Fjölrit RALA - 15.06.2004, Page 78

ratio of CEC, in present andic Brown soil. The andic diagnostic criteria are near the
acceptable limits to be an andic intergrade to Brown soils, but not in lahar alterite. To note in
Andic Brown soil the high ratio Feo/Alo and the very little allophane content (~ 1%).
Conclusion
The genetic processes, of present Brown soil, red paleosol and successive ‘alterites’ are :
The present soil is an eutrophic Brown soil intergrade to andic (Andic Eutrochrept). It was
forming during the Holocene time (< 10 000 years old), on preweathered trachybasalt
periglacial materials., under a semi-humid (ustic) temperate (mesic) climate.
The red paleosol, from pyroclastic quartziferous trachybasalt, is a ‘Fersialitic’ soil. It was
probably forming during a long weathering time (> 100 000 years) under a warm (thermic)
and semi-humid (ustic) climate.
The vermiculite alterites were probably formed immediately after the volcanic eruption under
hot and hydrated, hydromagmatic, conditions.
The poorly ordered smectites alterites could be due to a slow underground weathering,
without obvious soil formation, through a slow rainwater percolation.
References
Chesworth, W., Dejou, J., de Kimpe, C. et al., 1983. Etude de paleosols rouges développés sur
basaltes miocénes du Massif Central frangais. Principales caractéristiques physico-
chimiques de ces pédogénéses. Cah. ORSTOM Sér. Pédol. XX (3): 189-208.
Dejou, J., Chesworth, W., and Larroque, P. 1982. Données nouvelles sur l’évolution
superficielle fersiallitique subie par les basaltes pontiens du Bassin d’Aurillac (Cantal,
France). Cas du profil de Saint Etienne de Carlat et considérations paleoclimatiques.
Pédologie, Gand, 32 :67-83.
Hétier, J.M. 1975. Formation et évolution des andosols en climat tempéré. Thése Univ. Nancy
1: 194 p.
Moinereau J. 1977. Altération des roches, formation et évolution des sols sur basalte, sous
climat tempéré humide (Velay, Vivarais, Coirons). Thése Univ. Sci. Et Techn. Languedoc.
Montpellier:139 p.
Acknowledgements : to Dra Concepción Jimenez, of La Laguna University, for her contribution to soil analysis.
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