by Pleistocene glaciation, has both lithologic and pedologie characteristics. This paper reviews pedogenic features found in weathered rock substrates and interprets the pedologie processes and environmental roles of this regolith. Lithogenic features such as rock structure, texture, and composition strongly influence weathering and resulting weathered rock characteristics. Joint fractures provide access for infiltrating water and roots, which promote weathering. As rock weathers, it develops microporosity, thereby increasing its water-holding capacity, which further enhances weathering and water availability for plants. Plant roots can penetrate the matrix of saprolite, but in less weathered rock they follow fractures, producing localized organic C concentrations as large or larger than in overlying A horizons. Organic acids and CO2 from decomposing roots promote weathering, and K uptake by living roots causes the transformation of biotite to vermiculite, an important weathering mechanism that extensively fractures rocks. Root exploitation of the saprolite matrix diminishes the importance of lithogenic features by producing channels that more effectively conduct water. Water moving from soil into weathered rock carries colloids which are commonly deposited to form argillans in fractures, abandoned root channels, and intergranular pores within the matrix. These argillans are protected from physical disturbances that affect soils and may be better expressed than those in the solum. In arid and semiarid areas, CaCO3 and opaline silica commonly precipitate within the fractures and porous matrix of weathered rock underlying soils. These features can be used to help interpret past environmental conditions. The weathered rock zone is an important and somewhat neglected part of the soil-rock continuum. Research is needed to better understand how it evolves and functions in the environment.

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Copyright © 1994. Copyright © 1994 by the Soil Science Society of America, Inc., 5585 Guilford Rd., Madison, WI 53711 USA