Fjölrit RALA - 15.06.2004, Page 33

"Thermal" soils of Terceira island - the Azores
J. Pinheiro1, J. Madruga1, L. Matos1 and F. Monteiro2
1Departamento de Ciencas Agrarias, Universdade dos Agores Angra do Heroismo, Azores, Portugal;
2Departamento de Ciencias do Ambiente, Instituto Superior de Agronomia, Lisboa, Portugal
The volcanic nature of the Azores islands determines that hydrothermal activity occurs at
surface in some localized spots, generally associated to gas emissions of varied chemical
composition, and exerting a direct influence on pedogenesis. In the hydrothermal Fumas field
(Terceira island) and its vicinities we studied two pedons, distant from each other not more
then 50 metres, developed from the same parent material (basaltic lapilli) and showing a
similar horizon sequence, being only one of them under present hydrothermal influence as
observed from profile temperatures and gas emissions. The analytical results from these two
pedons show some dramatic differences conceming the weathering degree as inferred from
particle size distribution. The “thermal” pedon also shows drastic base depletion (aqua regia
extracted) as compared to the non-thermal one. Acid oxalate extractable A1 and Si also have
an intense decrease in the C horizons of the “thermal” soil which, consequently, has low
contents of allophonic materials.
Possible taxonomic implications of the hydrothermal effects are also addressed.
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